Effective Injury Prevention in Soccer Donald T. Kirkendall, PhD; Jiri Dvorak, MD

Effective Injury Prevention in Soccer Donald T. Kirkendall, PhD; Jiri Dvorak, MD Abstract: Sports participation is accompanied by risk of injury, and each specific sport has its own unique injury profile. One of the goals of a sports medicine professional is injury prevention, and the past decade has seen numerous reports on the outcomes of injury-prevention studies. Health care professionals have been particularly vigilant in attempting to reduce common injuries in soccer, beginning with work in the early 1980s to the rigorous randomized trials of today. The use of a structured, generalized warm-up program has been shown to be effective in preventing common soccer injuries, reducing overall injury rates by approximately 30%. Given the number of individuals who play soccer worldwide, any injury reductions will likely have an impact on public health. It is an important goal of the sports medicine community to inform physicians and other sports medicine professionals about the effectiveness of prevention programs to increase use and compliance. Keywords: soccer; injury prevention; randomized trials; ligament sprains; muscle strains Donald T. Kirkendall, PhD 1 Jiri Dvorak, MD 1 1 FIFA Medical Assessment and Research Center (F-MARC), Schulthess Clinic, Zürich, Switzerland Correspondence: Donald T. Kirkendall, PhD, 1103 Winwood Drive, Cary, NC 27511. Tel: 919-624-0663 Fax: 919-681-6357 E-mail: donald_kirkendall@yahoo.com Introduction Soccer is unquestionably the most popular participant and spectator sport in the world. The statistics representing soccer s influence and outreach are impressive: 1 There are > 250 million registered players worldwide. The international governing body of soccer, the Fédération Internationale de Football Association (FIFA), has more member nations than the United Nations. The FIFA World Cup Web site received 4.2 billion Web page views during the 1-month competition in 2006. The 2006 FIFA World Cup Germany was broadcast in 214 countries. The estimated cumulative number of television viewers for the entire 2006 FIFA World Cup Germany was 26.29 billion. 715 million viewers watched Italy s overtime win over France in the 2006 final. No other single sport or event comes close to the impact soccer has worldwide. In June 2010, the world s attention will again be riveted on soccer for the 2010 FIFA World Cup South Africa. Although the world s attention will be focused on a single tournament involving a few hundred professional players, professionals account for only a few hundred thousand of this massive number of participants. Like all other sports, playing soccer carries the risk of injury. Every sport has its unique profile of injuries, many of which are preventable. One of the goals of the sports medicine professional is injury prevention, and sports physicians should be doing everything possible to minimize the risk of injury to keep players in the competitive arena. In the past, sports physicians and coaches had to apply general concepts of prevention to any particular sport. Many of the methods may have made sense intuitively, but there continues to be debate on issues, such as the preventive benefits of pre-event stretching. 2 4 In the past 10 years, great strides have been made by the research community to provide strong evidence for the effectiveness of prevention programs. Most prevention trials follow a model proposed by van Mechelen 5 that follows 4 conceptual steps (Figure 1): 1) establish the incidence of injury; 2) determine the mechanism of injury; 3) devise prevention programs; and 4) test the program by gathering new incidence data. THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38 1

Donald T. Kirkendall and Jiri Dvorak Figure 1. Prevent research model. 1. Establish the extent of the injury problem: Incidence Severity 2. Establish the etiology and mechanisms of sports injuries 4. Assess effectiveness by repeating step 1 3. Introduce preventive measures Reproduced with permission from Sports Med. 5 Methods of Injury Prevention Research in Soccer Injury Incidence in Soccer Soccer is classified as a contact sport. Many consider soccer to be relatively safe because of the low incidence of serious injury. 6 However, it is difficult to measure soccer s ranking in terms of overall injury rate compared with other sports because of different study methods used in defining injury and severity, reporting of rates, and comparing different ages, genders, and performance levels. Although there is consensus on the methods of conducting injury surveillance research, 7 earlier research (which focused primarily on professionals) was difficult to compare, and there are few papers that have studied injuries in which there is the highest level of participation: recreational youth. One of the most reliable data sets on a large number of athletes in various sports is the National Collegiate Athletic Association (NCAA) s Injury Surveillance System. This rigorous survey project, sponsored by the NCAA, includes almost 20 years worth of collected injury statistics, at all levels of 15 intercollegiate sports. 8 A summary of the NCAA injury rates is shown in Table 1, with soccer ranked in the upper one-third of all sports observed. Since 1998, FIFA s Medical Assessment and Research Center (F-MARC) has been conducting an injury surveillance program at all FIFA competitions. 9,10 These data are gathered by each team physician at every match and reported to the research team. Over time, injury rates of 88.7 10 and 67.4 9 injuries per 1000 match hours have been reported for men and women, respectively, and most of these injuries are considered minor ( 7 days lost). 11 Most injuries involve contact during tackling, 12,13 and a little less than one-half 10 and one-third 9 of all injuries in men and women, respectively, are due to foul play. As data collected at world championship events come from the highest level of competition, the rates are higher than that reported for lower levels of play. 6,14 Common Injuries in Soccer Because of the nature of the game, in which the lower extremities are primarily used to control and advance the ball, the most common injuries are to the lower extremity. A common soccer injury is a contusion; however, a contusion rarely results in days lost from training or competition. Most studies report that the top time-loss injuries, regardless of level of play or gender, are ankle sprains, knee sprains, hamstring strains, and groin strains, 14 19 2 THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38

Preventing Common Soccer Injuries Table 1. Overall Rates of Injury per 1000 Athlete Exposures for National Collegiate Athletic Association Sports Game Practice Men s football 35.9 3.8 Men s wrestling 26.4 5.7 Men s soccer 18.8 4.3 Women s soccer 16.4 5.2 Men s ice hockey 16.3 2.0 Women s gymnastics 15.2 6.1 Women s ice hockey 12.6 2.5 Men s lacrosse 12.6 3.2 Men s basketball 9.9 4.3 Women s field hockey 7.9 3.7 Adapted from J Athl Train et al. 8 Table sorted on game injury rate. Data averaged over 16 years. which account for between one-half and two-thirds of all soccer injuries. The order changes by level of play, with ankle sprains being more common in youth and lower levels of play, 20,21 and hamstring strains being a leading cause of time loss in higher levels of play. 11,19,22 Mechanism of Common Soccer Injuries Ankle Sprain Noncontact ankle sprains can occur when stepping on uneven ground, landing on another player s foot after jumping, or while cutting. A mechanism that is specific to soccer occurs during tackling: a player can be dribbling the ball with the right foot when an opponent attempts a tackle from the opposite side, usually by sliding. Contact with the medial aspect of the dribbler s shin at the time of ground contact can force an inversion stress on the right ankle, spraining the lateral ankle ligaments. 23,24 Knee Sprain The other main ligament injury in soccer is a noncontact anterior cruciate ligament (ACL) tear a particular problem in the female player. It is believed that cutting or landing on a nearly straight hip and knee, 25 coupled with perturbations at the trunk, 26 places enough tension on the ACL to tear the ligament. The ACL injury rate in female soccer players is reported to be about 3 times that of males. 27,28 Hamstring Strain Higher levels of play can be accompanied by increased speed of running. For many, a hamstring strain is thought of as a sprinting injury (ie, a track injury). The higher frequency and length of high-speed sprints at top-level competition has resulted in hamstring strains increasing from 2.3 injuries per team in the inaugural season of the Major Soccer League, 18 to 5 per club in the English Football Association (FA), 11 and from 9% of all injuries in Swedish professionals 29 to 28% of all injuries in the Italian Serie A. 19 The mechanism of a muscle strain is well described (high muscle tension during muscle stretch 30 ), with the biceps femoris being the most frequently injured of the 3 hamstrings. 11,31 Groin Strains The basic mechanism of this muscle strain is the same as described for a hamstring strain a strong contraction during stretch. In soccer, a player reaches far to the side with his or her leg, usually in an attempt to tackle or deflect a ball. When contacting the ground with the lead leg, a groin muscle (usually the adductor longus) in the push-off leg tears when it contracts to help maintain balance, resulting in an injury in the trailing leg (W.E. Garrett, personal communication 2009). Method of Injury Prevention Research Although injury prevention has a long history of discussions, evidence-based research has become more commonplace in the past 10 to 15 years. A search of PubMed was conducted using injury prevention and sport as primary keywords (n = 2910). This list was limited to clinical trial or randomized controlled trial (n = 202). The list was then narrowed to studies on all children (0 18) and adults (18 44) (n = 172). The list was limited further to studies on preventing common soccer injuries (ankle sprain, knee sprains, muscle strains). Each title and abstract was then reviewed to ensure that, after random assignment, injuries in a control group and intervention group were compared. Studies that used a prophylactic intervention (eg, taping, bracing) were excluded because only training programs were of interest. Finally, while soccer was the sport of interest, other team sports (basketball, volleyball, team handball, and Australian Rules football) were also included. In total, there were 3 trials on ankle sprains, 3 trials on knee sprains, 3 trials on muscle strains, and 7 general injury prevention trials. Table 2 summarizes the trials that met the inclusion criteria. Ekstrand et al 32 34 conducted the first serious investigation in soccer injury prevention in the early 1980s. Many factors and interventions were studied using a medically supervised on-field program for Swedish professionals. The final tally was THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38 3

Donald T. Kirkendall and Jiri Dvorak Table 2. Summary of Randomized Control Trials on Injury Prevention in Sport Study Joint Focus Sex Group Membership Hupperets et al 41 Ankle Prevent recurrent ankle sprains Males and females Intervention 256 (120 females), controls 266 (128 females) Mohammadi 42 Ankle Prevent recurrent ankle sprains in soccer Males 80 males randomly assigned to 1 of 3 interventions or control Holme et al 63 Ankle Prevent recurrent ankle sprains Males and females 92 subjects matched for age, sex, level of play Myklebust et al 53 Knee ACL injury prevention in team handball Gilchrist et al 52 Knee Prevent ACL injury in collegiate soccer Females Control season 942, 2 intervention seasons of 855 and 850 Females Intervention 583; control 852 Mandelbaum et al 51 Knee Injury prevention in youth soccer Females Year 1: intervention, 1041; control, 1905; Year 2: intervention 844, control 1913 Gabbe et al 58 Muscle Hamstring injury prevention in amateur Australian Rules Football Males 220 males randomly assigned to control or intervention group Askling et al 60 Muscle Hamstring injury prevention in professional soccer Fredberg et al 64 Muscle Prevent Achilles and patellar tendon injury in professional soccer Scase et al 65 Overall Injury prevention in junior Australian Rules Football Males Males 30 players randomly assigned to hamstring specific eccentric training or control 209 (9 teams randomly assigned to intervention or control) Males Intervention 114, control 609 Soligard et al 36 Overall Injury prevention in youth soccer Females Intervention 1055, control 837 Steffen et al 54 Overall Injury prevention in youth soccer Females Intervention 1091, control 1001 Emery et al 66 Overall Injury prevention in youth basketball Males Intervention 494, control 426 Junge et al 61 Overall Injury prevention in youth soccer Males Intervention 194, controls 101 Ekstrand et al 33 Overall Multifactorial study of professional male soccer Emery et al 67 Overall Home-based balance training to prevent sports-related injury in adolescents Males 12 teams (180 players) randomly assigned to control or intervention groups Males and females Intervention 66, controls 81 4 THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38

Preventing Common Soccer Injuries Age Intervention Outcome Middle school-aged to adult Adult 8-wk home-based proprioception program Proprioceptive training, strength training, orthosis, control 35% risk reduction of recurrent ankle sprain Significant reduction in recurrent sprains in proprioceptive training group vs control Adult Balance training program Recurrent ankle sprain in 29% of control vs 7% of intervention Adult College-aged High school-aged Balance, neuromuscular control, planting and landing strategies Guided warm-up program focused on neuromuscular control Guided warm-up: stretching, strengthening, plyometrics, agility ACL injury risk was reduced in players who complied with the program 41% reduction in ACL injuries, 70% reduction in noncontact injuries. Significant reduction in recurrent ACL injury. 88% reduction in ACL injury in year 1, 74% reduction in year 2. Adult Adult Stretching (control) vs eccentric hamstring strengthening Mechanical eccentric overload of hamstrings No group differences in injury rates because of poor compliance. In those who complied with the program, the RR was 0.3 (P 0.1) Hamstring strains in 10 of 15 in control group vs 3 of 15 in intervention group Eccentric training and stretching of the Achilles and patellar tendons Intervention reduced risk of developing ultrasonic evidence of abnormalities, but no differences in injury rate High school-aged Landing and falling strategies 28% reduction in injuries in intervention group High school-aged High school-aged Middle school- to high school-aged High school-aged Adult High school-aged Progressive warm-up program for strength, awareness, neuromuscular control Exercises to improve core stability, lower extremity strength, neuromuscular control, agility Guided warm-up program and home-based balance program Exercises to improve core stability, lower extremity strength, neuromuscular control, agility Numerous interventions, including training correction, prophylactic measures, controlled rehabilitation, etc. Daily wobble board training (6 wks), then 6-month, home-based program 32% reduction in overall injury rate, 43% reduction in overuse injuries, 45% reduction in severe injuries No difference in injury rate, possibly due to poor compliance by intervention teams 29% reduction in acute injuries. Reported 20% reduction in all injuries, 17% reduction in lower extremity injuries, and 29% reduction in ankle sprains were not significant. There was poor compliance to the home-based program 36% injury reduction in intervention group, especially for mild, overuse, training injuries. Greater effect in low- vs high-skilled players 75% injury reduction in intervention group, particularly in ankle/knee sprains and muscle strains. Balance training had a protective effect of injury (RR = 0.2) Abbreviations: ACL, anterior cruciate ligament; RR, relative risk. THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38 5

Donald T. Kirkendall and Jiri Dvorak a 75% reduction in injuries, 33 and a number of observations (eg, role of the training/match ratio to injuries 34 ) have become a renewed topic of interest. 35 The designs of the projects and the number of interventions implemented, however, made it difficult to determine which prevention methods were the most critical. As previously mentioned, van Mechelen 5 outlined a protocol for conducting injury-prevention research (Figure 1). In a research setting, a large number of teams are randomly assigned to a control (no intervention) or an experimental group (the prevention program), and the injury rates of the 2 groups over a full competitive season are compared. A good example of such a study is the cluster randomized trial by Soligard et al. 36 These studies require a very large numbers of subjects (most involve 1000 players), are very time consuming, labor intensive, costly, and are conducted at a limited number of research centers. Some studies have been performed on limited numbers (Table 2). Many prevention studies and randomized controlled trial designs have been conducted on specific injuries. Ankle sprains are one of the most common ligament injuries in sports and require aggressive treatment to avert future disability. 37 This common injury is a primary target for prevention research. 38 44 The hypothesis is that if the muscles can be strengthened and proprioceptive function around the ankle can be improved, the athlete will be better positioned to respond to forces that attempt to force the ankle into inversion. Prevention activities include various exercises and protocols using an unstable platform (eg, cushioned pillows, various types of wobble boards) and strengthening of muscles that cross the ankle. In a recent trial, the rate of ankle sprains was reduced from 0.9 to 0.5 injuries/1000 player hours 44 in Dutch volleyball players. Notably, there was a 60% reduction in ankle injury in players with a history of a prior ankle sprain. 44 No prevention trial to date has been able to prevent the first ankle sprain. The only projects that have been shown to prevent the first ankle sprain are those that use external supports (eg, tape, braces, orthoses). 45 Prevention of ACL injuries, particularly in females, has come under intense scrutiny and has been the subject of several reviews that discuss the factors behind the gender inequity in ACL injury rates. 46 48 Although there are many risk factors of interest, 49 the prevention programs have primarily focused on basic motor skills, strength, balance, and proprioception, all of which are incorporated in the warm-up. These methods have been shown to improve neuromuscular power and control, 50 which should reduce ACL injury rates. Reduced ACL injury rates have been reported in the 2-year intervention trial at the Santa Monica Orthopaedic and Sports Medicine Foundation (A FIFA Medical Center of Excellence) by Mandlebaum et al, 51 who reported 88% and 74% reductions in years 1 and 2, respectively, in ACL injury rates in female soccer players aged 14 to 18 years. Gilchrist et al 52 demonstrated a 70% reduction in noncontact ACL injuries in collegiate female soccer players. When performed regularly, there are reports of significant reductions in ACL injury rates. However, compliance is critical because these programs can be ineffective if not performed on a consistent basis. 53,54 Further, the effectiveness may be sportspecific, with reduced injury rates being reported in soccer but not in basketball. 49 The risk of an overuse injury of the knee is increased in a player with a prior ACL tear, 55,56 making primary prevention of the first injury a major goal. The traditional way to prevent hamstring strains is by static stretching during warm-up (a practice that has come into question). 2 While the risk of this injury is greatly increased in players with a history of a previous injury (odds ratio [OR], 7.4 57 ), age 57,58 and thigh muscle strength imbalance 59 have also been identified as risk factors. 22 Strength can be increased by strength training in the weight room. In professional male soccer players, a 10-week strength training program (1 2 days/week) that emphasized an eccentric overload reduced the incidence of hamstring strains from 67% in the control group to 20% in the intervention group. 60 Like many other sports injuries, a history or a prior injury greatly increases the risk of a recurrent injury; thus, primary prevention of the first hamstring strain is critical. Generalized Warm-Up to Decrease Soccer Injuries In the spring of 2003, F-MARC brought together a group of international injury-prevention experts and developed a prevention program for amateur players based on their knowledge. The 11 is a program of 10 simple, catchy, and time-efficient exercises plus the promotion of fair play to reduce injuries from fouls. The previous version of this prevention program 61 was studied in males (aged 14 19 years), in which the overall injury rate was reduced by 36%. When the results were reported as injuries per player, there were 43% fewer mild injuries, 41% fewer overuse injuries, 42% fewer noncontact injuries, 55% fewer training injuries, and 80% fewer groin injuries. 61 The 11 has been successfully implemented in countrywide campaigns in Switzerland and New Zealand. A new, more advanced version of this program called The 11+ is a complete generalized warm-up program. 6 THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38

Preventing Common Soccer Injuries Figure 2 shows that The 11+ program has 3 parts: running exercises and dynamic stretching (part 1) followed by 6 exercises (part 2), each with 3 levels of increasing difficulty to improve strength, balance, motor control, and core stability. This is then followed by further running exercises (part 3). Coaches and players can individually adapt the program as fitness improves. The entire warm-up takes about 20 minutes to complete. A cluster randomized trial of The 11+ conducted by researchers at the Oslo Sports Trauma Research Center (a FIFA Medical Center of Excellence) was recently published and reports evidence substantiating the effectiveness of the program at reducing common soccer injuries. 36 The research group collected information on all injuries, both acute and overuse. The 11+ was used in every training session instead of a traditional warm-up. Prior to a match, only parts 1 and 3 (the running exercises) were performed in advance of more soccer-specific, pre-match warm-up activities. This trial followed nearly 1900 female players (aged 13 17 years) from 125 Norwegian clubs that were randomly assigned to either a traditional warm-up (control group) or The 11+ (intervention group). Each coach and team captain of the intervention teams received face-to-face instruction and then introduced the program to their respective team. Injury incidence for the entire 8-month season was documented in each group. The overall risk of injury in the teams that performed The 11+ was about 30% less than that of the control group. Other rate ratios can be found in Table 3. According to the authors,... the overall rate of injuries, as well as the rate of match injuries, training injuries, overuse injuries, and acute injuries differed significantly. The rate of severe injuries, severe overuse injuries, and severe acute injuries was significantly lower in the intervention group. 36 The research paper, 36 videos of all the exercises, the poster, and more are available at the FIFA 11+ Web site. 62 Furthermore, fewer players in the intervention group had 2 injuries. There was an inverse relationship between compliance and injury rate: the teams that performed the program the most had the lowest injury rates. Summary Based on results from current randomized controlled trials, there is good evidence that prevention programs are effective at preventing common soccer injuries. The data show that significant reductions in ankle and knee ligament sprains as well as muscle strains are possible when complying with such Table 3. Injury Rate Ratios Between Teams with Traditional Warm-up and Teams Performing The 11+ Prevention Program Rate Ratio b All injuries 0.68 Match injuries 0.71 Training injuries 0.63 Severe injuries a 0.54 All overuse injuries 0.44 Severe a overuse injuries 0.30 All acute injuries 0.76 Severe a acute injuries 0.65 Contact injuries 0.64 a Severe injuries = 28 days time loss. b Rate ratio = intervention group injury rate/control group injury rate. Adapted with permission from BMJ. 36 a program. Although there are protocols for specific injuries, a generalized warm-up plan that incorporates aspects of the injury-specific programs, like The 11+, addresses many of the common injury problems in soccer. The sports medicine community needs to be diligent in promoting simple and effective injury prevention programs to the teams under their care. Educating the on-the-field coach on how such programs are effective at reducing the time lost to injury should encourage coaches to incorporate injury prevention as an essential component of their team s training program. Conflict of Interest Statement Donald T. Kirkendall, PhD, and Jiri Dvorak, MD disclose no conflicts of interest. References 1. FIFA. 2006 FIFA World Cup in numbers. http://www.fifa.com/aboutfifa/ marketing/factsfigures/numbers.html. Accessed December 12, 2009. 2. Shrier I. Stretching before exercise does not reduce the risk of local muscle injury: a critical review of the clinical and basic science literature. Clin J Sports Med. 1999;9(4):221 227. 3. Witvrouw E, Mahieu N, Danneels L, McNair P. Stretching and injury prevention: an obscure relationship. Sports Med. 2004;34(7):443 449. 4. Woods K, Bishop P, Jones E. Warm-up and stretching in the prevention of muscular injury. Sports Med. 2007;37(12):1089 1099. 5. van Mechelen W. Sports injury surveillance systems. One size fits all? Sports Med. 1997;24(3):164 168. 6. Froholdt A, Olsen OE, Bahr R. Low risk of injuries among children playing organized soccer: a prospective cohort study. Am J Sports Med. 2009;37(6):1155 1160. 7. Fuller CW, Ekstrand J, Junge A, et al. Consensus statement on injury definitions and data collection procedures in studies of football (soccer) injuries. Br J Sports Med. 2006;40(3):193 201. THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38 7

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Preventing Common Soccer Injuries 51. Mandelbaum BR, Silvers HJ, Watanabe DS, et al. Effectiveness of a neuromuscular and proprioceptive training program in preventing anterior cruciate ligament injuries in female athletes: 2-year follow-up. Am J Sports Med. 2005;33(7):1003 1010. 52. Gilchrist J, Mandelbaum BR, Melancon H, et al. A randomized controlled trial to prevent noncontact anterior cruciate ligament injury in female collegiate soccer players. Am J Sports Med. 2008;36(8):1476 1483. 53. Myklebust G, Engebretsen L, Braekken IH, Skjølberg A, Olsen OE, Bahr R. Prevention of noncontact anterior cruciate ligament injuries in elite and adolescent female team handball athletes. Instr Course Lect. 2007;56:407 418. 54. Steffen K, Myklebust G, Olsen OE, Holme I, Bahr R. Preventing injuries in female youth football a cluster-randomized controlled trial. Scand J Med Sci Sports. 2008;18(5):605 614. 55. Alentorn-Geli E, Myer GD, Silvers HJ, et al. Prevention of non-contact anterior cruciate ligament injuries in soccer players. Part 2: a review of prevention programs aimed to modify risk factors and to reduce injury rates. Knee Surg Sports Traumatol Arthrosc. 2009;17(8):859 879. 56. Waldén M, Hägglund M, Ekstrand J. High risk of new knee injury in elite footballers with previous anterior cruciate ligament injury. Br J Sports Med. 2006;40(2):158 162. 57. Arnason A, Sigurdsson SB, Gudmundsson A, Holme I, Engebretsen L, Bahr R. Physical fitness, injuries, and team performance in soccer. Med Sci Sports Exerc. 2004;36(2):278 285. 58. Gabbe BJ, Bennell KL, Finch CF. Why are older Australian football players at greater risk of hamstring injury? J Sci Med Sport. 2006;9(4):327 333. 59. Orchard J, Marsden J, Lord S, Garlick D. Preseason hamstring muscle weakness associated with hamstring muscle injury in Australian footballers. Am J Sports Med. 1997;25(1):81 85. 60. Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer players after preseason strength training with eccentric overload. Scand J Med Sci Sports. 2003;13(4):244 250. 61. Junge A, Rösch D, Peterson L, Graf-Baumann T, Dvorak J. Prevention of soccer injuries: a prospective intervention study in youth amateur players. Am J Sports Med. 2002;30(5):652 659. 62. F-MARC. https://extranet.fifa.com/medical. Accessed January 30, 2010. 63. Holme E, Magnusson SP, Becher K, Bieler T, Aagaard P, Kjaer M. The effect of supervised rehabilitation on strength, postural sway, position sense and re-injury risk after acute ankle ligament sprain. Scand J Med Sci Sports. 1999;9(2):104 109. 64. Fredberg U, Bolvig L, Andersen NT. Prophylactic training in asymptomatic soccer players with ultrasonographic abnormalities in Achilles and patellar tendons: the Danish Super League Study. Am J Sports Med. 2008;36(3):451 460. 65. Scase E, Cook J, Makdissi M, Gabbe B, Shuck L. Teaching landing skills in elite junior Australian football: evaluation of an injury prevention strategy. Br J Sports Med. 2006;40(10):834 838. 66. Emery CA, Rose MS, McAllister JR, Meeuwisse WH. A prevention strategy to reduce the incidence of injury in high school basketball: a cluster randomized controlled trial. Clin J Sport Med. 2007;17(1):17 24. 67. Emery CA, Cassidy JD, Klassen TP, Rosychuk RJ, Rowe BH. Effectiveness of a home-based balance-training program in reducing sports-related injuries among healthy adolescents: a cluster randomized controlled trial. CMAJ. 2005;172(6):749 754. THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38 9

Clinical features Figure 2. The 11+. Donald T. Kirkendall and Jiri Dvorak 10 THE PHYSICIAN AND SPORTSMEDICINE ISSN 0091-3847, April 2010, No. 1, Volume 38

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