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Opportunities for Prevention of Concussion and Repetitive Head Impact Exposure in College Football Players : A Concussion Assessment, Research, and Education (CARE) Consortium Study

• 1 Department of Neurosurgery, Medical College of Wisconsin, Milwaukee
• 2 Department of Biomedical Engineering, Virginia Tech, Blacksburg
• 3 School of Public Health-Bloomington, Department of Epidemiology and Biostatistics, Indiana University, Bloomington
• 4 Department of Psychiatry, Indiana University School of Medicine, Indianapolis
• 5 Michigan Concussion Center, University of Michigan, Ann Arbor
• 6 UCLA Steve Tisch BrainSPORT Program, Department of Neurosurgery, University of California at Los Angeles
• 7 UCLA Steve Tisch BrainSPORT Program, Department of Pediatrics, University of California at Los Angeles
• 8 Department of Family Medicine and Orthopedic Surgery, University of California at Los Angeles
• 9 John A. Feagin Jr Sports Medicine Fellowship, Keller Army Hospital Military Academy, West Point, New York
• 10 Department of Physical Medicine and Rehabilitation, Uniformed Services University, Bethesda, Maryland
• 11 Air Force Academy, Colorado
• 12 Matthew Gfeller Sport-Related Traumatic Brain Injury Research Center, Department of Exercise and Sport Science, University of North Carolina at Chapel Hill
• 13 Department of Orthopedics and Rehabilitation, School of Medicine and Public Health, University of Wisconsin, Madison
• 14 Department of Biomedical Engineering, Medical College of Wisconsin, Milwaukee
• Editorial Who Will Protect the Brains of College Football Players? Christopher J. Nowinski, PhD; Robert C. Cantu, MD JAMA Neurology

Question   Where might there be opportunities to do the greatest good toward reducing overall concussion incidence and head impact exposure (HIE) in collegiate football?

Findings   In this cohort study, concussion incidence and HIE were disproportionately higher in the preseason than the regular season, and most concussions and HIE occurred during football practices.

Meaning   These findings point to specific areas where public policy, education, and other prevention strategies could be targeted to make the greatest overall reduction in concussion incidence and HIE in college football, which has important implications for protecting the safety and health of collegiate football players.

Importance   Concussion ranks among the most common injuries in football. Beyond the risks of concussion are growing concerns that repetitive head impact exposure (HIE) may increase risk for long-term neurologic health problems in football players.

Objective   To investigate the pattern of concussion incidence and HIE across the football season in collegiate football players.

Design, Setting, and Participants   In this observational cohort study conducted from 2015 to 2019 across 6 Division I National Collegiate Athletic Association (NCAA) football programs participating in the Concussion Assessment, Research, and Education (CARE) Consortium, a total of 658 collegiate football players were instrumented with the Head Impact Telemetry (HIT) System (46.5% of 1416 eligible football players enrolled in the CARE Advanced Research Core). Players were prioritized for instrumentation with the HIT System based on their level of participation (ie, starters prioritized over reserves).

Exposure   Participation in collegiate football games and practices from 2015 to 2019.

Main Outcomes and Measures   Incidence of diagnosed concussion and HIE from the HIT System.

Results   Across 5 seasons, 528 684 head impacts recorded from 658 players (all male, mean age [SD], 19.02 [1.25] years) instrumented with the HIT System during football practices or games met quality standards for analysis. Players sustained a median of 415 (interquartile range [IQR], 190-727) recorded head impacts (ie, impacts) per season. Sixty-eight players sustained a diagnosed concussion. In total, 48.5% of concussions (n = 33) occurred during preseason training, despite preseason representing only 20.8% of the football season (0.059 preseason vs 0.016 regular-season concussions per team per day; mean difference, 0.042; 95% CI, 0.020-0.060; P  = .001). Total HIE in the preseason occurred at twice the proportion of the regular season (324.9 vs 162.4 impacts per team per day; mean difference, 162.6; 95% CI, 110.9-214.3; P  < .001). Every season, HIE per athlete was highest in August (preseason) (median, 146.0 impacts; IQR, 63.0-247.8) and lowest in November (median, 80.0 impacts; IQR, 35.0-148.0). Over 5 seasons, 72% of concussions (n = 49) (game proportion, 0.28; 95% CI, 0.18-0.40; P  < .001) and 66.9% of HIE (262.4 practices vs 137.2 games impacts per player; mean difference, 125.3; 95% CI, 110.0-140.6; P  < .001) occurred in practice. Even within the regular season, total HIE in practices (median, 175.0 impacts per player per season; IQR, 76.0-340.5) was 84.2% higher than in games (median, 95.0 impacts per player per season; IQR, 32.0-206.0).

Conclusions and Relevance   Concussion incidence and HIE among college football players are disproportionately higher in the preseason than regular season, and most concussions and HIE occur during football practices, not games. These data point to a powerful opportunity for policy, education, and other prevention strategies to make the greatest overall reduction in concussion incidence and HIE in college football, particularly during preseason training and football practices throughout the season, without major modification to game play. Strategies to prevent concussion and HIE have important implications to protecting the safety and health of football players at all competitive levels.

• Editorial Who Will Protect the Brains of College Football Players? JAMA Neurology

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McCrea MA , Shah A , Duma S, et al. Opportunities for Prevention of Concussion and Repetitive Head Impact Exposure in College Football Players : A Concussion Assessment, Research, and Education (CARE) Consortium Study . JAMA Neurol. 2021;78(3):346–350. doi:10.1001/jamaneurol.2020.5193

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• Volume 58, Issue 18
• Subsequent musculoskeletal injury after concussion in National Football League players
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• http://orcid.org/0000-0001-5957-5878 Erin B Wasserman 1 ,
• Alexandra Chretien 1 ,
• http://orcid.org/0000-0002-3670-6609 Kimberly G Harmon 2 ,
• http://orcid.org/0000-0002-1478-8068 Margot Putukian 3 ,
• David Okonkwo 4 ,
• http://orcid.org/0000-0002-1842-9759 Gary S Solomon 5 , 6 ,
• Javier Cardenas 6 , 7 ,
• Mackenzie M Herzog 1 ,
• Allen Sills 5 , 6 ,
• Christina D Mack 1
• 1 IQVIA Inc , Durham , North Carolina , USA
• 2 University of Washington , Seattle , Washington , USA
• 3 Major League Soccer , New York , New York , USA
• 4 University of Pittsburgh , Pittsburgh , Pennsylvania , USA
• 5 Vanderbilt University , Nashville , Tennessee , USA
• 6 National Football League , New York , New York , USA
• 7 West Virginia University , Morgantown , West Virginia , USA
• Correspondence to Dr Christina D Mack; christina.mack{at}iqvia.com

Objective To assess whether National Football League (NFL) players diagnosed with a concussion have an increased risk of injury after return to football.

Methods A retrospective cohort study analysed the hazard of subsequent time-loss lower extremity (LEX) or any musculoskeletal injury among NFL players diagnosed with a concussion in 2015–2021 preseason or regular season games compared with: (1) all non-concussed players participating in the same game and (2) players with time-loss upper extremity injury. Cox proportional hazards models were adjusted for number of injuries and concussions in the prior year, player tenure and roster position. Additional models accounted for time lost from participation after concussion.

Results There was no statistical difference in the hazards of LEX injury or any musculoskeletal injury among concussed players compared with non-concussed players, though concussed players had a slightly elevated hazard of injury (LEX injury: HR=1.12, 95% CI 0.90 to 1.41; any musculoskeletal injury: HR=1.08, 95% CI: 0.89 to 1.31). When comparing to players with upper extremity injuries, the hazard of injury for concussed players was not statistically different, though HRs suggested a lower injury risk among concussed players (LEX injury: HR=0.78, 95% CI: 0.60 to 1.02; any musculoskeletal injury: HR=0.82, 95% CI: 0.65 to 1.04).

Conclusion We found no statistical difference in the risk of subsequent injury among NFL players returning from concussion compared with non-concussed players in the same game or players returning from upper extremity injury. These results suggest deconditioning or other factors associated with lost participation time may explain subsequent injury risk in concussed players observed in some settings after return to play.

• Brain Concussion
• Sporting injuries
• Cohort Studies

Data availability statement

No data are available.

https://doi.org/10.1136/bjsports-2023-107970

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WHAT IS ALREADY KNOWN ON THIS TOPIC

There are contrasting reports regarding the risk of subsequent musculoskeletal injury following concussion, with few studies having robust data on potential confounders (eg, prior injury history, time at risk), detailed follow-up, or a comparative group of athletes without concussion but with time loss from sport.

WHAT THIS STUDY ADDS

This study identified time lost due to injury, and not necessarily concussion, as a potential driver of risk for subsequent musculoskeletal injury after return to participation among professional American football players.

HOW THIS STUDY MIGHT AFFECT RESEARCH, PRACTICE OR POLICY

Clinicians should consider the increased risk of subsequent injury from deconditioning due to time away from sport, regardless of the cause of the time loss. These findings suggest that concussed athletes as well as those returning from an upper extremity injury may benefit from more robust conditioning prior to reintroduction to sport; the impact of such an approach on all injured athletes should be further investigated.

Introduction

Concussion can be associated with transient alterations in neuromuscular control, cognitive processing speed, and vestibular and dynamic proprioceptive function. 1 2 This has led to analyses of risk of subsequent injury after return to play following concussion recovery. Examining the association between concussion and future injury risk is complex. While some studies have reported an increased risk of subsequent injury post-concussion, others have failed to support this association. 3–21 Despite mixed findings, three published systematic reviews of the relationship between concussion and lower extremity (LEX) injuries have all concluded that odds or risk of sustaining a LEX injury is higher after a concussion, with effect sizes from 1.60 to 2.49 (ORs and incidence rate ratios). 22–24

Most published studies do not use data sources generated from robust reporting from medical staff on injuries that occur across the entire year, rather than during the competitive season only, which limits the ability to account for one of the main predictors of injury in sport: previous injury. 25–27 Additionally, other factors such as player age, experience and timing during the competitive season are not taken into account. Many studies also do not accurately account for time at risk after concussion and instead include the time the player misses due to the concussion in the time at risk for a subsequent injury. Selection of the comparison (control) group is also important, as inclusion of only a non-concussed, primarily uninjured comparison group may result in bias since concussed players are removed from sports participation for a period of time while non-concussed, uninjured controls are not. 28 29

This study aimed to assess whether National Football League (NFL) players who were diagnosed with concussion had increased risk of subsequent time-loss injury (LEX-specific and overall injury risk) using internal, medically reported data and robust methodology, including detailed injury information, athlete-specific information and multiple comparison groups.

Data sources and study population

NFL player injury data were obtained from Club medical staff and entered into the League-wide electronic health record (EHR). Data entry for all sports-related injuries that receive medical treatment is mandatory for all 32 Clubs, using a customised coding system of diagnosis terms specifically tailored to NFL player injuries and adhering to standardised collection guidelines. Data quality reviews, including checks for completeness and consistency, are conducted on a weekly basis to ensure accurate and timely reporting. 30 In addition to reporting injury diagnosis, athletic trainers record additional details of the injury, including setting, timing during the season, player tenure, player position and activity. Data reporting for injuries that result in the player being restricted or limited from their normal level of participation in any activity (ie, ‘time-loss injuries’) also includes dates of removal, participation status, and the date of return to partial and full football activities. Exposure information on the game and play-level (type of play and total number of plays) for individual athletes is collected from the NFL Game Statistics and Information System for all NFL games and linked to the NFL EHR.

The Concussion in Sport Group’s definition of concussion was used throughout the study period. 31 32 Consistent with the NFL Head, Neck and Spine Committee’s Concussion Diagnosis and Management Protocol, all players diagnosed with concussion follow a stepwise progression back to full participation which requires final clearance by the Club Team Physician and confirmation by an independent neurological consultant. 33 34 To assess subsequent injury risk after concussion, this study retrospectively compared the hazard of subsequent injury among concussed players to two different comparator groups: (1) all other non-concussed players who participated in the same game (Comparison Group 1) and (2) players who sustained a time-loss upper extremity injury (Comparison Group 2). Concussed players included all players who were diagnosed with a concussion during a 2015–2021 preseason or regular season game, including during pre-game activities. Comparison Group 1 included all players from the same Club as the concussed player who participated in at least one play in the game in which a concussion was sustained, regardless of whether they sustained an injury other than a concussion during that game. This group represented individuals with a similar follow-up time, schedule and Club environment as the concussed group. Comparison Group 2 included players who sustained at least one in-game time-loss upper extremity injury, defined as a sprain, strain or fracture to the arm, clavicle, elbow, fingers, forearm, hand, shoulder or wrist. This group was selected with the hypothesis that these players lost time after their upper extremity injury, similar to the concussed group, but in most cases the upper extremity injury would not be a risk factor for the study outcome of LEX injury. To minimise potential bias, players (in any group) were excluded if they had sustained a concussion within the same season prior to study entry (eg, during practice), did not return to football participation after the index injury or had a missing return from injury date in the EHR (most often due to the player leaving the Club with which the index injury occurred before injury resolution). Players in their rookie season were excluded due to limited prior injury data, which is a strong risk factor for the subsequent injury outcomes of this study.

Outcome measurements

The hazard of subsequent injury was measured for two different musculoskeletal injury outcomes: (1) time-loss LEX injury, defined as a sprain, strain or fracture to the ankle, foot, heel, hip, knee, leg, thigh or toe that resulted in the player being restricted or limited from their normal level of participation and (2) time-loss injury, defined as a sprain, strain or fracture that resulted in being restricted or limited from the player’s normal level of participation. These outcome injuries could occur in any football-related setting (eg, practice, game, conditioning session). Outcome injuries were restricted to musculoskeletal injuries (sprain, strain or fracture) because these were considered the injuries on which post-concussion neuromuscular control deficits would have an impact.

Study follow-up

Players entered the study (index date) on the game date of their injury (Concussed Group, Comparison Group 2 and injured players from Comparison Group 1; figure 1 , Players 1–4) or the game date in which another player from the same Club sustained a concussion (uninjured players from Comparison Group 1; figure 1 , Players 5–7). Follow-up time for injured players began on the day they returned to full participation; the days the player missed due to the concussion or other injury were excluded from follow-up time but accounted for in adjusted models to account for time they were not participating. The hazard of subsequent injury was measured through the remainder of the preseason and regular season (referred to as full-season follow-up), as well as through the 30-day and 60-day periods following the index date.

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Inclusion and exclusion criteria and calculation of time to event for different cohorts and timing scenarios. a a Secondary analyses that censored at 30 days and 60 days after the index time-point were also conducted.

Study follow-up time was calculated as the number of plays in which the player participated in games during the follow-up period (player-plays), which accounts for exposure to injury risk at a more granular level than follow-up time in days or number of games played. For analyses with any injury as the outcome, study follow-up time ended at the time of injury; if a player was not injured, follow-up was stopped (censored) at the end of the time-period of interest (full-season, 60-day or 30-day) or removal from the team’s roster, whichever occurred first. Injured players who were removed from the roster, sustained a subsequent injury prior to return to play or returned to play after the end of the regular season (the end of study follow-up) were excluded from analyses.

For analyses examining time to LEX injury, study follow-up time was calculated the same as described above, except follow-up was stopped for players who sustained an injury other than the outcome injury. Follow-up was not stopped for non-time-loss upper extremity injuries, as occurrence of this injury likely would not influence subsequent risk of LEX injury.

Statistical methods

Descriptive statistics were calculated for all variables of interest including means, medians, SD and IQRs. HRs and 95% CIs were calculated using Cox Proportional Hazards models for the two comparator groups (Comparison Group 1 and Group 2) across both outcomes (time-loss LEX musculoskeletal injury and any time-loss musculoskeletal injury), resulting in four primary statistical models. Sensitivity analyses examined the 30-day and 60-day follow-up period to determine whether there was an impact of concussion on injury risk within the time immediately following concussion, when one would expect a larger deficit in neuromuscular control. A 95% CI exclusive of 1.0 was considered statistically significant.

All models adjusted for number of football-related injuries (all or LEX only in models with LEX injury as the outcome) sustained during the 365 days prior to the index time point (regardless of the setting during which they occurred), the number of football-related concussions during the 365 days prior to the index time point (regardless of the setting during which they occurred), player tenure (years of experience) in the NFL and roster position. Analyses that included number of days missed due to injury from the index date as a potential mediator were also conducted to assess the impact of time missed from participation on the outcome. To account for multiple observations among the same player, we added a frailty term to the models.

Patient and public involvement

Patients and/or the public were not directly involved in the design, conduct or reporting of this research; however, the NFL Players Association approved this work through the NFL Player Scientific and Medical Research Protocol and the Players Association provided feedback on the findings of the work. 35

Equity, diversity and inclusion statement

All players in the NFL were eligible for inclusion in analyses. All players are men and are racially and ethnically diverse. The research team comprises a diverse group of clinicians and epidemiologists (60% women). Authors were invited based on their expertise, as demonstrated by previous research and clinical experiences with musculoskeletal injury and concussions.

Study population and player characteristics

The dataset included 766 player-observations (defined as the number of non-distinct players included in the analysis) among 641 unique players with a concussion, 51 572 player-observations among 4878 unique players in Comparison Group 1 (non-concussed players from the same game) and 789 player-observations among 653 unique players from Comparison Group 2 (time-loss upper extremity injury). Player age, weight, NFL tenure and number of prior injuries were similar among groups; roster position varied slightly by group ( table 1 ).

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Player characteristics by exposure/comparator group, National Football League, preseason and regular season, 2015–2021

Comparison with all non-concussed players from the same game

Concussed players had a slightly smaller proportion of players sustain a subsequent LEX injury across all follow-up time points ( table 2 ). When accounting for potential confounders and taking into account time at risk, there was not a statistically significant difference in the hazard of sustaining a subsequent LEX injury within the same season among concussed players compared with non-concussed players (Comparison Group 1), although the hazard was 9% higher (HR=1.09, 95% CI: 0.88 to 1.35; table 3 ). Findings were consistent at the 60-day follow-up time period, with similar but slightly larger effect estimate. At the 30-day follow-up, when not accounting for time missed due to injury, the concussed group had a significantly higher hazard of subsequent LEX injury (HR=1.39, 95% CI: 1.07 to 1.82; table 3 ).

Players who sustained a sprain, strain or fracture requiring lost playing time during follow-up by group, National Football League preseason and regular season, 2015–2021

Comparison Group 1: hazard of subsequent injury following concussion compared with all other non-concussed players who played in the same game, by outcome; National Football League, preseason and regular season, 2015–2021

When expanding to all time-loss musculoskeletal injuries, concussed players and Comparison Group 1 had the same proportion of players sustain a subsequent time-loss injury when followed through the full-season and at 60 days of follow-up ( table 2 ). At 30 days of follow-up, concussed players had a smaller proportion of players sustain a subsequent time-loss injury compared with Comparison Group 1 ( table 2 ). The hazard of sustaining a subsequent time-loss injury within the same season was not significantly different compared with Comparison Group 1, though it was 6% higher (HR=1.06, 95% CI: 0.88 to 1.27; table 3 ). At the 30-day follow-up, when not accounting for time missed due to injury, the concussed group had a significantly higher hazard of subsequent musculoskeletal injury (HR=1.36, 95% CI: 1.08 to 1.70; table 3 ).

Importantly, concussed players lost time due to the concussion before they returned to participation and started their follow-up period for subsequent injury (mean=10.5 days, SD=7.2; median=10.0 days, IQR: 0–81), whereas non-concussed players in Comparison Group 1 only lost time after their index date if they sustained a non-concussion injury during the same game (mean=0.5 days, SD=34; median=0.0 days, IQR=0–93). For the 30-day and 60-day follow-up periods, accounting for this difference shifted the hazard observed for subsequent time-loss injury closer to the null, and the effect at 30 days was no longer statistically significant ( table 3 ).

Comparison with players with time-loss upper extremity injury

In terms of time lost due to the upper extremity injury before the player returned to participation and started their follow-up period for subsequent injury, players in Comparison Group 2 lost an average of 13.7 days (mean=13.7, SD=15.0; median=7.0, IQR=0.0–121.0), which was similar to the 10.5 day average among concussed players.

The hazard of a subsequent time-loss LEX injury within the same season among concussed players was not significantly different compared with players in Comparison Group 2, although the hazard was 22% lower (HR=0.78, 95% CI: 0.60 to 1.02; table 4 ). At the 60-day follow-up, there was a statistically significantly lower hazard among concussed players (HR=0.74, 95% CI: 0.56 to 0.98). These results were consistent with a non-statistically significant lower hazard of a subsequent time-loss injury for concussed players when compared with players returning after an upper extremity injury (Comparison Group 2; HR=0.82, 95% CI: 0.65 to 1.04; table 4 ). Results were consistent among 60-day and 30-day follow-up time periods ( table 4 ).

Comparison Group 2: hazard of subsequent injury following concussion compared with time-loss upper extremity injury, National Football League 2015–2021

In this study of NFL athletes, risk of subsequent musculoskeletal injury was elevated compared with non-concussed athletes but lower compared with players who sustained an upper extremity injury, though neither association was statistically significant. Comparison Group 2 players, who, similar to the concussed players, lost an average of 10–15 days between their upper extremity injury and their return to participation, had a higher hazard of subsequent injury compared with athletes returning from concussion.

There has been significant interest in the relationship between concussion and subsequent injury risk, with literature focused largely on post-concussive LEX injuries. A number of these studies report an increase in injury after concussion, but these frequently use an uninjured control group, which does not account for days lost due to concussion, nor other unmeasured residual confounding (eg, exposure to contact). Other methodological limitations in prior reports include use of publicly available or incomplete injury data, which does not contain confirmed or granular return to participation information, accurate (or any) medical history or necessary player-specific information. Importantly, other studies either do not account for follow-up time when the player is at risk for sustaining a subsequent injury or measure this in days rather than using a metric that incorporates true player exposure such as player-plays. 36 37 Of note, Wilson et al 8 used injured comparison groups and did not observe a statistically significant elevated risk of subsequent lower extremity injury among concussed players, similar to our findings.

The current consensus within the literature is that concussion interferes with neurological function (eg, cognition, oculomotor, vestibular and balance), leading athletes to greater risk of subsequent injury because of residual deficits in these domains; however, our data suggest that the observed increased risk could be due to the time missed due to concussion. After accounting for time missed due to injury, we observed a slightly elevated risk of subsequent musculoskeletal injury compared with non-concussed players, but this was not statistically significant. If residual neurocognitive effects of concussion are the driving cause for increased subsequent musculoskeletal injury risk, we would have expected to see an increased risk compared with players with upper extremity injuries, which we did not. 1

Other factors such as physical deconditioning may explain the elevated hazard of injury after injury; however, we neither defined nor measured deconditioning in this study. The models in the current analysis that included days lost following the concussion showed an attenuated subsequent injury risk among concussed players, compared with models that did not include this factor. Early exercise is encouraged after concussion as well as any upper extremity injury; however, concussion return to participation protocols in the NFL specifically require a league-standardised, rigorous, highly-monitored stepwise progression back to full sport, which provides some level of conditioning and, in the football setting, introduction to contact. 34 Practices for return from upper extremity injuries may also be rigorous, but they are more variable based on the type of injury, player position and the medical practitioner. This may explain the observed slightly lower hazard of subsequent time-loss injury among concussed players compared with those with an upper extremity injury.

It is important to note that this study was among elite, male American football athletes, who may have a different risk of subsequent injury than other cohorts. However, our results showed a smaller magnitude of association than reported in other elite sports including European soccer 4 and rugby union, 3 and our results were not statistically significant. In previous studies of NFL players, most authors used publicly available data to assess subsequent risk injury post-concussion and had varying findings, likely due to differences in methods. 17–19 21 Buckley et al 18 reported that 21.4% of concussed players had subsequent musculoskeletal injuries in the remainder of the season compared with 26.4% of control athletes, and there was no observed difference in time to subsequent musculoskeletal injury between groups. Similarly, Jildeh et al 19 analysed publicly available data for the 2012–2016 seasons and did not observe a statistically significant difference in the odds of LEX injury during the 90-day period after return to play between concussed and control athletes (OR=0.573, 95% CI: 0.270 to 1.217). However, Baker et al 17 evaluated all active NFL players during the 2016–2019 seasons and did find players with a single concussion and multiple concussions to have an increased odds of sustaining a LEX injury compared with non-concussed players (OR: 2.92, SD: 1.7–4.9 and OR: 2.28, SD: 1.5–3.6, respectively). These studies did not have the robust data derived from the prospectively maintained and audited injury reports and electronic medical record mandated by NFL policy and procedures. Therefore, they were likely unable to accurately capture all subsequent injuries or account for player history of prior concussion or musculoskeletal injury, nor were they able to accurately measure the time lost due to injury. Pietrosimone et al 20 surveyed former NFL athletes about their injury history and showed that players with a history of concussions had higher odds of musculoskeletal injury; however, this did not account for when the injuries occurred relative to each other or time at risk.

Clinical implications

Findings of the current study may suggest that time lost due to injury, and not necessarily lingering neurologic dysfunction, may be the driving factor in subsequent injury risk after return to play. This observation was also noted in athletes returning from time lost due to an upper extremity injury in the absence of concussion. Taken together, our findings suggest that concussed athletes may benefit from rigorous, protocolised approach to reintroduction to sport; we recommend further research to determine whether such an approach could have a beneficial impact on all injured athletes.

Limitations

Limitations of the current study include differences in follow-up time across the athletes after return to play, especially for those athletes in Comparison Group 1 who were not injured at the start of follow-up. Athletes were censored at the end of regular season or removal from the team’s roster, which resulted in follow-up for some athletes being short of the 30-day, 60-day or full-season follow-up time-periods; however, these censorship points are accounted for by the model and are considered to be uninformative of the outcome of interest. The use of in-game player-plays as a measurement of follow-up is a strength of our study, as it accounts for the player’s true exposure to football participation; however, it does not account for exposure across all types of football activities (eg, practices, pre-game). Additionally, injury history prior to the NFL (such as collegiate injury history) was not available; as a result, rookie players were excluded from these analyses and models were adjusted for prior injury only in the 365 days prior to the index time point. This study covers a wide time range, during which concussion detection and management changed; however, sensitivity analyses did not demonstrate a change in results interpretation when restricting to more recent years ( online supplemental table ). This study was able to demonstrate the impact of using different comparison groups and control for a number of potential confounding factors; however, we could not fully account for intrinsic injury risk or potential for different injury risk based on game schedule (eg, opponent, weather). The applicability of these findings to other sports with a different season schedule, non-elite athletes or women is not clear and could not be assessed with this study.

Supplemental material

Results demonstrate that, in elite male American football athletes, risk of subsequent musculoskeletal injury was elevated compared with non-concussed athletes but lower compared with players who sustained an upper extremity injury, though neither association was statistically significant. These findings are distinct from some previous studies due to the ability to account for prior injuries as well as time out of sport during recovery and time at risk following return. The results highlight the need for increased attention to reintroduction to sport after any injury, across all sports, as time lost due to injury may be an important factor in risk of subsequent injury.

Ethics statements

Patient consent for publication.

Not applicable.

Ethics approval

This study was approved through the NFL Player Scientific and Medical Research Protocol and the Mt. Sinai Institutional Review Board (Study# 19-NFL05-CR001).

Acknowledgments

The authors thank the NFL athletic trainers for diligent reporting of NFL player injuries. We wish to acknowledge the contributions of the IQVIA Injury Surveillance and Analytics team, for data curation efforts. We also appreciate the collaboration of the NFL Players Association in the completion of this study.

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Supplementary materials

Supplementary data.

This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.

• Data supplement 1

X @DrKimHarmon, @Mputukian

Contributors EW, AC, MH and CM had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis, EW as guarantor. All authors contributed to the concept and design of the manuscript, results interpretation drafting of the manuscript and critical revision of the manuscript for important intellectual content. EW, MH and AC completed the acquisition, analysis or interpretation of data as well as the statistical analysis. EW, MH, CM and AC provided administrative, technical or material support. CM, MH and EW provided analytic supervision.

Funding The curation, build and analytics in the NFL Injury Database are funded by the NFL/NFLPA.

Competing interests EW, AC, MH and CM are employees of IQVIA, which is in a paid consultancy with the NFL. GSS is a paid consultant to the NFL. MP is a Senior Advisor for the NFL Head Neck & Spine Committee, a member of the NFL General Medical Committee and a paid consultant to MLS. JC is the Vice Chair of the NFL Head Neck & Spine Committee and a paid consultant to the NFL. AS is a full-time employee of the NFL.

Patient and public involvement Patients and/or the public were not involved in the design, or conduct, or reporting, or dissemination plans of this research.

Provenance and peer review Not commissioned; externally peer reviewed.

Supplemental material This content has been supplied by the author(s). It has not been vetted by BMJ Publishing Group Limited (BMJ) and may not have been peer-reviewed. Any opinions or recommendations discussed are solely those of the author(s) and are not endorsed by BMJ. BMJ disclaims all liability and responsibility arising from any reliance placed on the content. Where the content includes any translated material, BMJ does not warrant the accuracy and reliability of the translations (including but not limited to local regulations, clinical guidelines, terminology, drug names and drug dosages), and is not responsible for any error and/or omissions arising from translation and adaptation or otherwise.

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• Short Report
• Open access
• Published: 30 January 2024

Publicly available data sources in sport-related concussion research: a caution for missing data

• Abigail C. Bretzin   ORCID: orcid.org/0000-0003-3730-2849 1 ,
• Bernadette A. D’Alonzo 3 , 4 ,
• Elsa R. van der Mei 5 ,
• Jason Gravel 6 &
• Douglas J. Wiebe 1 , 2  

Injury Epidemiology volume  11 , Article number:  3 ( 2024 ) Cite this article

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Metrics details

Researchers often use publicly available data sources to describe injuries occurring in professional athletes, developing and testing hypotheses regarding athletic-related injury. It is reasonable to question whether publicly available data sources accurately indicate athletic-related injuries resulting from professional sport participation. We compared sport-related concussion (SRC) clinical incidence using data from publicly available sources to a recent publication reporting SRC using electronic health records (EHR) from the National Football League (NFL). We hypothesize publicly available data sources will underrepresent SRC in the NFL. We obtained SRCs reported from two publicly available data sources (NFL.com, pro-football-reference.com) and data reported from the NFL’s published EHR. We computed SRC per 100 unique player signings from 2015–2019 and compared the clinical incidence from publicly available data sources to EHR rates using clinical incidence ratios (CIR) and 95% confidence intervals (CI).

From 2015–2019, SRC counts from published EHR record data ranged from 135–192 during the regular season, whereas SRC counts ranged from 102–194 and 69–202 depending on the publicly available data source. In NFL.com the SRC clinical incidence was significantly and progressively lower in 2017 (CIR: 0.73, 95% CI: 0.58–0.91), 2018 (CIR: 0.66, 95% CI: 0.50–0.87), and 2019 (CIR: 0.48, 95% CI: 0.35–0.64) relative to the gold-standard EHR. In the pro-football-reference.com data, the documented SRCs in publicly available data sources for other years were ~ 20–30% lower than the gold-standard EHR numbers (CIRs 0.70–0.81).

Conclusions

Publicly available data for SRCs per 100 unique player signings did not match published data from the NFL’s EHR and in several years were significantly lower. Researchers should use caution before using publicly available data sources for injury research.

Background/context

Across many sports, findings from large epidemiology and surveillance studies continue to inform sport-related concussion (SRC) prevention efforts, including policy and rule changes. American football offers several examples. For example, injury surveillance research identified the protective impact of progressive limitations to collision practices (Bretzin et al. 2022 ; Pfaller et al. 2019 ; Broglio et al. 2016 ). Other research using injury surveillance found that rule changes, including the Ivy League Kickoff Rule (Wiebe et al. 2018 ) and penalizing targeting (Baker et al. 2021 , 2022 ) were associated with reduced rates SRC, albeit sometimes with unintended consequences (e.g., increasing likelihood of lower extremity injury) (Hanson et al. 2017 ; Westermann et al. 2016 ). Such studies demonstrate the importance of valid documentation of incident SRC; if the surveillance data underlying analyses is inaccurate the conclusions may be, too.

Certainly SRC poses a concern in the National Football League (NFL), where SRC accounts for nearly 7% of injuries (Bedard and Wyndham Lawrence 2021 ). Recent estimates for the likelihood of sustaining of a single SRC in the NFL ranged from 6.2–8.3% per player per season during the 2015–2019 seasons (Mack et al. 2021 ; Cools et al. 2022 ), and the likelihood of repeat concussion during the same season ranged from 5.3–8.3% (Cools et al. 2022 ). Descriptive analyses based on surveillance data identified in-game factors including time during the game, player position, location on the field, level of anticipation, type of play, and helmet location and impact type as important correlates of SRC occurrence in the NFL (Casson et al. 2010 ; Clark et al. 2017 ). These findings may help identify players at risk for SRC, and inform rule changes aimed at improving player safety; although, continued investigation of these and other factors is warranted. In parallel, evidence continues to accumulate, but is mixed, regarding negative short- and long-term outcomes of SRC among former NFL players such as worse cognitive function and mood-related (e.g., anxiety and depression) symptoms (Walton et al. 2021 ; Brett et al. 2021 ) and neurodegenerative disease (e.g., chronic traumatic encephalopathy) (Mez et al. 2017 ). As such, American football remains ripe with opportunities for developing and implementing prevention efforts aimed at reducing SRCs.

Notably, accessing high-quality surveillance data from the NFL requires approval through multiple levels of independent review (Dreyer et al. 2019 ), which represents a potential deterrent for some investigators. This issue may prompt researchers to find novel opportunities using publicly obtainable data sources. For example, injury reports published by the NFL available on NFL.com which reports players’ injuries by week during the season, or pro-football-reference.com which is designed to democratize sports-related data for users to understand and share the sports they love, are obtainable data sources used to test associations between hypothesized risk factors and injury rates. For example, in a recent study Bedard and Wyndham Lawrence ( 2021 ) described the epidemiology of injury using the NFL.com injury reports and suggested a decrease in SRC rates between the 2014 to 2015 seasons. Others used these publicly obtainable data sources to separately test the associations between SRC rates and introduction of Playing Rule Article 8 (Baker et al. 2021 ), unconventional game schedules (i.e., Thursday game day, overseas play) (Teramoto et al. 2017 ), and the association between in subsequent musculoskeletal injury after sustaining a SRC (Buckley et al. 2022 ).

Importantly, a recent systematic review identified publicly obtained data sources of SRC in the NFL only captured 70% of medically reported SRC (Inclan et al. 2023a ). Another systematic review of studies of the NFL found that publicly obtainable data sources only captured 66% of anterior cruciate ligament (ACL) injuries that were reported by team medical records, the gold standard for documented injury (Inclan et al. 2022 ). Evidently then, interpreting findings from studies using publicly obtainable data sources warrants caution as incomplete data sources may yield inaccurate results due to measurement bias. Accordingly, we aimed to describe injury counts captured in publicly available data sources and compare injury rates relative to published SRC data originating from the NFL’s electronic health records (EHR). We hypothesize that studies based on publicly available data will underestimate the number of concussions when compared the NFL EHR data.

Materials and methods

Data sources.

We obtained data using publicly available data sources and previous published aggregate data. Institutional review from the University of Michigan approved this study as exempt; therefore, approval of consent was waived. All methods were performed in accordance with the ethical standards as laid down in the Declaration of Helsinki and its later amendments or comparable ethical standards.

Publicly available data sources

Teams are required by the NFL to list all reportable injuries in a practice report, game status reports, and are responsible for reporting in-game injuries (NFL Communications Department 2017 ). We identified concussions sustained by NFL players across five athletic seasons (2015–2019) using the Pro Football Reference database (pro-football-reference.com) and NFL injury reports (NFL.com). We captured all injuries given the designation “concussion” across the five seasons during the 17 regular season games each year. As this data is only reported weekly and does not give the exact injury date, we determined an incident SRC as the first reported “concussion” for each player. Instances where a player was removed from the injury report for at least one week before reappearing on the report with a concussion were labeled as a subsequent incident concussion. Otherwise, they were considered as ongoing concussions. We also determined the number of players that sustained a concussion if an athlete was given the designation “concussion” at least once during the season. For sensitivity analyses, we additionally captured all injuries designated as “head” using the same methodology. This was completed in R version 4.2.0 (2022-04-22) “Vigorous Calisthenics”.

Electronic health records

A recent study on the epidemiology of SRC in the NFL reported that 1,302 concussions occurred among 1,004 NFL players across the 2015–16 through 2019–20 seasons. In this retrospective, observational study Mack et al. ( 2021 ) described it is mandated that all NFL player injury data across the 32 clubs requiring medical treatment are reported into the league’s electronic health record (EHR). Due to mandatory injury reporting, standardized training and reporting of injury data, and audits against internal and external sources, we determined these were the best available data to compare against publicly available data sources. In addition, Inclan et al. ( 2023a ) described this data source as the “gold standard” for NFL injury data. Furthermore, these data match an available news release disseminated by the NFL (NFL Player Health and Safety 2015 ), and additional analyses investigating subsequent or repeat concussions occurring in the same year (Cools et al. 2022 ). From this data source, we obtained injury counts overall and by time during season (off-season, pre-season, regular season, post-season), and the number of players injured with SRC each year reported in the study. Further, as a measure of exposure, we also obtained the number of unique player signings from this published record.

Statistical analyses

We used descriptive statistics including frequencies and percentages to describe data from each available data source. After combining data sources, we report the percent of medically documented SRC in the EHR that were accounted for in the publicly available data source(s). We replicate the findings by Mack et al. ( 2021 ), in which we computed the clinical incidence of SRC, defined as the number of SRCs divided by the unique player signings per 100 athletes during the regular season (Knowles et al. 2006 ). Note that our numerator includes multiple concussions across the same player, but our denominator only counts each player once. To compare the clinical incidence between publicly available data and published EHR, we computed clinical incidence ratios (CIR) and 95% confidence intervals (CI). We determined statistical significance if 95% CI excluded one. We performed all analyses in Stata (Stata Statistical Software: Release 17. College Station, TX: StataCorp LLC).

Sensitivity analysis

Additionally, we identified novel instances of “head” injury reported in both publicly available data sources during the study period. We did this to account for the possibility that the information available to each of these data sources may lead the curators to classify some SRCs as a head injury in their data rather than as “concussion” specifically. We report the incidence of such injuries annually to assess how including them as suspected SRCs would make the data from public data sources compare to the EHR data gold standard.

Previously reported EHR data indicated a total of 834 SRC occurring in the regular NFL seasons from 2015–2019. Overall, in the pro-football-reference.com and NFL.com data sources, we identified 694 and 660 incident SRCs across the study period, respectively. Tables 1 and 2 report the SRC data by season and data source. Data from pro-football-reference.com captured 101.0% of SRC in 2015, 81.4% in 2016, 81.1% in 2017, 77.0% in 2018, and 70.3% in 2019 relative to published EHR data. Data from NFL.com captured 105.2% of SRC in 2015, 93.0% in 2016, 72.6% in 2017, 65.9% in 2018, and 47.6% in 2019 relative to published EHR data.

We report the clinical incidence per 100 athletes by year in Fig.  1 . For the published EHR data, annual clinical incidence was 5.86, 5.23, 5.69, 4.05, and 4.58, respectively. The publicly available data from pro-football-reference.com had annual clinical incidences that ranged from 3.12 to 5.92. The publicly available data from NFL.com had similar clinical incidence ranges from 2.18 to 6.16.

Annual clinical incidence per unique player signings and 95% confidence intervals of sport-related concussion (SRC) reported for a gold-standard source and two sources of publicly available data by year

Figure  2 compares annual clinical incidence in each publicly available dataset relative to the clinical incidence in the published EHR data. When comparing pro-football-reference.com to published EHR data, SRC clinical incidence was similar in 2015 (CIR: 1.01, 95% CI: 0.82–1.24) and then were progressively lower annually with an CIR of 0.48 (95% CI: 0.35–0.64) in 2019. When comparing NFL.com to published EHR data, SRC rates were again similar in 2015 (CIR: 1.05, 95% CI: 0.86–1.29) and were progressively lower annually with an CIR of 0.70 (95% CI: 0.54–0.91) in 2019.

Clinical incidence ratios (CIRs) comparing the annual SRC clinical incidence per 100 unique player signings in two publicly available data sources relative to a gold-standard data source (Mack et al. 2021 )

The annual number of head injuries, respectively, over the study period as reported by pro-football-reference.com was 7, 8, 0, 3, and 4 and was 4, 3, 0, 2, and 5 as reported by NFL.com.

We used publicly available data sources of injury reports from 2015 through 2019 in the NFL to compare SRC clinical incidence relative to data from the NFL EHR. Our results indicate that publicly available data may underrepresent SRC clinical incidence based on the year and sources of data. For example, pro-football-reference.com and NFL.com only captured 70% and 48% of SRCs in 2019, 77% and 66% in 2018, 81% and 73% in 2017, 81% and 94% in 2016, and 101% and 105% in 2015, respectively, relative to data from published EHR (Mack et al. 2021 ). Thus based on these results, investigators should question whether publicly available data sources can be used to accurately complete their research. The number of “head” injuries reported in publicly available data sources each year was small—ranging from 0 to 8—relative to the number of concussion injuries reported annually, indicating that misclassified concussions or coding definitions do not account for the differences between the publicly available and gold-standard data.

Our findings, of inconsistencies in medically documented SRC in EHR (Mack et al. 2021 ) and publicly available data sources, are perhaps unsurprising when considering the findings of recent studies noted above. Specifically, the recent report by Inclan et al. ( 2023a ) that identified that publicly obtainable data sources only accounted for 70% of medically documented SRC in the NFL. Also the systematic review by Inclan et al. ( 2022 ) revealed that only 66% of ACL injuries were captured in publicly available data sources. Furthermore, the authors reported that the discrepancies in the percentage of ACL injuries reported in publicly available data sources varied by certain factors (e.g., position, type of play) (Inclan et al. 2022 ). Therefore, our findings that publicly available data from 2015–2019 only captured 79–83% of medically documented SRC in EHR depending on the source appear to reveal a genuine reason for concern.

The current study provides meaningful findings, considering the prevalence of research reports that disseminate results based on analyses using publicly available data. In fact, a recent systematic review and bibliometric analysis identified an exponential increase in manuscripts using this methodology (Inclan et al. 2023b ). One example is a recent research report that identified no increased risk of subsequent musculoskeletal injury after sustaining SRC in NFL players (Buckley et al. 2022 ), which were in contrast to findings reported at other levels of participation (Lynall et al. 2017 ; Nusbickel et al. 2022 ; Herman et al. 2017 ). Notably, those findings are based on a publicly available data source that only captured less than half of the medically documented SRCs based on the reported descriptives. Therefore, our findings in the current study paired with interpretations of publicly available data by Inclan et al. ( 2022 ) and Inclan et al. ( 2023a ) suggest that both data on SRC and musculoskeletal injury reported in prior work may be incomplete or invalid. Another research report found that a change in SRC rate occurred within the study period (Bedard and Wyndham Lawrence 2021 ); however, based on the reported descriptive statistics, the authors only captured approximately 63% of medically documented SRCs. Moreover, some researchers using publicly available data reported in their discussion that despite well-intentioned rule changes by the NFL the game may not be getting safer (Sheth et al. 2020 ). Note, working with a partial sample size put the authors at risk for a type-II error. Additionally, if the extent of missing data differed before and after the rule change, that could have produced not only imprecision, but bias as well. Thus, we caution interpretations and policy implications based on publicly available data, due to lower percentages of SRC captured in publicly available data relative to EHR (Mack et al. 2021 ) found in the current study, and the findings reported by Inclan et al. ( 2022 ) and Inclan et al. ( 2023a ).

The above examples are not the only studies using publicly available injury reports to ask data-driven research questions. A recent report evaluated the impact of Playing Rule Article 8—yielding a penalty for a player if they lower the head and use the helmet during a tackle—in the NFL, in which the authors reported decreased SRC rates after enactment of the policy change using publicly available injury reports (Baker et al. 2021 ). Furthermore, another study reported no significant associations between SRC and playing an unconventional game schedule—that is playing a Thursday game day, or overseas play—that might yield less rest time between games due to a shortened week or travel (Teramoto et al. 2017 ). However, these findings are based on publicly available injury reports from the PBS Frontline Concussion Watch ( http://apps.frontline.org/concussion-watch ). Importantly, due to the findings in the current study suggesting differences in SRC rates based on injury reports that are accessible through publicly available websites, it is likely that the data sets utilized in the aforementioned studies may not include all injuries, both SRC and subsequent musculoskeletal injuries (Inclan et al. 2022 , 2023a ).

There may be many causes for discrepancies between data sources. One such reason may be due to player privacy rights, in which all specific medical information is not released on public domains. Second, there may be competition-related or economic consequences of individual player-level medical records to be publicly accessible. It is also important to note perhaps minor injuries that do not impact practice or game availability do not need to be reported to the public per NFL rules, impacting overall injury data that is publicly available. Therefore, although there are discrepancies in available data sources, we suggest that the inconsistencies are also expected. Still, the discrepancies between data sources found in the current study may indicate either imprecision in a study’s findings if the sample is consistent but small, or bias toward or away from the null hypothesis if missing data are inconsistent across a study period when examining associations in prior work and should be a consideration in future research investigations (e.g., equipment, rules).

Limitations

This study is not without limitations. The publicly available injury data sources provided few details about each SRC, which aligns with our overall message that research questions that can be pursued with these data sets may be limited. In particular, date of injury was not included, which limits what can be deduced about time out of competition. As an alternative, there was a weekly report indicating a player sustained an SRC, and therefore the player missed out on subsequent play time due to injury. Furthermore, given that the EHR records are not publicly available, we could not access them directly and instead relied on EHR injury counts reported from Mack et al. ( 2021 ). Therefore, we could not validate SRC injury counts at the case level in the current study. As noted above, our analyses included injuries designated as “concussion” in the two publicly obtainable data sources, whereas some concussions might be classified as “head” injuries. Our sensitivity analysis indicated this is not an explanation for variability between data sources. Last, it is important to recognize that although EHR data are considered the “gold-standard” source, to date, it may itself have imperfections and may over or underreport SRC which may be due to athlete injury non-disclosure (Jacks et al. 2022 ; Kerr et al. 2018 ; Monseau et al. 2021 ) or even overdiagnosis (Ware and Jha 2015 ).

Large surveillance studies play a critical role in enabling researchers to monitor disease and injury trends and identify a need for closer study or prevention efforts (Rothman et al. 2008 ). In effort to use large data sources that at the surface may seem to be a complete capture, researchers often report having utilized the “most readily available data” including publicly available data sources to answer specific injury-related research questions. This study demonstrates commonly utilized methods for identifying SRC may underestimate the incidence of injury. Specifically, we found publicly obtainable data sources captured a range of 48% to 105% of SRC from the “gold-standard” source depending on the source and year, which represents a critical weakness within the methodology. Therefore, investigators should exercise caution when using publicly available data sources to answer research questions as partial data may yield biased results if the missingness is related to any other variable being investigated, or inappropriate interpretations of results, promoting incomplete findings as stakeholders (e.g., clinicians and policy makers) translate evidence-based findings to clinical practice and policy. Future researchers and journals should understand the limitations associated with use of these sources and seek to use alternative data sources or research strategies.

Availability of data and materials

This datasets analyzed during the current study are available using injury reports from the pro-football-reference.com and NFL.com.

Abbreviations

Sport-related concussion

National Football League

Injury rate ratios

Confidence intervals

Anterior cruciate ligament

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Time for analyses and writing for AB, DW, BD, JG was supported by the Penn Injury Science Center (CDC R49CE 003083). The funder had no role in interpretation of results.

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Abigail C. Bretzin & Douglas J. Wiebe

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Douglas J. Wiebe

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Bernadette A. D’Alonzo

Penn Injury Science Center, University of Pennsylvania, Philadelphia, USA

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AB analyzed and interpreted data and drafted and provided final review of the manuscript; BD interpreted analyses and drafted and provided final review of the manuscript; EvdM analyzed data and interpreted data and drafted and provided final review of the manuscript; JG collected data and interpreted data and provided final review of the manuscript; DW interpreted analyses and provided final review of the manuscript.

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Bretzin, A.C., D’Alonzo, B.A., van der Mei, E.R. et al. Publicly available data sources in sport-related concussion research: a caution for missing data. Inj. Epidemiol. 11 , 3 (2024). https://doi.org/10.1186/s40621-024-00484-7

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Results: Concussion, Playing Experience, and Long-Term Health

One of our Study’s priorities is to better understand the health impacts of concussion. Another is to identify how particular NFL playing experiences affect health outcomes. The findings below shed light on both of these topics, showing how concussions, NFL career length, and playing position may impact your long-term cognitive and mental health. You will also find some action steps and resources to help you be proactive about these aspects of your health.

What the Science Says Our analysis of 3,500 former NFL players looked at individuals’ current cognitive and mental health alongside the specific exposures they encountered during their NFL careers, using the health and playing data that former players reported on themselves in our First Health and Wellness questionnaire (Q1). Here’s what we found:

• Concussion Symptoms : Former players who reported more concussion symptoms during their NFL playing years (loss of consciousness, disorientation, nausea, etc.) were significantly more likely to report having cognitive impairment 1 , depression, and anxiety later in life.
• Playing Position : In comparison to men who played positions with the lowest concussion risk (kicker, punter, and quarterback), running backs, linebackers and special teams 2 positions were over twice as likely to report having cognitive impairment and 40% more likely to report depression. Wide receivers, defensive backs, linemen and tight ends were 70% more likely to report cognitive impairment and 40% more likely to report depression when compared to the lowest risk group.
• Years of Play : Having a longer NFL career significantly increased risk of cognitive impairment and depression later in life. With each five years of play, risk for cognitive impairment increased by 20%, while risk for depression rose by 9%.

1 In this study, cognitive impairment was defined as frequent and regular problems with memory, concentration, and attention, as well as difficulty processing and understanding basic information.

2 This group includes all special teams’ positions except for kickers and punters. Kickers and punters were assessed as a separate group, alongside quarterbacks.

This is the first large-scale study to quantify the specific risks associated with NFL career length and playing position, and an important step in beginning to understand how playing exposures may impact health. However, more research is needed on this topic before any definitive conclusions can be drawn. Additionally, it is important to understand that there are a number of different variables that affect cognitive health , and that many individuals who had the playing exposures outlined above will not experience cognitive impairment or poor mental health. As the graph below illustrates, even among those players who experienced the highest levels of exposure (i.e., individuals who played for seven seasons or more in high-risk positions), the percentage of men who reported cognitive impairment did not exceed 16%.

Potential Action Steps Although you cannot change your NFL playing experience, there are things you can do to be proactive about your cognitive and mental health now:

• Depression and anxiety are common and treatable health conditions that can negatively impact cognitive function. If you think you may be experiencing depression or anxiety, talk to your doctor about these concerns and how you can address them. If you need to speak with someone immediately or need help finding a doctor, contact the NFL Life Line , described in detail below.
• If you think you may be experiencing symptoms of cognitive impairment, talk to your doctor about getting a comprehensive neurocognitive evaluation. With proper diagnosis and management, people with cognitive impairment can live happy and fulfilling lives. If you don’t currently have a doctor, contact the NFL Life Line to get help finding a physician in your area.
• There are many ways to maintain and improve your cognitive health. For example, getting good-quality sleep, regular exercise, and maintaining a healthy diet and weight are all known to enhance cognitive function. Continually challenging your brain to stay active and learn new skills is also known to positively impact cognitive health. If you would like to learn more about specific techniques and strategies for enhancing cognitive health, contact us for a copy of Harvard’s Guide to Cognitive Fitness or consult your physician.
• Cognitive function and mental health can be negatively affected by certain physical conditions that disproportionately impact former players, such as sleep apnea and heart disease. Talk with your doctor about getting a comprehensive health evaluation so that you can be evaluated for any conditions that may be impacting these and other important areas of your health.
• NFL Life Line : This is a free, independent, and confidential phone consultation service that is available to former players and their families 24 hours a day, 7 days a week. It is designed to help individuals with any mental or physical health matters that they need support on, and to connect them with the resources they need. The Life Line is run by professionals who are trained to assist individuals experiencing personal or emotional crises. Contact: (800) 506-0078.
• Players Assistance & Counseling Services : This benefit provides eligible former players and their families with up to eight free counseling sessions a year for matters ranging from family/marital concerns to depression. Contact: (866) 421-8628.
• The Trust: Information on Depression : This resource provides an overview of depression symptoms and treatments, as well as resources to help former players experiencing depression.
• NFLPA: Myths about Depression and Anxiety : This resource discusses and debunks common myths about depression and anxiety.

If you have questions about the information above or would like to learn more about the Study, please call us (617.432.5000) or  email us .

Read the paper:  Exposure to American Football and Neuropsychiatric Health in Former National Football League Players

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July 11, 2023

How football raises the risk for chronic traumatic encephalopathy

At a glance.

• The force of blows to the head that football players experienced over their lives better predicted chronic traumatic encephalopathy than the number of concussions.
• A better understudying of the causes of this deadly type of neurodegenerative disease could help change how football players practice and play.

Blows to the head during contact sports like football, boxing, and soccer can cause injury to the brain, called traumatic brain injury. Studies of American football players have identified a serious consequence of repeated traumatic brain injuries. In a condition called chronic traumatic encephalopathy, or CTE, tangles of a protein called tau build up in the brain after repeated head impacts. The resulting brain damage is similar to that seen in Alzheimer’s disease. CTE can lead to dementia and eventually death.

To help prevent CTE in people who play contact sports, scientists are investigating the types of head impacts that confer the most risk. An NIH-funded research team led by Dr. Daniel Daneshvar from Massachusetts General Hospital and Dr. Jesse Mez from Boston University set out to better understand the relationship between head impacts and CTE.

The team collected data from 34 previous studies of helmet accelerometers used in youth, high school, and college football players. These devices measure the number, speed, and direction of impacts to the head during play. The researchers used this data to create what they called a positional exposure matrix, or PEM. This estimated the average number and types of blows to the head a person would experience during a season for a particular playing position and level of play, including professional athletes.

The team then looked at the relationships between these estimated impacts and CTE in 631 male brain donors who had previously played football. Results were published on June 20, 2023, in Nature Communications .

On average, the brain donors had played about 12 years of football and died at age 60. About 28%, or 180 of them, didn’t have evidence of CTE in their brains. Another 163 had low-stage CTE, and 288 had high-stage CTE. As seen in previous studies, the number of reported concussions wasn’t associated with CTE incidence or severity.

However, the number of years playing football as well as several factors measured by the PEM were associated with CTE. Every additional year playing football was associated with 15% increased odds of a CTE diagnosis and, for those with CTE, 14% increased odds of severe CTE.

Every 1,000 additional estimated blows to the head conferred 21% increased odds of a CTE diagnosis, and 13% increased odds of developing severe CTE. Analyses that took into account the linear and rotational accelerations experienced during head blows were better at predicting CTE than models that only included the number of blows.

These associations held when the researchers took other potential sources of head injury over a lifetime into account. These included military service and other contact sports.

“These results provide added evidence that repeated non-concussive head injuries are a major driver of CTE pathology rather than symptomatic concussions,” Mez says.

“This study suggests that we could reduce CTE risk through changes to how football players practice and play,” adds Daneshvar. “If we cut both the number of head impacts and the force of those hits in practice and games, we could lower the odds that athletes develop CTE.”

—by Sharon Reynolds

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References:  Leveraging football accelerometer data to quantify associations between repetitive head impacts and chronic traumatic encephalopathy in males. Daneshvar DH, Nair ES, Baucom ZH, Rasch A, Abdolmohammadi B, Uretsky M, Saltiel N, Shah A, Jarnagin J, Baugh CM, Martin BM, Palmisano JN, Cherry JD, Alvarez VE, Huber BR, Weuve J, Nowinski CJ, Cantu RC, Zafonte RD, Dwyer B, Crary JF, Goldstein LE, Kowall NW, Katz DI, Stern RA, Tripodis Y, Stein TD, McClean MD, Alosco ML, McKee AC, Mez J. Nat Commun . 2023 Jun 20;14(1):3470. doi: 10.1038/s41467-023-39183-0. PMID: 37340004.

Funding:  NIH’s National Institute of Neurological Disorders and Stroke (NINDS), National Institute on Aging (NIA), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), and National Center for Advancing Translational Sciences (NCATS); Department of Veterans Affairs; Department of Defense; Alzheimer’s Association; Nick and Lynn Buoniconti Foundation; Concussion Legacy Foundation; Andlinger Foundation; World Wrestling Entertainment, Inc.

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Concussions and heading in soccer: a review of the evidence of incidence, mechanisms, biomarkers and neurocognitive outcomes

Affiliation.

• 1 Institute of Medical Sciences, University of Toronto , Toronto, Ontario , Canada .
• PMID: 24475745
• DOI: 10.3109/02699052.2013.865269

Background: Soccer is currently the most popular and fastest-growing sport worldwide. Similar to many sports, soccer carries an inherent risk of injury, including concussion. Soccer is also unique in the use of 'heading'. The present paper provides a comprehensive review of the research examining the incidence, mechanisms, biomarkers of injury and neurocognitive outcomes of concussions and heading in soccer.

Methods: Seven databases were searched for articles from 1806 to 24 May 2013. Articles obtained by the electronic search were reviewed for relevance, with 229 selected for review. Ultimately, 49 articles met criteria for inclusion in the present review.

Results: Female soccer players have a higher incidence of concussions than males. The most frequent injury mechanism is player-to-player contact for both genders. Few studies examined the effects of concussion in soccer players; however, neurocognitive outcomes were similar to those reported in the larger sport concussion literature, while the effect of heading is less clear.

Conclusion: Despite variation in research designs and study characteristics, the outcomes of concussions in soccer align with the greater concussion literature. This review makes recommendations for future research to increase standardization of research for improved understanding of concussions in soccer as well as the effects of heading.

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New research suggests concussion risks can be outweighed by the benefits of playing sport

Conjoint Lecturer, UNSW Sydney

Disclosure statement

Matt Lennon does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

UNSW Sydney provides funding as a member of The Conversation AU.

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CTE: Chronic Traumatic Encephalopathy .

It is the now popularised term that makes athletes, parents, sports administrators and insurers ’ knuckles white with anxiety as sports codes struggle to come to grips with the risks and impacts of concussion.

In 2005, Bennet Omalu, a United States-based neuropathologist, published a case of an NFL player who had suffered from CTE .

Over the following years, public anxiety about head injuries grew , with the 2015 movie Concussion bringing it to the forefront of public attention.

Omalu, famously played by Will Smith in that movie, wrote at the time that “high-impact contact sports […] place athletes at risk of permanent brain damage” and argued kids should not be allowed to participate in contact sports.

Since then, sports codes from across the world have tried to reduce the risks of concussion , and athletes and parents have pondered whether contact sports are worth the risk of brain injury.

The risks and benefits of sport

Over the past few decades, rates of physical activity and sporting participation in Australian children have declined .

In Australia, according to the Australian Bureau of Statistics’ 2022 data, only one in 20 (5.6%) young people (5–17 years) met the physical activity guidelines .

According to a report by the Australian Sport Commission, around 40% of young people do not participate in any organised sport.

There are many reasons why childhood physical activity may be declining, including more online entertainment options, reduced time allocation of physical education in schools and higher rates of obesity.

But having had many conversations with worried mothers and fathers, the fear that their child will have permanent brain damage, a stunted lifespan or a more rapid onset of dementia is real and a likely driver to avoid contact sport.

But what evidence is there that this is the case?

The honest answer is very little – which is why the 2022 International Consensus Statement of Concussion in Sport identified that studies looking at long-term outcomes of sports-related concussions was the highest priority area for future study.

The most studied area within the field is the long-term outcomes of active and retired professional American football players.

Clearly, the genuine and well-founded health concerns about 160kg NFL players clashing heads day in, day out cannot be generalised to your average school boy or girl playing rugby, Aussie rules or basketball.

This grey area prompted a team of researchers, including myself, from the UK, Australia and the US to take on the largest study to date examining the long-term cognitive outcomes of sports-related concussions.

The findings, just published , turn the field on its head.

What we found

In a longitudinal study of 15,000 UK-based adults between the ages of 50 and 90, we found those who had experienced a sports-related concussion at some point in their lives had better working memory and verbal reasoning than those who had never experienced one.

In a further analysis, we found those who had experienced two or three (or more) sports-related concussions did not perform better than those without a concussion history, but equally they did not perform any worse on any measure.

By contrast, those who had experienced three or more other types of concussions (such as from falling, motor vehicle accidents or assaults) had significantly worse processing speed and attention.

Importantly, the study examined “everyday” adults and the results are not applicable to professional athletes whose head injuries tend to be more frequent, debilitating and severe.

Getting the balance right

While at first glance you may feel puzzled by these results, a brief consideration of some modern realities makes them seem sensible.

Rates of obesity in Australia have been steadily climbing over the past few decades and as of 2022 , 31.1% of Australians are obese and 33.7% are overweight.

Similarly, rates of diabetes and high blood pressure are at historic highs.

Many Australians are physically inactive, with teenagers and young adults clocking up an average of 6.6–7.5 hours of screen-based activity a day and reporting concerningly high levels of isolation and loneliness .

While many sports come with risks of head injuries, the beneficial effects on physical activity, obesity, diabetes, hypertension, and social connectedness likely more than mitigate the damaging cognitive effects of a head injury.

The obvious reply to this is we should encourage “safer” physical activity options and do everything we can to protect those in contact sports from injuries.

This is a sensible point of view but requires a few qualifications.

The first is, sports that are more physically demanding (and therefore possibly deliver greater benefits from physical activity) often have greater contact and risk of concussion (for example, golf has fewer concussions than Australian football).

Secondly, even “lower-risk” sports such as cycling or soccer carry a risk of head injury – I have personally been concussed while cycling, although I don’t remember it all that well.

It goes without saying that sporting bodies should continue to minimise the risk of head injuries because the individual effects of concussion can be devastating.

But what our study suggests is perhaps the greater risk is to attempt to take no risks at all – and that the benefits from playing sport may outweigh the effects of concussions.

• Physical activity
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• Youth concussions
• Brain health
• sports concussions
• concussion in sport
• physical activity in kids
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Scientists Say Concussions Can Cause a Brain Disease. These Doctors Disagree.

As another major medical institution acknowledged the link between concussions and the brain disease C.T.E., a group of scientists who guide many of sports’ top governing organizations dismissed the research at its conference.

By Ken Belson

AMSTERDAM — For the first time since 2016, one of the most influential groups guiding doctors, trainers and sports leagues on concussions met last month to decide, among other things, if it was time to recognize the causal relationship between repeated head hits and the degenerative brain disease known as C.T.E.

Despite mounting evidence and a highly regarded U.S. government agency recently acknowledging the link , the group all but decided it was not. Leaders of the International Consensus Conference on Concussion in Sport, meeting in Amsterdam, signaled that it would continue its long practice of casting doubt on the connection between the ravages of head trauma and sports.

C.T.E., or chronic traumatic encephalopathy, was first identified in boxers in 1928 and burst into prominence in 2005, when scientists published their posthumous diagnosis of the disease in the N.F.L. Hall of Fame center Mike Webster, creating an existential crisis for sports such as football and rugby that involve players hitting their heads thousands of times a year.

Scientists have spent the past decade analyzing hundreds of brains from athletes and military veterans, and the variable evident in nearly every case of C.T.E. has been their exposure to repeated head trauma. Researchers have also established what they call a dose response between the severity of the C.T.E. and the number of years playing collision sports.

After playing down an association between head injuries and brain damage for years, the N.F.L. in 2016 acknowledged that there was a link between football and degenerative brain disorders such as C.T.E. Just days before the conference in Amsterdam, the National Institutes of Health, the biggest funder of brain research in the United States, said that C.T.E. “is caused in part by repeated traumatic brain injuries.”

But in one of the final sessions of the three-day conference, one of the leaders of the conference, a neuropsychologist who has received $1.5 million in research funding from the N.F.L. , dismissed the work of scientists who have documented C.T.E. in hundreds of athletes and soldiers because he said their studies thus far did not account for other health variables, including heart disease, diabetes and substance abuse.

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• v.5(3); 2020 Sep

Concussion in soccer: a comprehensive review of the literature

James mooney.

1 Department of Neurosurgery, University of Alabama at Birmingham, 1813 6th Ave S #516, Birmingham, AL 35233, USA

Mitchell Self

Karim refaey.

2 Department of Neurosurgery, Mayo Clinic, 4500 San Pablo Rd S, Jacksonville, FL 32224, USA

Galal Elsayed

Gustavo chagoya, joshua d bernstock.

3 Department of Neurosurgery, Brigham & Women’s Hospital, Harvard Medical School, 75 Francis St, Boston, MA 02115, USA

James M Johnston

Sports-related concussion has been examined extensively in collision sports such as football and hockey. However, historically, lower-risk contact sports such as soccer have only more recently garnered increased attention. Here, we review articles examining the epidemiology, injury mechanisms, sex differences, as well as the neurochemical, neurostructural and neurocognitive changes associated with soccer-related concussion. From 436 titles and abstracts, 121 full texts were reviewed with a total of 64 articles identified for inclusion. Concussion rates are higher during competitions and in female athletes with purposeful heading rarely resulting in concussion. Given a lack of high-level studies examining sports-related concussion in soccer, clinicians and scientists must focus research efforts on large-scale data gathering and development of improved technologies to better detect and understand concussion.

Sports-related concussion (SRC) and mild traumatic brain injury (TBI) have become topics of major public health interest with the US Centers for Disease Control and Prevention (GA, USA) declaring that SRC is reaching “ epidemic levels ” and deserves further investigation [ 1–3 ]. Significant focus has been given to SRC in American football and ice hockey, with soccer only garnering increased attention within the past decade due to its worldwide popularity. Currently, soccer is the most popular and fastest-growing sport worldwide [ 4 ]. While soccer has historically been considered a lower risk contact sport, the American Academy of Pediatrics (IL, USA) has recently ranked soccer equivalent to American football and ice hockey, with a comparable frequency of head injury [ 5 , 6 ]. Head trauma in soccer is frequently underdiagnosed with potential consequences neglected due to heterogeneous and often mild symptoms. The consensus statement adopted by the Federation Internationale de Football Association (FIFA; Zürich, Switzerland) from the 2012 and 2016 International Conference on Concussion in Sport, defines SRC as an injury that represents “ immediate and transient symptoms of traumatic brain injury (TBI) ” [ 7 ]. The consensus states that players in question must be immediately evaluated for any feature of concussion with a side-line diagnostic assessment evaluating state of consciousness, orientation, cranial nerve function and balance, and then be withdrawn from play if concussion has occurred [ 7 , 8 ]. Various diagnostic tools including the Sport Concussion Assessment Tool (SCAT) have been developed that has most recently been updated to the SCAT-5.

This comprehensive review provides a summary of the literature examining the epidemiology, mechanisms of injury, sex differences, short- and long-term neurochemical, neurostructural and neurocognitive consequences, as well as recommendations for prevention of SRC in soccer.

Search strategy

A literature search was developed by the primary author (J Mooney) and conducted in May 2020, using PubMed and Medline, limited to English language. Search phrases including ‘soccer; concussion; mild TBI; repetitive subconcussive head impact (RSHI); sex differences; repetitive heading; neurochemistry; chronic traumatic encephalopathy (CTE); neuropsychiatric; epidemiology; neurocognitive; injury mechanisms and prevention’ were combined using the operators ‘AND’ and ‘OR’. Titles and abstracts were screened by the primary author, and independent authors reviewed relevant full-text articles. Studies were grouped into categories based on specific areas of focus. 121 full texts were initially screened for inclusion based on the relevance of the title and abstract. Bibliographies of full texts were surveyed for additional pertinent studies. Ultimately, 64 unique articles were chosen for final inclusion. Figure 1 demonstrates the study selection process. Articles divided by category and study type are shown in Table 1 . Several articles were included in multiple categories if parts of the articles pertained to different categories. Studies utilized in several categories are bolded in Table 4 .

Table 1. 

RCT: Randomized control trial.

Table 4. 

Bolded studies appear in multiple categories.

CTE: Chronic traumatic encephalopathy.

Eligibility criteria

Manual review of all articles was carried out for relevance by the primary author (J Mooney) and approved by all study authors. Using the criteria listed below, eligible articles were selected for full review.

Inclusion criteria

Included articles were published in the English language and included literature reviews, retrospective, prospective, descriptive, cross-sectional and randomized control trials containing greater than five soccer athletes.

Exclusion criteria

Articles examining sports other than soccer were excluded or only data examining soccer were extracted from those studies. Case reports and case series were excluded.

Data extraction

Data relating to epidemiology, mechanisms, sex differences, neurochemical/structural/psychiatric, CTE and prevention were extracted from studies examining soccer-related concussion. Level of evidence and study types for the various categories is reported in Tables 1–3 .

Table 2. 

NA: Not available; RCT: Randomized control trial.

Table 3. 

Epidemiology

4 to 22% of all soccer injuries are head/neck injuries with a reported incidence of 1.7 injuries per 1000 playing hours [ 9 ]. The incidence of concussions has been estimated at 0.5 injuries per 1000 playing hours; however, the accuracy of this estimate is difficult to determine given inconsistencies in the interpretation and reporting of concussions [ 9–11 ].

Over 80% of all soccer-related injuries reported in the USA occur among those under the age of 25 [ 12 ]. Faude et al . examined injuries to child and adolescent soccer players (aged 5–19 years) and reported that 5% of all injuries in these age groups were to the head [ 13 ]. In a more recent online survey of 8104 youth soccer teams including 101,699 players aged 7–14 years, the overall reported concussion incidence rate was 0.85/1000 athlete exposures (AEs) with concussions 5.7-times more likely to occur in games (1.73/1000 AEs) than practices (0.27/1000 AEs) [ 14 ].

The overall rate of SRC in high school soccer in the USA from 2005 to 2014 was estimated to be 0.36/1000 AEs (girls = 0.45/1000; boys = 0.28/1000 AEs). The rate of concussion in games versus practices was higher in girls when compared with boys [ 15 ].

Marar et al . reported a competition concussion rate of 0.92 per 1000 AEs among female high school soccer athletes, with a rate of 0.53 per 1000 AEs among male players [ 16 ]. Among youth and high school players, concussion is the second most common game injury representing approximately 24% of all injuries [ 17 ].

One of the earliest surveillance studies on concussions in college age male and female players reported the concussion rate to be 0.6/1000 AE for men and 0.4/1000 AE for women [ 18 ].

Data based on professional players come from both the domestic professional season and tournament settings. Concussions are the fifth most common injury in the US Major League Soccer, although injury rates vary by team and season [ 19 ]. Junge and Dvorak summarized the injury data collected during 51 FIFA-sponsored tournaments and four Olympic Games from 1998 to 2012, noting that 15% of injuries affected the head or neck [ 20 ].

Mechanisms of injury

Mechanisms of head injury in soccer include unintentional hits to the head through contact with body parts of other players, against the ground, soccer goal frame or from the ball to an unprepared player [ 10 , 21 , 22 ]. Unique to soccer are the repetitive forces or subconcussive trauma involved in heading the ball [ 10 , 23 , 24 ]. Subconcussive head impact is defined as an impact to the head that does not induce clinical symptoms of concussion and has emerged as a complex public health issue. Rotational acceleration, linear acceleration and carotid artery injuries have been reported by researchers as having an impact on the development of neuropathological changes in acute brain trauma suffered in boxing as well as in soccer [ 21 , 25 , 26 ].

Player-to-player contact has been noted to be the mechanism responsible for the greatest proportion of concussions in both male and female soccer athletes [ 27 ]. One prospective study of collegiate female and male players over 2 years found that about 70% of concussions occurred during games, and that head-to-head contact was the most frequent mechanism of injury, followed by head to ground and head to other body parts. None of the concussions in this study resulted from intentional heading of the ball [ 18 ]. Additionally, the majority of concussions in youth and high school players occur when the player is unaware of the oncoming contact or collision [ 28 ]. The risk of concussive events is increased in games due to a greater quantity of head impacts and not necessarily greater severity [ 29 ].

In another retrospective study of high school and youth soccer players between 2005 and 2014, nearly 60% of concussions involved contact or collision with another player and the player behavior of heading the ball represented 28% of all concussions, 70% of which were from player-to-player contact, not ball-to-player contact [ 15 ].

Andersen et al . examined 192 head injury incidents involving player-to-player contact in Norwegian and Icelandic professional players and reported that 41% of concussions resulted from contact by an elbow, arm or hand to the head [ 9 ]. With respect to playing situation, 58.3% of head injuries were sustained while the player was engaged in a heading duel [ 9 ]. In a similar cohort of high school athletes, Marar et al . reported concussions accounted for 60.0% of all injuries sustained while heading the ball [ 16 ], while another study reported a proportion of 64.1% [ 30 ].

In an examination of 69 concussions in 60 collegiate athletes, Delaney et al . found that blows to the temporal region of the head were most likely to result in concussion for soccer [ 31 ]. Defenders and goalkeepers have also been reported to sustain more concussive injuries than forwards or midfielders [ 32–34 ].

Sex differences

Among female athletes, concussions are reported most often among soccer players with concussion incidence as high as 1.9 per 1000 competition-related athlete-exposures and 0.6 per 1000 overall athlete-exposures [ 35–37 ]. This compares with 0.53 per 1000 competition-related athlete-exposures and 0.28 per 1000 overall AEs in males [ 15 , 16 ]. Chandran et al. examined sex differences in head injuries and concussions among collegiate soccer players between 2004 and 2009, finding a higher rate of head injuries in women than men with the rate of injury due to contact with an apparatus (ball/goal) nearly 2.5-times higher and the rate due to contact with a playing surface over two-times higher in women [ 38 ].

Sex differences in anthropometrics and heading kinematics have been examined among Division I soccer athletes, with women found to display lower anthropometry measures compared with men, resulting in increased head impact kinematics during soccer heading. Decreased neck girth and strength among female players were found to be potentially associated factors [ 39 ]. Similarly, in a study of controlled soccer headers, Caccese et al. found higher peak linear and rotational acceleration in females compared with males, suggesting that females may be exposed to greater traumatic shearing forces associated with brain injury [ 40 ].

Sex may also play a role in determining recovery after concussion, with female soccer players performing worse on neurocognitive testing and also reporting more symptoms than male soccer players [ 41 ]. Covassin et al. reported that female concussed soccer players conveyed more total concussion symptoms at 8 days compared with male concussed athletes [ 42 ]. In addition, significant effects for sex on verbal and visual memory were documented, with female athletes reporting lower scores and more symptoms on the migraine-cognitive-fatigue and sleep clusters, respectively [ 42 ].

In contrast, Zuckerman et al . examined 40 male and 40 female soccer players using the Immediate Postconcussion Assessment and Cognitive Testing, finding no difference based on sex in the acute response to concussion among high school soccer players [ 43 ].

Biochemical & structural implications of RSHI & concussion in soccer

As previously discussed, subconcussive impact due to repetitive heading, acceleration, deceleration and rotational forces on the brain may also result in structural and functional changes. Such changes have been identified in both former and active soccer players, and studies have postulated that these neurochemical and neurostructural changes may occur before evidence of neurobehavioral change [ 44 , 45 ].

Using diffusion tensor imaging (DTI), Myer et al. described the ability of a specialized neck collar to dampen the effects of subconcussive impact on white matter integrity through bilateral jugular vein compression, diverting blood flow to vertebral veins and promoting venous engorgement. The authors reported pre- to postseason changes in mean diffusivity, axial diffusivity and radial diffusivity in a noncollar group versus collar group, finding significant correlation between head impact exposure and DTI changes over the season in the noncollar group [ 46 ].

In a study of 37 amateur soccer players, Lipton et al. studied the effects of repetitive heading on fractional anisotropy and cognitive function, and reported that heading was associated with lower fractional anisotropy at three locations in temporo-occipital white matter and that lower levels of fractional anisotropy were associated with poorer memory scores with a threshold of 1800 headings per year [ 47 ].

Magnetic resonance spectroscopy has also been used to examine the effects of RSHI on neurochemistry. Koerte et al . found significant increases in both choline (a membrane marker) and myo-inositol (a marker of glial activation) in 14 former professional soccer players without a history of clinically diagnosed concussion compared with control athletes [ 48 ].

Researchers have also reported electroencephalographic abnormalities and protracted postconcussive symptoms in a group of 37 former Norwegian national team players [ 49–51 ].

Other technologies, including transcranial sonographic measurement of optic nerve sheath diameter (a noninvasive technique to predict raised intracranial pressure), and various cerebrovascular biomarkers have been proposed to assess the effects of RSHI in soccer players [ 52 , 53 ]. Sadrameli et al . observed an increase in optic nerve sheath diameter that was independent of concussions in 24 female collegiate soccer players [ 52 ].

Several cerebrospinal fluid (CSF) biomarkers for RSHI and concussion have also been analyzed in former and active soccer players. Biomarkers such as neurofilament light protein, glial fibrillary acidic protein and S-100B have provided insight into the pathophysiological mechanisms of TBI and are indicators of neuronal, axonal and astroglial damage resulting from TBI [ 54 ].

Zetterberg et al . investigated the effects of repetitive heading on CSF concentrations of neurofilament light protein, total tau, glial fibrillary acidic protein, S-100B and albumin, with the results showing no correlation between the CSF biomarkers and number of headers performed. Dorminy et al . found no correlation between S-100B, concussion assessment test scores and repetitive heading in 16 division 1 soccer players across a range of ball velocities [ 55 ]. Conversely, Mussack et al. observed S-100B levels of a controlled amateur heading group, and reported transient increases 60–360 min post-training that were significantly elevated compared with normal exercise [ 56 ]. Wallace et al. also found elevated levels of neurofilament light chain at 1 h and 1 month following a repetitive heading session, as well as elevated total symptoms and symptom severity on the SCAT3 test [ 57 ]. These results suggest heading may lead to biochemical signs of axonal damage in serum, at least in the short term. Larger-scale studies that correlate serum biomarker findings with long-term neuropsychological development and clinical symptoms are warranted.

Soccer & neurodegenerative disease

Data regarding incidence and prevalence of CTE pathology and neurodegenerative disease in soccer are limited due to the small number of case studies and the retrospective nature of brain bank specimen analysis [ 45 , 58–62 ]. Koerte et al. identified greater cortical thinning with age in 15 male former professional soccer players, specifically in the right inferolateral–parietal, temporal and occipital cortex compared with controls [ 63 ].

A recent study tracked 14 retired soccer players with dementia from 1980 to 2010 and reported neuropathologic findings of septal abnormalities in all six postmortem cases, supportive of a history of chronic repetitive head impacts. Four of the cases had pathologically confirmed CTE [ 45 ].

Mackay et al . conducted a recent retrospective cohort study comparing mortality from any neurodegenerative disease among 7676 former professional Scottish soccer players with 23,028 general population controls. They found that mortality from neurodegenerative disease was higher (1.7 vs 0.5%) and mortality from other common diseases lower in the soccer players compared with matched controls [ 64 ].

The Boston University CTE Center and Brain Bank (MA, USA) has published several studies examining CTE, with the majority of cases including former boxers (85%), although also including cases from former American football players, soccer players and wrestlers [ 65 ]. The center recently announced a new study on Soccer, Head Impacts and Neurological Effects that is enrolling 20 former female professional soccer players to study the long-term cognitive effects of RSHI [ 66 ].

At present, no data beyond autopsy studies exist to support the hypothesis that soccer participation is a risk factor for the development of neurodegenerative disease like CTE.

Neuropsychological implications of RSHI & concussion in soccer

Recent research suggests that exposure to both concussive and subconcussive events can result in persistent cognitive and neuropsychologic impairments [ 67 ]. For soccer, studies examining subconcussive impacts have demonstrated no effects of heading on cognitive function after a 15-min heading session [ 19 ], after two practices or games, after one season and through assessment with a cross-sectional analysis [ 68 ]. Forbes et al . found no association between neuropsychological test performance and concussive impacts in soccer [ 69 ], while others have demonstrated no significant relationship between computerized neurocognitive test performance and an athlete’s exposure to increased concussive and subconcussive impacts through soccer [ 70 , 71 ]. Conversely, data from Di Vigilio et al . using transcranial magnetic stimulation suggested changes in cognitive performance and increased intracortical inhibition following a single exposure to subconcussive head impacts from routine soccer heading [ 72 ]. These results echo those found by Webbe et al . in which ball-to-head contact was associated with at least transient cognitive impairment [ 73 ]. Levitch et al . demonstrated that soccer RSHI affected neurophysiologic function differentially based on the chronicity of exposure, with recent heading impacting psychomotor tasks and long-term heading exposure affecting verbal memory and learning [ 74 ].

Overall, studies investigating the neuropsychological implications of RSHI and concussion in soccer are lacking due to their retrospective nature and low number of subjects [ 19 , 67–77 ]. The limited available data do not support the hypothesis that there are intermediate or long-term adverse neurocognitive effects from heading the ball in soccer.

Injury prevention

Recognition and awareness of concussions through external observation and removal of athletes from play is of paramount importance. Educational resources have been published by many organizations, including the Center for Disease Control and Prevention, the National Collegiate Athletic Association (IN, USA), US Soccer (IL, USA) and the United Soccer Coaches (MO, USA) [ 78 ]. Current guidelines, including those from FIFA, state that athletes diagnosed with SRC should be removed from play and evaluated by a healthcare provider trained in concussion management, and should not return to play the same day [ 79 ]. An initial period of cognitive and physical rest is recommended [ 80 ]. Return to academic and sport process then includes a stepwise symptom-limited progression of increased physical, cognitive, work or school-related activity [ 79 ].

Unfortunately, as many as 44% of elite female youth soccer players in one study indicated that they would not report their concussion symptoms [ 81 ], while another study of elite female soccer players aged 12–15 demonstrated that the majority (59%) of concussed players continued to play with concussive symptoms [ 82 ]. The use of in-game spotters who can review live game film or gameplay to identify indicators of possible concussion may be beneficial; however, costs and logistics of implementation are not feasible for most nonprofessional/elite organizations.

Currently, US Soccer’s policy on age-related heading prohibits heading for 10 years old and under, with ages 11–12 limited to a maximum of 30 min of heading training per week [ 40 , 83 , 84 ]. The danger of repetitive heading in soccer has been called into question multiple times previously with several groups more recently calling for a ban on heading among soccer players younger than 14 years of age [ 5 , 85 , 86 ]. While these efforts have served to decrease the number of head-to-ball contacts in practices and games, contact with another player has consistently shown to be the most common mechanism of injury in heading-related concussions among boys and girls [ 10 , 15 , 22 , 62 , 77 ]. Thus, reducing athlete contact through rule changes or stricter enforcement may be a more effective way to prevent concussions and other injuries [ 15 ]. Stricter rules punishing aerial challenges that involve elbows to the head, head-to-head or hand-to-head contact (e.g., goalkeeper) may diminish rates of head injury. Andersen et al . reported that strict enforcement and interpretation of the laws of the game was associated with a lower incidence of head injury caused by player-to-player contact [ 9 ].

The role of headgear for injury prevention has been examined with results demonstrating no reduction in impact accelerations or the incidence and severity of SRC in soccer [ 87 , 88 ]. Concerns also exist that universal use of headgear might lead to more aggressive heading and head challenges, resulting in a paradoxical increased risk of injury [ 89 ]. In contrast, Delaney et al . reported that female adolescent soccer players who did not wear headgear were more susceptible to concussion [ 90 ].

Educating players on the biomechanics of heading as well as strength training and conditioning may be other important areas of focus for injury prevention. Behavioral skills training for heading has previously been described [ 91 ]. Babbs et al . examined the safety of heading a soccer ball by calculating head accelerations, reporting that heading is usually safe, although occasionally dangerous, depending on characteristics of the player and ball. Low head–neck segment mass predisposes athletes to higher head acceleration. Head–neck–torso alignment during heading and follow-through after contact can be used to decrease head acceleration [ 92 ]. Safety is also improved when players head the ball with greater effective body mass. Thus, younger players with smaller bodies are at risk of potentially more dangerous headers. As discussed, there are sex differences in heading kinematics, with women exposed to higher rotational head acceleration and resultant shearing forces due to relative neck weakness compared with men. Strength and awareness training may increase an athlete’s ability to prepare and brace for an impact to the body or head and is an important area of future research [ 93 ]. The use of lower ball inflation pressures, teaching of proper heading technique and redesign of age-appropriate balls for young players may also mitigate the risks of dangerous head accelerations [ 94 ].

More recently, specialized technologies have been developed to both record AE to head impacts and to diminish the consequences of these impacts. The earlier discussed specialized neck collar, designed to reduce intracranial energy absorption, has been shown to preserve white matter integrity in studies of football and hockey athletes [ 95 , 96 ]. In female high school soccer athletes, Myer et al . found microstructural changes in the white matter tracts on DTI in athletes who did not wear the collar device in comparison to athletes who wore the device [ 46 ]. Correlation of these imaging changes with neurocognitive outcomes is unclear.

In an attempt to better quantify head impact exposures in soccer, Press et al . used linear accelerometer sensors worn on the mastoid and compared impact exposures recorded by the devices to exposures recorded via video. They found that the sensors reported a much greater total number of impacts than that identified through video analysis [ 97 ]. Improvements in sensitivity and specificity of monitoring technologies may eventually lead to earlier identification of potential concussive head impacts during competitions.

Soccer-related concussion represents a significant proportion of SRC worldwide. While sports such as hockey and American football have received substantial attention and research in regards to concussion, studies exclusively examining soccer are lacking.

After review of the soccer concussion literature, studies suggest that women experience a greater rate of concussion in both practices and games compared with men, and that female soccer players have the highest rates of concussion when compared with female athletes in other sports. Concussions in soccer most commonly occur due to player-to-player contact rather than ball-to-head contact, with purposeful heading rarely resulting in concussion. The majority of studies examining the biochemical, structural and cognitive implications of RSHI and concussion in soccer have included only small numbers of athletes. At present, no data support the hypothesis that soccer participation is a risk factor for the development of CTE, though CTE has been found in small autopsy studies of former professional soccer players. Further large-scale studies are warranted.

While there have already been efforts to decrease the prevalence of soccer-related concussion through the use of headgear, limitations on heading, strength training and player education, it is unclear how these efforts have impacted concussion rates. Future attempts to quantify concussion frequency and severity more accurately with mastoid sensors, spotters and other novel technologies will lead to a greater understanding and ultimately ability to prevent concussions in soccer. For example, limiting athlete contact during soccer practices and games may have a greater impact on concussion rate than limiting heading of the ball. Future studies examining the effects of preventative interventions are warranted.

Limitations of this study include the low number of high-level studies included with only one randomized control trail ( Table 1 ), as well as the small number of athletes included in these studies. Additionally, a large proportion of the studies represented only North American soccer players, highlighting a lack of high-level and large-scale international studies. While using the search term soccer instead of football may have limited the proportion of international studies, using the term football resulted in many nonrelevant articles solely examining American Football.

Given the lack of high-level research examining SRC and RSHI in soccer, as well as the popularity and growing number of athletes who play soccer around the world, scientists and clinicians should focus their efforts to better understand the specific epidemiology, pathophysiology, injury biomechanics, as well as the short- and long-term consequences of soccer-related concussion. This more nuanced understanding will be necessary to formulate and institute effective interventions that will make the sport safer for its participants.

Future perspective

In the near future, with ongoing research and technological improvements, electronic sensors and spotters may acquire the sensitivity and specificity to detect concussive impacts better than humans, resulting in prompt removal from play of affected individuals for evaluation by medical personnel.

Studies seeking to identify blood and even salivary markers for mild TBI and SRC are promising and expanding. One could envision a future where a suspected concussed athlete is diagnosed on the sideline using an objective and validated blood or salivary biomarker.

Additionally, stricter enforcement of existing rules and new rules limiting player-to-player contact, particularly aerial challenges where head impact risk is greatest, will serve to diminish concussion risk in soccer in the future. Age- and gender-specific research and recommendations will also serve to protect those populations most at risk. While American football and hockey have been subjected to rigorous and focused scientific attention in regard to concussion, the research examining soccer-related concussion remains in its infancy. Future large-scale basic scientific and prospective studies will expose the true consequences of RSHI and concussion in soccer and reveal new avenues for improvements in the safety of the game.

Executive summary

• The majority of soccer-related injuries, including head injuries and concussions, occur among those under the age of 25.
• Concussions are more likely to occur in soccer games than in practices and have a higher incidence in female versus male athletes.
• Player-to-player contact is the most common mechanism resulting in concussions in both male and female soccer athletes.
• Purposeful heading rarely results in concussion.
• Female soccer players have the highest concussion rate of all female athletes and may have a prolonged recovery compared with male soccer athletes.
• Imaging changes on diffusion tensor imaging, magnetic resonance spectroscopy, transcranial sonography and electroencephalographic have been noted in active and former soccer players.
• Studies examining biomarkers for concussion in soccer have yielded mixed results with some reporting elevated S-100B and neurofilament light chain after soccer RSHI.
• No data at present support the hypothesis that soccer is a risk factor for development of neurodegenerative diseases.
• Studies are lacking but do not demonstrate intermediate or long-term adverse neurocognitive effects from heading the ball in soccer.
• Recognition and awareness of concussions during soccer play and removal of those affected is of paramount importance and officials should be trained to recognize signs and symptoms with referral to appropriate medical care. Technologies designed to identify potential concussive impacts will prove beneficial once refined.
• Reduction of player contact along with biomechanical/behavioral education, strength/awareness training and restructuring/enforcement of rules will all serve to reduce concussion incidence in soccer.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.

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