Risk factors for hamstring injuries in community level Australian football

Objectives: To identify risk factors for hamstring injury at the community level of Australian football.

Methods: A total of 126 community level Australian football players participated in this prospective cohort study. To provide baseline measurements, they completed a questionnaire and had a musculoskeletal screen during the 2000 preseason. All were monitored over the season. Injury surveillance and exposure data were collected for the full season. Survival analysis was used to identify independent predictors of hamstring injury.

Results: A hamstring injury was the first injury of the season in 20 players (16%). After adjustment for exposure, increasing age and decreased quadriceps flexibility were identified as significant independent predictors of the time to sustaining a hamstring injury. Older age (23 years) was associated with an increased risk of hamstring injury (RR 3.8; 95% confidence interval (CI) 1.1 to 14.0; p = 0.044). Players with increased quadriceps flexibility (as measured by the modified Thomas test) were less likely to sustain a hamstring injury (RR 0.3; 95% CI 0.1 to 0.8; p = 0.022).

Conclusions: The findings of this study can be used in the development of hamstring injury prevention strategies and to identify Australian football players at increased risk of hamstring injury.

     

Why are older Australian football players at greater risk of hamstring injury?

INTRODUCTION: Increasing age is a commonly identified predictor of hamstring injury but is not modifiable to reduce injury risk. Why increasing age is a risk factor for hamstring injuries in athletes has not been studied to date. This study aimed to identify potentially modifiable age-related changes that predict hamstring injury in a population of Australian football players. METHODS: One hundred and one young (< or =20 years), and 73 older (> or =25 years), Australian football players, without a history of hamstring injury in the past 12 months were studied prospectively. Players underwent screening of anthropometric, flexibility and lower extremity range of movement tests during the pre-season period and were followed-up for a full season with respect to injury and match participation. Comparisons of the age groups were performed to identify differences related to age. Logistic regression analysis was undertaken to determine whether the observed differences were predictors of hamstring injury. RESULTS: There were significant differences between the age groups with respect to body weight, body mass index, hip flexor flexibility, hip internal rotation and ankle dorsiflexion range of movement. Body weight and hip flexor flexibility were significant independent predictors of hamstring injury in players aged > or =25 years. None of the observed differences were predictors of injury in the younger age group. CONCLUSIONS: There are age-related changes that are potentially modifiable to reduce injury risk in older athletes and these factors should be considered in the development of hamstring injury prevention programs for this high risk group.     

Clinical risk factors for hamstring muscle strain injury: a prospective study with correlation of injury by magnetic resonance imaging

OBJECTIVE: To prospectively establish risk factors for hamstring muscle strain injury using magnetic resonance imaging (MRI) to define the diagnosis of posterior thigh injury. METHOD: In a prospective cohort study using two elite Australian Rules football clubs, the anthropometric characteristics and past clinical history of 114 athletes were recorded. Players were followed throughout the subsequent season, with posterior thigh injuries being documented. Hamstring intramuscular hyperintensity on T2 weighted MRI was required to meet our criteria for a definite hamstring injury. Statistical associations were sought between anthropometric and previous clinical characteristics and hamstring muscle injury. RESULTS: MRI in 32 players showed either hamstring injury (n = 26) or normal scans (n = 6). An association existed between a hamstring injury and each of the following: increasing age, being aboriginal, past history of an injury to the posterior thigh or knee or osteitis pubis (all p<0.05). These factors were still significant when players with a past history of posterior thigh injury (n = 26) were excluded. Previous back injury was associated with a posterior thigh injury that looked normal on MRI scan, but not with an MRI detected hamstring injury. CONCLUSIONS: Hamstring injuries are common in Australian football, and previous posterior thigh injury is a significant risk factor. Other factors, such as increasing age, being of aboriginal descent, or having a past history of knee injury or osteitis pubis, increase the risk of hamstring strain independently of previous posterior thigh injury. However, as the numbers in this study are small, further research is needed before definitive statements can be made.     

Association between eccentric knee flexor strength and hamstring injury risk in 185 elite Gaelic football players

Hamstring strains are the most common time-loss injury in elite Gaelic football affecting over 20% of players per season. Thus, there is a need to identify factors contributing to the onset of hamstring injuries in order to inform injury risk management strategies. The current study investigated whether eccentric knee flexor strength and between-limb imbalances were associated with increased risk of sustaining a time-loss hamstring injury in elite Gaelic football players. A total of 185 elite male players (26.9 ± 2.7 years, 86.4 ± 6.2 kg, 183.4 ± 5.6) were prospectively followed for 12 weeks from the day of testing. Injury data were provided by the team medical staff. Twenty-eight players (16%) sustained a time-loss hamstring injury following testing. Players that did not sustain a hamstring injury had greater average between-limb asymmetries (uninjured = 9.1%, 95% CI 7.8-10.1; injured = 5.1%, 95% CI 3.7-6.7; P = .001). Eccentric knee flexor strength profiles were not associated with increased or decreased risk of sustaining a hamstring injury and did not alter the post-test probability of sustaining a hamstring injury across the investigation period. These findings do not support the use of eccentric knee flexor strength metrics in managing hamstring injury risk in elite male Gaelic football players.     

Isokinetic strength testing does not predict hamstring injury in Australian Rules footballers

OBJECTIVE: To determine the relation of hamstring and quadriceps muscle strength and imbalance to hamstring injury using a prospective observational cohort study METHOD: A total of 102 senior male Australian Rules footballers aged 22.2 (3.6) years were tested at the start of a football season. Maximum voluntary concentric and eccentric torque of the hamstring and quadriceps muscles of both legs was assessed using a Kin-Com isokinetic dynamometer at angular velocities of 60 and 180 degrees/second. Twelve (11.8%) players sustained clinically diagnosed hamstring strains which caused them to miss one or more matches over the ensuing season. RESULTS: There were no significant differences for any of the isokinetic variables comparing the injured and non-injured legs in players with unilateral hamstring strains (n=9). Neither the injured nor the non-injured leg of injured players differed from the mean of left and right legs in non-injured players for any isokinetic variable. The hamstring to opposite hamstring ratios also did not differ between injured and non-injured players. A hamstring to opposite hamstring ratio of less than 0.90 and a hamstring to quadriceps ratio of less than 0.60 were not associated with an increased risk of hamstring injury. A significantly greater percentage of players who sustained a hamstring strain reported a history of hamstring strain compared with non-injured players (p=0.02). However, this was not related to muscle weakness or imbalance. CONCLUSIONS: Isokinetic muscle strength testing was not able to directly discriminate Australian Rules football players at risk for a hamstring injury.


     

The epidemiology of injury for an elite junior Australian Football cohort

ObjectivesTo implement an injury recording protocol in a junior elite Australian Football competition and determine the injury profile of this population.DesignLongitudinal cohort study.MethodsPlayers from an elite Under 18 Australian Football competition were tracked throughout one football season in terms of participation or non-participation in the football competition. Injury reporting forms were collected for all players who were not available for selection as a result of injury.ResultsThe cohort consisted of 532 players who provided consent for inclusion in the study (100% of players in the competition). There were 256 injuries sustained during the season. Results were standardised to a 40 man team to allow comparison with results from the Australian Football League. The injury incidence was 17.1 new injuries per club (95% CI 14.1–19.4), and prevalence 63.3 missed matches per club (95% CI 59.1–67.1). The category “Ankle joint injuries” was the most commonly reported (n = 34) and “Collision with another player” was the main injury mechanism (n = 75).ConclusionsThe most commonly injured region in junior elite Australian Football was the ankle and collision with another player was the most common injury mechanism. As with previous reports on junior Australian Football, injury incidence was low in comparison to the senior elite competition. Defining the injury profile guides injury prevention strategies. Analysis of injury in junior elite football may provide a unique opportunity to affect both junior and senior injury rates.     

A pilot randomised controlled trial of eccentric exercise to prevent hamstring injuries in community-level Australian Football

Hamstring injuries are the most common injury sustained by Australian Football players. Eccentric training has been proposed as a potential preventative strategy. This pilot randomised controlled trial (RCT) evaluated the effectiveness of a pre-season eccentric training program for preventing hamstring injuries at the community level of Australian Football. Seven amateur clubs (n=220 players) were recruited. Players were randomised within clubs to the intervention (eccentric exercise) or control (stretching) groups and randomisation was stratified according to previous history of hamstring injury. Five exercise sessions were completed over a 12-week period, three during the pre-season and two during the first 6 weeks of the season. Compliance was recorded and players were monitored for the season to collect injury and participation data. There was no difference between the control (n=106) or intervention (n=114) groups with respect to baseline characteristics. Only 46.8% of all players completed at least two program sessions. Compliance was poorest for the intervention group. Intention-to-treat analysis suggested that players in the intervention group were not at reduced risk of hamstring injury (RR 1.2, 95% CI: 0.5, 2.8). When only control and intervention group players who participated in at least the first two sessions were analysed, 4.0% of intervention and 13.2% of control group players sustained a hamstring injury (RR 0.3, 95% CI: 0.1, 1.4; p=0.098). The findings suggest that a simple program of eccentric exercise could reduce the incidence of hamstring injuries in Australian Football but widespread implementation of this program is not likely because of poor compliance.     

Motor control and strength as predictors of hamstring injury in elite players of Australian football

Objectives. To investigate motor control as a possible contributing factor in hamstring injuries using a test movement similar to the injury-critical phase during running. Leg swing movement discrimination (MD), thigh muscle strength as tested by Cybex isokinetic dynamometry and previous hamstring injury history were all assessed to determine any association with subsequent hamstring injury.Design. Prospective and retrospective observational and analytical cohort study.Setting. A professional football club and a university laboratory.Participants. Twenty elite level players of Australian football.Results. In the 2 years prior to testing, 7 subjects had sustained a hamstring injury. At the time of testing, there were no significant differences between subjects never injured and those previously injured. In the two years following testing, 6 players sustained a hamstring injury. Mean MD score and concentric hamstring-to-quadriceps strength ratio of subsequently injured subjects were significantly worse than those uninjured (p<0.05). Optimum cutoffs on these measures were determined.Conclusion. Both a lower MD ability score for the backward swinging leg and an imbalance of thigh muscle strength were predictive of hamstring injury.     

No association between rate of torque development and onset of muscle activity with increased risk of hamstring injury in elite football

Hamstring injuries remain a significant burden in sports that involve high‐speed running. In elite male football, hamstring injury has repeatedly been identified as the most common non‐contact injury, representing 12% of all injuries. As the incidence remains high, investigations are aimed at better understanding how to improve prevention efforts. Intrinsic risk factors such as strength have been investigated extensively in a cohort of professional football players; however, other intrinsic measures of neuromuscular function have not been studied in this cohort. This study aims to investigate the association between timing of hamstring muscle activity onset and the rate of torque development during the early phase of isokinetic strength testing with risk of hamstring injury in professional football players in a prospective cohort study. All teams (n = 18) eligible to compete in the premier football league in Qatar underwent a comprehensive strength assessment during their annual periodic health evaluation at Aspetar Orthopaedic and Sports Medicine Hospital in Doha, Qatar. Variables included rate of torque development and timing of muscle activity onset. A total of 367 unique players (60.6% of all QSL players) competed for 514 player seasons (103 players competed both seasons) and sustained 65 hamstring injuries. There was no difference in the onset of muscle activity between the biceps femoris and medial hamstrings comparing the injured to uninjured players. For both onset of muscle activity and rate of torque development, there were no significant differences between any of the variables (P  > .05), with small effect sizes detected across all the different variables (d < 0.3). Rate of torque development and onset of muscle activity were not associated with a risk of future hamstring injury. The use of these measures as part of a periodic health evaluation to identify risk of hamstring injury is unsupported.     

Community level australian football: a profile of injuries

Identification of injuries and their risk factors is required in order to develop risk controls within the context of sports injury prevention. The Australian Football Injury Prevention Project (AFIPP) was a randomised controlled trial (RCT) examining the effects of protective equipment on injury rates in Australian Rules Football. The aim of this paper is to describe the general injury profile of community level Australian Football players over one season, as recorded during this RCT. Players were recruited from the largest community football league in Victoria, Australia, during the 2001 playing season. A total of 301 players participated (64% response rate) and all injuries occurring during training and games were recorded. The overall injury incidence rate was 12.1/1000 player hours. Bruises/soft tissue injuries made up more than a quarter of all injuries (28%) and the leg (lower leg, ankle, thigh/hamstring and knee) was the most commonly injured body region. Most injuries occurred at the beginning of the season (April-May, 53% of injuries), during competition (77%) and through body contact (49.9%). Midfielders (OR = 3.39, 95% CI: 1.13, 10.14) and players aged at least 25 years (OR = 2.15, 95% CI: 1.06, 4.34) were significantly more likely to experience an injury than other playing positions and younger players. Although the injury rate in this study was lower than that in previous studies, the results are consistent with the finding that injuries tend to occur earlier in the season and more commonly during competition. Injury prevention efforts should be particularly targeted at midfielders and older players.     

Self-reported psychological characteristics as risk factors for injuries in female youth football

Identifying and understanding injury risk factors are necessary to target the injury-prone athlete and develop injury prevention measurements. The influence of psychological factors on injuries in football is poorly documented. The purpose of this 8-month prospective cohort study therefore was to examine whether psychological player characteristics assessed by a self-administered questionnaire represent risk factors for injury. At baseline, female football players (14–16 years) were asked to complete a detailed questionnaire covering player history, previous injuries, perception of success and motivational climate, life stress, anxiety and coping strategies. During the 2005 season, a total of 1430 players were followed up to record injuries. A history of a previous injury [odds ratio (OR)=1.9 (1.4; 2.5), P <0.001] increased the risk of a new injury to the same region. There were significant differences in disfavor for previously injured compared with non-injured players for ego orientation ( P =0.007), perception of a performance climate ( P =0.003) and experienced stressful life events ( P <0.001). However, only high life stress ( P =0.001) and perception of a mastery climate ( P =0.03) were significant risk factors for new injuries. In conclusion, a perceived mastery climate and a high level of life stress were significant predictors for new injuries in a cohort of young female football players.     

Previous injury as a risk factor for injury in elite football: a prospective study over two consecutive seasons

Background

Previous injury is often proposed to be a risk factor for football injury, but most studies rely on players reporting their own medical history and are thus potentially subject to recall bias. Little is known about the natural variation in injury pattern between seasons.

Objectives

To study whether prospectively recorded injuries during one season are associated with injuries sustained during the following season, and to compare injury risk and injury pattern between consecutive seasons.

Methods

The medical staffs of 12 elite Swedish male football teams prospectively recorded individual exposure and time loss injuries over two full consecutive seasons (2001 and 2002). A multivariate model was used to determine the relation between previous injury, anthropometric data, and the risk of injury.

Results

The training and match injury incidences were similar between seasons (5.1 v 5.3 injuries/1000 training hours and 25.9 v 22.7/1000 match hours), but analysis of injury severity and injury patterns showed variations between seasons. Players who were injured in the 2001 season were at greater risk of any injury in the following season compared with non‐injured players (hazard ratio 2.7; 95% confidence interval 1.7 to 4.3, p<0.0001). Players with a previous hamstring injury, groin injury, and knee joint trauma were two to three times more likely to suffer an identical injury in the following season, whereas no such relation was found for ankle sprain. Age was not associated with an increased injury risk.

Conclusions

This study confirmed previous results showing that previous injury is an important risk factor for football injury. Overall injury incidences were similar between consecutive seasons, indicating that an injury surveillance study covering one full season can provide a reasonable overview of the injury problem among elite football players in a specific environment. However, a prolonged study period is recommended for analyses of specific injury patterns.     

Acute hamstring injuries in Danish elite football: A 12‐month prospective registration study among 374 players

The purpose of the present study was to examine the incidence rates of acute hamstring injuries in Danish elite football sustained during training or match play. Furthermore, it was our intention to document details about the recurrence, severity and the injury seasonal distribution. Hamstring injuries among 374 elite football players were registered prospectively during a 12‐month period. A total of 46 first‐time and eight recurrent hamstring injuries were registered. The incidence rates for incurring a first‐time hamstring injury showed a significantly (P<0.01) greater incidence rate per 1000 h during match play compared with training. Of 32 players who reported a hamstring injury in the 12‐month period before the study, eight (25%) incurred an injury that fulfilled the criteria for a recurrent injury. In 69% of the injuries, the severity of injury was categorized as moderate (8–28 days from injury to injury free) and 18% as severe (>28 days from injury to injury free). Each team sustained a mean of 3.4 hamstring injuries per season, with a mean of 21.5 days missed per injury (range 3–136; median 16 days per injury). The seasonal distribution showed an accumulation of injuries in the first 2 months after a 3.5‐month mid‐season winter break.     

Australian football: Injury profile at the community level

Successful injury prevention relies on injury surveillance to establish the extent of the problem, to monitor injury patterns and to evaluate prevention strategies. Despite the popularity of participation in Australian football at the community level, few injury surveillance studies have been published describing the pattern of injuries at this level of participation. In contrast, ongoing injury surveillance at the elite-level is well established. Reliance on injury data from the elite-level of Australian football to guide injury prevention at the community level may not be appropriate due to differences across the levels with respect to exposure, fitness and skill level. Therefore, specific injury surveillance at the community level of Australian football is warranted. This study describes the epidemiology of community level Australian football injuries. Injury surveillance was undertaken in five amateur Australian football clubs over the 1999 season. The 320 participating players sustained 421 injuries over the season. The overall rate of injury was 27 injuries per 1000 player hours. Injuries were most commonly sustained at the start of the season and during the second quarter of match participation. Hamstring muscle strains were the most common injury sustained, followed by thigh haematomas and lateral ligament sprains of the ankle. The injury surveillance system used in this study was well accepted by the clubs and provides detailed data for the prioritisation of future injury prevention research at the community level of Australian football.     

Muscle flexibility as a risk factor for developing muscle injuries in male professional soccer players. A prospective study

BACKGROUND: Muscular tightness is frequently postulated as an intrinsic risk factor for the development of a muscle injury. However, very little prospective data exist to prove this. HYPOTHESIS: Increased muscle tightness identifies a soccer player at risk for a subsequent musculoskeletal lesion. STUDY DESIGN: Prospective cohort study. METHODS: We examined 146 male professional soccer players before the 1999-2000 Belgian soccer competition. None of the players had a history of muscle injury in the lower extremities in the previous 2 years. The flexibility of the hamstring, quadriceps, adductor, and calf muscles of these players was measured goniometrically before the start of the season. All of the examined players were monitored throughout the season to register subsequent injuries. RESULTS: Players with a hamstring (N = 31) or quadriceps (N = 13) muscle injury were found to have significantly lower flexibility in these muscles before their injury compared with the uninjured group. No significant differences in muscle flexibility were found between players who sustained an adductor muscle injury (N = 13) or a calf muscle injury (N = 10) and the uninjured group. CONCLUSIONS: These results indicate that soccer players with an increased tightness of the hamstring or quadriceps muscles have a statistically higher risk for a subsequent musculoskeletal lesion. CLINICAL SIGNIFICANCE: Preseason hamstring and quadriceps muscle flexibility testing can identify male soccer players at risk of developing hamstring and quadriceps muscle injuries.     

The Football Association Medical Research Programme: an audit of injuries in professional football--analysis of hamstring injuries

OBJECTIVE: To conduct a detailed analysis of hamstring injuries sustained in English professional football over two competitive seasons. METHODS: Club medical staff at 91 professional football clubs annotated player injuries over two seasons. A specific injury audit questionnaire was used together with a weekly form that documented each clubs' current injury status. RESULTS: Completed injury records for the two competitive seasons were obtained from 87% and 76% of the participating clubs respectively. Hamstring strains accounted for 12% of the total injuries over the two seasons with nearly half (53%) involving the biceps femoris. An average of five hamstring strains per club per season was observed. A total of 13 116 days and 2029 matches were missed because of hamstring strains, giving an average of 90 days and 15 matches missed per club per season. In 57% of cases, the injury occurred during running. Hamstring strains were most often observed during matches (62%) with an increase at the end of each half (p<0.01). Groups of players sustaining higher than expected rates of hamstring injury were Premiership (p<0.01) and outfield players (p<0.01), players of black ethnic origin (p<0.05), and players in the older age groups (p<0.01). Only 5% of hamstring strains underwent some form of diagnostic investigation. The reinjury rate for hamstring injury was 12%. CONCLUSION: Hamstring strains are common in football. In trying to reduce the number of initial and recurrent hamstring strains in football, prevention of initial injury is paramount. If injury does occur, the importance of differential diagnosis followed by the management of all causes of posterior thigh pain is emphasised. Clinical reasoning with treatment based on best available evidence is recommended.     

Intrinsic and extrinsic risk factors for muscle strains in Australian football

Muscle strains are common injuries in Australian football and other sports involving sprinting. Between 1992 and 1999, 83,503 player-matches in the Australian Football League were analyzed for risk of muscle strain injuries using logistic regression analysis. There were 672 hamstring, 163 quadriceps, and 140 calf muscle strain injuries. All three types of muscle strains were associated with significant risk factors. For all injuries, the strongest risk factor was a recent history of that same injury and the next strongest risk factor was a past history of the same injury. History of one type of muscle strain increased the risk for certain types of other muscle strains. Age was a risk factor for hamstring and calf muscle strains (even when adjusted for injury history) but was not a risk factor for quadriceps muscle strains. Quadriceps muscle injuries were more common in shorter players and were more likely when there had been less rainfall at the match venue in the previous week. Quadriceps muscle injuries were significantly more common in the dominant kicking leg, whereas hamstring and calf muscle injuries showed no difference in frequency between the dominant and nondominant legs.     

How valid is a self reported 12 month sports injury history?

BACKGROUND: A past injury history is one of the most commonly cited risk factors for sports injury. Often, injury history data are collected by self report surveys, with the potential for recall bias. OBJECTIVE: To assess the accuracy of a 12 month injury history recall in a population of 70 community level Australian football players. METHODS: The retrospective, self reported injury histories of 70 community level Australian football players were compared with prospective injury surveillance records for the same 12 month period. The accuracy of the players' recall of the number of injuries, injured body regions, and injury diagnosis was assessed. RESULTS: Recall accuracy declined as the level of detail requested increased. All players could recall whether or not they were injured during the previous year. Almost 80% were able to accurately recall the number of injuries and body regions injured, but not the diagnoses, whereas only 61% were able to record the exact number, body region, and diagnosis of each injury sustained. DISCUSSION: The findings of this study highlight the difficulty of using retrospectively collected injury data for research purposes. Any injury research relying on self reported injury history data to establish the relation between injury history and injury risk should consider the validity of the self report injury histories.     

Risk factors for noncontact ankle sprains in high school football players: the role of previous ankle sprains and body mass index

BACKGROUND: In a previous study, we noted a possible connection between an athlete's weight and risk of ankle sprain. HYPOTHESIS: A high body mass index and a history of a previous ankle sprain increase the risk of a subsequent noncontact sprain. STUDY DESIGN: Cohort study; Level of evidence, 2. METHODS: One hundred fifty-two athletes from 4 football teams were observed (2 varsity and 2 junior varsity). Two teams were observed for 3 seasons, and 2 teams were observed for 1 season. Before each season, body mass, height, history of previous ankle sprains, and ankle tape or brace use were recorded. RESULTS: There were 24 ankle sprains, of which 15 were noncontact inversion sprains (11 grade I, 3 grade II, 1 grade III; incidence, 1.08 per 1000 athlete-exposures). Injury incidence was higher in athletes with previous ankle injuries (2.60 vs 0.39; P < .001). Body mass index was also a risk factor (P < .05): injury incidence was 0.52 for players with a normal body mass index, 1.05 for players at risk of overweight, and 2.03 for overweight players. Injury incidence was 0.22 for normal-weight players with no previous ankle sprain compared with 4.27 for overweight players who had a previous sprain. CONCLUSION: An overweight player who had a previous ankle sprain was 19 times more likely to sustain a noncontact ankle sprain than was a normal-weight player with no previous ankle sprain. CLINICAL RELEVANCE: Ankle sprain prevention strategies should be targeted at football players with a high body mass index and a history of previous ankle sprains.     

Risk factors for injury in high school football players

BACKGROUND: Many studies have reported the frequency and types of injuries in high school football players. However, few have assessed the relationship between player characteristics and risk of injury. PURPOSE: To describe the epidemiologic characteristics of and risk factors for injury in high school football players and to determine whether players' characteristics could be used to predict subsequent injury. STUDY DESIGN: Prospective cohort study. METHODS: This study was part of a 2-year prospective investigation (1998 to 1999) of risk factors for injury in 717 (343 in the 1998 season and 374 in the 1999 season) high school football players in the Oklahoma City, Oklahoma, School District. Player characteristics (playing experience, position, injury history) and physical parameters (body mass index, weight, height, grip strength) were measured at the beginning of each season. Logistic regression analysis was used to determine whether any of the baseline variables were associated with the odds of subsequent injury. RESULTS: The physical characteristics of players, such as body mass index and strength, were not associated with risk of injury. More playing experience and a history of injury in the previous season were significantly related to increased risk. Linemen were at the highest risk of injury, particularly knee injuries and season-ending injuries. CONCLUSIONS: Future research should focus on decreasing the risk of injury to linemen.