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Describe acute lower extremity injuries and evaluate extrinsic risk factors in female youth soccer
Nested case-control study
Youth soccer clubs in Washington State, USA.
Female soccer players (N= 351) ages 11 to 15 years randomly selected from 4 soccer clubs from which 83% of their players were enrolled with complete follow-up for 92% of players.
Injured players were interviewed regarding injury, field surface, shoe type, and position. Uninjured controls, matched on game or practice session, were also interviewed.
The association between risk factors and acute lower extremity injury using logistic regression to estimate odds ratios (OR) and 95% confidence intervals (CI).
One hundred seventy-three acute lower extremity injuries occurred involving primarily the ankle (39.3%), knee (24.9%), and thigh (11.0%). Over half (52.9%) recovered within 1 week, while 30.2% lasted beyond 2 weeks. During practices, those injured were approximately 3-fold ( OR 2.83, 95% CI 1.49-5.31) more likely to play on grass than artificial turf and 2.4-fold (95% CI 1.03-5.96) more likely to wear cleats on grass than other shoe and surface combinations. During games injured players were 89% (95% CI 1.03-4.17) more likely to play defender compared to forward.
Half of the acute lower extremity injuries affected the ankle or knee. Grass surface and wearing cleats on grass increased training injuries.
Soccer is one of the most popular sports for girls in the U.S. with U.S. Soccer reporting 1.44 million girls participating.1 World-wide, the number of female participants doubled between 2000 and 2006, reflecting the exploding popularity of the women's game.2 Previous studies have shown that amongst girl's sports, soccer has the highest acute contact injury rate with only boy's football and wrestling having higher injury rates.3 In youth soccer, the existing literature shows high game injury rates predominantly affecting the lower extremity.4-5 However, evidence based injury prevention strategies in youth soccer are lacking.4
Injury prevention efforts require identification of modifiable intrinsic and extrinsic risk factors and playing surface is one extrinsic variable gaining researchers’ interest as artificial turf has replaced natural grass. First generation turf consisted of short fibers attached to base material with later versions of artificial turf having longer fibers with sand fill (second generation) or composite fill using sand and rubber particles (third generation) which is most common today. Field surface has been studied as an injury risk factor primarily in adults and early studies of first generation turf suggested the artificial surface was associated with more injuries.6 More recent studies conducted primarily at the professional level find no difference in injury risk comparing grass and third generation turf with the exception of more ankle sprains on turf.7-9 There is still some concern that increased friction between shoe and field surface places elite athletes in particular at higher risk on artificial surfaces10,11 and one recent review suggested elite athletes may be safer on grass while lower level athletes have fewer injuries on artificial surface.6
Many communities are turning to artificial turf for their youth sports programs because compared to grass it is more durable and easier to maintain thus reducing cost, but the potential for turf to increase or decrease risk of injuries in this population is largely unstudied. The purpose of this study was to evaluate extrinsic risk factors including field surface and footwear and describe the characteristics of acute lower extremity injuries in a sample of 11-15 year old elite female youth soccer players in the United States.
We enrolled 351 female youth soccer players ages 11 to 15 years (U13-U15) in a prospective cohort study, randomly selecting 36 of 72 elite (select and premier level) teams from four youth soccer clubs in the xxx region of Washington State. Of the 36 teams we contacted, 33 (91.7%) agreed to participate and we recruited 83.1% (351/422) of their players. The top two teams in each age group are designated premier level and the next level teams are designated select level. Eight teams were enrolled during the 2008-2009, 2009-2010, and 2010-2011 season and 9 teams during the 2011-2012 season, for a total of 20 premier and 13 select teams. We followed each team for at least one year-long season with 6 teams followed for two seasons. We had complete follow-up through the end of the season on 92.4% of players. At the start of the soccer season, all subjects provided data on age, race, ethnicity, length of soccer career, and prior history of soccer-related injuries. We also obtained data on each subject's soccer shoes (type of shoe – cleats or turf) at the start of the season and updated this information during the course of the soccer season when players bought new shoes. All subjects provided assent and their parents provided informed consent. Our study was approved by the xxx Institutional Review Board.
We performed a case-control study of episodes (games and practices) nested within our cohort study to identify episode-specific risk factors for acute lower extremity injuries among our soccer players. We implemented a validated injury surveillance system12 in which we sent a weekly email to each player's parent with a link to an internet-based survey which included the question, “Did your daughter have an acute injury in the past week that resulted in loss of playing time?” The email system sent an automated reminder after two days of non-response and, if not completed, study staff telephoned all non-responders and obtained the information. A case was defined as an episode (game or practice) for a soccer player who experienced an acute lower extremity injury that resulted in loss of playing time (left game or practice and could not return). A lower extremity injury was defined as a sudden onset injury to the groin, hip, thigh, knee, lower leg, ankle or foot. The reference date for cases was the game or practice in which the injury occurred. A control was defined as an episode (game or practice) for a soccer player who played without an injury during the episode. Thus, a game in which 14 players had playing time (rotating through the 11 positions) would generate 14 episodes of interest to the study. If, for example, only one of the episodes involves an injury with a loss of playing time, that episode would become a case; the other 13 episodes would be eligible to be sampled as controls. Control episodes were randomly selected every week from all injury-free episodes played that week. The selection was stratified on game/practice and took into account the number of episodes for that week and the cumulative number of all episodes and of control episodes in the study up to that week. The reference date for controls was the selected injury-free game or practice episode.
Trained study personnel performed a standardized telephone interview with each case and control. Cases were asked injury-related questions including the location and type of injury, if the injury was caused by contact and object contacted. Our sports medicine physician (xxx) assigned each diagnosis after reviewing all interview data and any available medical records. If the diagnosis was in question, all players were offered a free clinical evaluation by (xxx) as well as any required imaging. Players were interviewed weekly until the injury resolved and full participation resumed. At the final interview, we obtained the date of return to play. For the control interviews, players were asked to respond to interview questions using the reference date described above. Median time to telephone interview for all players was 3.0 days. Both cases and controls were interviewed about episode-specific risk factors that were present on the reference date. These risk factors included pre-versus post menarche status as well as field conditions such as moisture level (dry, wet) type of field surface (artificial turf, grass), the combination of moisture level and type of field surface, and shoe type/field surface combinations (cleats worn on artificial turf, cleats worn on grass, turf shoes worn on artificial turf, other combinations). The shoe type assessed in the shoe type/field surface risk factor represented the shoe the player was wearing at the time of injury (cases) or game /practice (control) using the updated shoe data when new shoes were purchased. We evaluated the position played at the time of the injury (cases) or during the selected game (controls) as a risk factor for games only.
We compared baseline demographic and soccer characteristics among cases and controls including age, race, ethnicity, soccer level, years playing soccer, and prior injuries. Among injured players, we described the injury.
We assessed the association between risk factors and acute lower extremity injury using weighted logistic regression to estimate odds ratios (OR) and 95% confidence Intervals (CI, determined by the non-parametric bootstrap). We hypothesized that the risk factors would differ based on the type of soccer event because training is more likely to be friendly and players prioritize not injuring teammates while games are likely to be played more aggressively. Since the association between risk factors and lower extremity injury differed by type of soccer event, we built separate regression models for training and games. The models used weighting. Cases had a weight of one (all cases were sampled) and controls had the weight determined by dividing the number of available players for a week by the number of selected controls. The final model for each risk factor was adjusted for selected confounders based on a >10% change in the risk factor OR comparing the crude and adjusted models. Shoe type/field surface for practices was adjusted for age and level of play and shoe type/field surface for games and position were adjusted for age. All calculations were performed using Stata, version 12 (StataCorp, College Station, TX) and R, version 2.15.0 (The R Foundation for Statistical Computing, Vienna, Austria).
During the study period, 173 acute lower extremity injuries occurred among 129 players. Cases with acute lower extremity injuries and controls were similar with respect to race, Hispanic ethnicity, soccer level, and years playing soccer, although cases were more likely to be younger than controls (Table 1). There were 296 unique players who served as controls for acute injuries at least once, for a total of 805 control interviews.
Among injured players, the most common type of injury was a sprain/strain (108/173, 62.4%), with ankle, knee, and thigh the most commonly affected body region (Table 2). Contusions were next most common (52/173, 30.0%) affecting the knee, ankle, and foot. The most common body regions injured were ankle (39.3%), knee (24.9%), and thigh (11.0%). The majority (52.9%) of injuries lasted from 1 to 7 days, although 30.2% lasted 14 days or longer (Table 3). Two-thirds (66.5%) of the injuries involved contact, with most injuries (78.1%) involving contact with another player.
We found no association between risk of lower extremity injury and pre-compared to post-menarche ((pre-menarcheal vs. post-menarcheal), OR 1.29, 95% CI 0.84-1.92). During practices, players with a lower extremity injury were 2.83-fold (95% CI 1.49-5.31) more likely to have played on grass compared to uninjured players (Table 4). Injured players during practice were also 2.40-fold (95% CI 1.03-5.96) more likely to wear cleats on grass vs. cleats worn on artificial turf compared to uninjured players. During games, injured players were 89% (OR 1.89, 95% CI 1.03-4.17) more likely to play defender position vs. forward, the reference category, compared to uninjured players. We found no significant association between injury and a wet surface for games or practices.
Injury patterns in this study are similar to those previously reported in both men and women across age groups. Junge et al.13 using men's and women's FIFA tournament data noted 80% of injuries occurred from player-to-player contact. Several investigators have noted the ankle as the most frequently injured body part among adult male and female players. 14,15 The few studies on youth players have similar results to our findings with higher incidences of ankle and knee injuries.4 The ankle is the most injured body region in high school girls soccer3 as well as among elite players under 15 years of age.16 Player to player contact accounts for 60- 80% of injuries in a number of studies. 4,17,13,18
Defenders had a higher risk of game injury than other positions in this study. Only one other study of female youth soccer players stratified risk by position played and found no field position at increased risk relative to the other positions.16 Another female youth study did find that more highly skilled players were at increased risk of injury but they did not stratify by position.19 A number of studies of professional players have stratified risk by position and found that goalies are at lower risk with no difference among the field players,20,17 with one study finding higher injury risk for forwards and central defenders.21 Why female youth defenders are at higher risk than other positions is unclear, but it may be that they are less skilled or more willing to tackle and are coached harder to tackle at an early age thus placing themselves at higher risk.
We evaluated the shoe type and field surface as episode-specific risk factors and found cleats worn on grass were associated with a 2.4 fold increased risk for training injuries. Several studies have found that cleats increase injury risk. Renstrom et al.22 found that cleated shoes posed an increased injury risk in soccer on first generation artificial turf. Studies in American football have found that independent of surface, longer cleats increase injury risk.23,24 Many experts suggest increased shoe-surface friction can improve performance but also increases injury risk.6,25 Presumably, cleats dig into the surface increasing friction and our finding may be explained by cleats doing that to a greater extent on grass than turf. If increased friction is the explanation, other factors likely to contribute in a positive or negative fashion include the length of the turf, age of the turf, general condition of the grass, and weather variables including moisture and temperature.26
We found training on grass associated with a 2.8-fold increased risk of lower extremity injury while game injuries did not vary significantly comparing turf and grass. Most of the existing literature on this topic involves the study of elite adult athletes outside of the U.S. Ekstrand et al.9 followed two cohorts of elite male soccer players over 2 years, one group playing on grass and one playing on artificial turf. The overall (training + game) incidence of injury did not vary between the groups but in games, the incidence of ankle sprains was higher on turf while the overall injury rate was higher on grass. In a subsequent study, they followed 20 elite teams over a year and found no difference in overall injury risk between the surfaces but did find that the male players were more likely to sprain an ankle on turf.8 Bjorneboe et al.27 followed male professional soccer players for 3 years and found no difference in injury risk. Kristenson et al.28 found no difference in player game injury rates but did find that clubs with turf on their home fields had higher practice injury rates.29 Among youth soccer players, two studies, 30,31 have noted no overall increased risk of injury associated with surface type, although Steffen et al.31 found serious game injuries, most of which were ankle sprains, were more common on artificial turf. In one of the few U.S. studies, Myers et al.29 found that female college soccer players had fewer injuries in game play on turf.
Our finding that training on artificial turf is safer than grass contradicts some of the existing literature although the majority of that literature reports on high-level play in European adults.8,9,27 The explanation for more training injuries on grass in our study is speculative, but grass surfaces available to youth soccer players receive less maintenance and are in poorer condition than those at higher levels and irregularities in the surface may contribute to injuries in non-contact training situations. We found no difference in games. Possibly in games where the entire time is spent in full contact play, the contribution of the surface to acute injuries may diminish, as the physical nature of the sport becomes the more dominant risk factor. One professional level study supports this theory finding that game injuries are twice as likely to involve contact compared to training injuries.32
Our study had several limitations. Results may have been affected by recall bias since cases may have been more likely to remember type of surface and shoes at time of injury compared to memory of the conditions at the specified game or practice for controls. We performed our interviews with controls and cases within a median of 3 days of the reference or injury date, respectively to decrease this bias.
Another weakness was not having every athlete evaluated by a medical professional. Our goal was to develop a system for injury tracking in youth soccer that was cost-efficient yet accurate and our pilot study33 demonstrated high agreement for injury rate, type, and laterality between on-site certified athletic trainers (ATC) and the internet-based survey. Our study noted that among players with acute injuries, 48% were reported by both systems, 63% were reported by team ATC and 85% by the internet-based survey. Full time medical personnel are desirable but cost prohibitive in youth sports and our desire was to identify an accurate, viable alternative for injury tracking in youth soccer. Lastly, our results could be explained by uncontrolled confounding by player fitness or skill level. We did not measure these factors so were unable to adjust for them in our multivariate analysis.
In conclusion, our findings suggest that injury patterns among female youth soccer players are similar to those among older, higher level players, with ankle sprains most prevalent. Neuromuscular training programs34 along with ankle braces35,36 have been proven to reduce the risk of primary and secondary ankle sprains and should be considered in this population. Acute injuries overall are more common with training on grass and cleated shoes are riskier on grass than turf. This study supports third generation artificial turf as a safe playing surface for U.S. female youth soccer players particularly when compared to the grass fields available to this population.
The majority, 64%, of female youth soccer players’ acute injuries involve the ankle and knee and injury prevention strategies in this age group should target this areas. When considering playing surfaces for training, communities and soccer organizations should consider third generation artificial turf a safe alternative to grass.
We would like to thank the soccer clubs and the players and parents who participated in this study. We would also like to thank Christopher Mack, MS for assistance with data management. Research reported in this publication was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under Award Number R01AR051059. Dr. Schiff had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
None of the authors report any conflicts of interest.