Anterior cruciate ligament injuries in soccer:Loading mechanisms, risk factors,and prevention programs

Boyi Di,Dewei Mo,Willim E.Grrett,Bing Yu

aDivision of Kinesiology and Health,University of Wyoming,Laramie,WY 82071,USA

bShandong Sports Science Research Center,Jinan 250102,China

cDuke Sports Medicine Center,Duke University,Durham,NC 27710,USA

dDivision of Physical Therapy,University of North Carolina at Chapel Hill,Chapel Hill,NC 27599,USA

Review

Anterior cruciate ligament injuries in soccer:Loading mechanisms, risk factors,and prevention programs

Boyi Daia,Dewei Maob,William E.Garrettc,Bing Yud,*

aDivision of Kinesiology and Health,University of Wyoming,Laramie,WY 82071,USA

bShandong Sports Science Research Center,Jinan 250102,China

cDuke Sports Medicine Center,Duke University,Durham,NC 27710,USA

dDivision of Physical Therapy,University of North Carolina at Chapel Hill,Chapel Hill,NC 27599,USA

Anterior cruciate ligament(ACL)injuries are common in soccer.Understanding ACL loading mechanisms and risk factors for ACL injury is critical for designing effective prevention programs.The purpose of this review is to summarize the relevant literature on ACL loading mechanisms,ACL injury risk factors,and current ACL injury prevention programs for soccer players.Literature has shown that tibial anterior translation due to shear force at the proximalend of tibia is the primary ACL loading mechanism.No evidence has been found show ing thatknee valgus moment is the primary ACL loading mechanism.ACL loading mechanisms are largely ignored in previous studies on risk factors for ACL injury.Identified risk factors have little connections to ACL loading mechanisms.The results of studies on ACL injury prevention programs for soccer players are inconsistent.Current ACL injury prevention programs for soccer players are clinically ineffective due to low compliance. Future studies are urgently needed to identify risk factors for ACL injury in soccer that are connected to ACL loading mechanisms and have cause-and-effect relationships w ith injury rate,and to develop new prevention programs to improve compliance.

ACL injuries;Biomechanics;Injury prevention;Soccer

1.Introduction

Soccer is the most popular sport in the world.1Playing soccer can improve musculoskeletal,metabolic,and cardiovascular functions.2However,soccer is one of the sports that have the highest risk of anterior cruciate ligament(ACL) injury.3,4The incidence rates of ACL injury in soccer range 0.15%—3.67%per person per year and 0.07—1.08 per 1000 sports exposures across various age and competition levels.5,6Female soccer players are 2—3 times more likely to suffer ACL injuries compared to male soccer players.5,7The majority of ACL injuries occur w ithout external contact to the knee joint.4,8—15

ACL injuries have brought financial burden to society,and caused devastating consequences to patients’quality of life. Based on an estimated 200,000 cases of ACL tears in US each year,annual costof ACL injury isestimated to be US$4 billion for surgical treatmentalone.16The lifetime financialburden of these injuries to society is estimated to be US$7.6 billion annually when treated w ith ACL reconstruction and US$17.7 billion when treated with rehabilitation.17Even w ith ACL reconstructions,individuals after reconstructed ACLs usually have abnormal strength,proprioception,balance,and neuromuscular control patterns18as well as increased risks for reinjury.19—21Many of these individuals are notable to return to theirpre-injury levelofactivity.22Fifty-nine percent to 70%of these individuals would develop radiographically diagnosed knee osteoarthritis;16%—19%would have symptomatic knee osteoarthritis over their lifetime,and 13%—15%would need total knee arthroplasty.17Tremendous research and clinicalefforts have been made in the last 2 decades to prevent ACL injuries and improve the rehabilitation after ACL reconstruction surgeries,23—25however ACL injury rates have not been reduced.10,26,27

van Mechelen et al.28proposed a sequence for preventing sports injuries.In this sequence,prevention of sports injury should follow four steps:(1)descriptions of the extent of injuries,(2)understanding of injury mechanisms and identification of risk factors,(3)development of injury prevention strategies,and(4)evaluation injury prevention strategy.The problem of ACL injury has been well described,however,the injury mechanisms and risk factors for ACL injury are stillnot well understood and identified.Consequently current ACL injury prevention programs have lim itations that prevent them from being effective.Therefore,the purpose of this review is to summarize the relevant literature on ACL loading mechanisms,ACL injury risk factors,and current ACL injury prevention programs for soccer players.We hope this review w ill contribute to the developmentof future ACL injury prevention programs in soccer as well as in other sports.

2.Loading mechanism s

The ACL is a primary restraint to anterior translation of the tibia relative to the femur.29In vitrostudies demonstrated that an anterior shear force applied on the tibia was the primary ACL loading mechanism.30,31The magnitude of anteriorshear force applied on the tibia and its effect on ACL loading are largely affected by the posterior ground reaction force and knee flexion angle during a movement.The posterior ground reaction force on the footduring a movement creates a flexion moment at the knee that needs to be balanced by an extension moment at the knee.The quadriceps is the primary generator of knee extension moments.While generating knee moments, the quadriceps applies an anterior shear force at the proximal end of the tibia that is a primary cause of anterior tibial translation and ACL loading mechanism.30,31A previous study demonstrated that the peak impact posterior ground reaction force was significantly correlated to the peak impact knee extension moment and proximal tibial anterior shear force during the landing of a stop-jump task.32

Knee flexion angle affects ACL loading through its relationships w ith patella tendon-tibia shaft angle and ACL elevation angle.33—35Studies showed that ACL loading decreased when knee flexion angles increased.30,36Taylor etal.37recently quantifiedin vivoACL length during a landing task using a combined fluoroscopic,magnetic resonance imaging(MRI),and videographic technique.They found that knee flexion angle and ACL length were negatively correlated, and that the peak ACL length actually occurred prior to landing when the knee flexion angle was m inimal.Taylor et al.38also found that knee flexion angles explained 61%of the variance in ACL length,and that peak ACL length occurred in m id-stance during walking when the knee was close to full extension.Using the same technique,Brown et al.39found that landing with an increased initial knee flexion angle decreased peak ACL length during both prelanding and landing phases of a drop vertical jump task. Kim et al.40recently estimated knee kinematics at the time of ACL injury for eight patients follow ing ACL injuries through reconstruction of the relative positions of the femur and tibia at the time of ACL injury by maximizing the contactof bone bruise areas between the femurand tibia in MRI.Their results showed a mean tibial anterior translation of 22 mm,a mean knee flexion angle of 12°,and a mean knee valgus angle of 5°at the time of ACL injury.These findings clearly demonstrate thatanterior translation of the tibia relative to the femur is the primary mechanism of ACL injury,and that a small knee flexion angle is responsible for an increased anterior shear force at the knee and thus anterior translation of the tibia relative to the femur.

The ACL can also be loaded by a compressive force along the longitudinal axis of the tibia through a posterior tilted tibial plateau.41,42W ith the presence of a posterior tibial plateau slope,a compressive force can generate an anterior shear force to cause the tibia to translate anteriorly and load the ACL.41,42Anin vitrostudy showed thatanterior translation of the tibia relative to the femur increased when the posterior titled tibial plateau slope increased from 8.8°to 13.2°under a 200 N compressive force loading.43Anin vivostudy showed that female patients w ith ACL injuries had significantly greater posterior tibia plateau slopes than the uninjured individuals.44These results provided a plausible explanation of the mechanism of ACL injury occurring in vertical landing tasks in which the external forces on the lower extrem ity are mainly in the vertical direction.

Knee“valgus collapse”was repeatedly proposed to be the major ACL injury mechanism especially in women based on the observation of ACL injury video records.9,45Quatman and Hewett45proposed sex-specific mechanism of ACL injuries. The investigators indicated that a primarily“sagittal plane”ACL injury mechanism m ightbe correct for male athletes,but female athletes sustained ACL injuries by a predominantly“valgus collapse”mechanism.However,evidences from quantitative studies did not support“valgus collapse”as the injury mechanism for either males or females.In vitrostudies demonstrated that,although knee valgus,varus,and internal rotation moments affected ACL loading,their effects were significantonly when an anteriorshear force was presentat the knee.30,31A recentin vivostudy demonstrated that the knee valgus collapse did not increase ACL length when the knee was in a flexion position.46A lso,studies demonstrated that medial collateral ligament was the primary structure resisting knee valgus moment in an intact knee,and thata pure valgus moment could not rupture ACL until the medial collateral ligamentwas completely ruptured.47—49Only 6%patients who had ACL injuries completely ruptured their medial collateral ligaments.50Further,anin vitrostudy found that the knee valgus motion significantly increased only after the ACL had been injured,42which indicated that the increased knee valgus motion observed in injury video records was likely a consequence instead of a cause of ACL injuries.

Current literature suggests that anterior translation of the tibia relative to the femur is the primary mechanism of ACLloading.Increased anterior shear forces at the knee due to a small knee flexion angle and increased compression forces on a posteriorly tilted tibialplateau are primary causes of anterior translation of the tibia relative to the femur.Although knee valgus/varus and internal rotation moments affect ACL loading when combined w ith significant anterior shear forces at the knee,current literature does notsupport them as primary ACL loading mechanisms relevant to ACL injuries.Future studies are encouraged to employ non-invasive methods37to quantifyin vivoACL loading during athletics tasks to determ ine ACL loading mechanisms under the loading that is close to cause ACL injuries.

3.Risk factors

Most ACL injuries occur during athletic tasks w ithout external contact to the knee joint.4,8—15The non-contactnature suggests that these injuries are likely caused by abnormal movementpatterns which m ight be modified through training. Understanding the risky movement patterns for non-contact ACL injuries can provide valuable information for developing training strategies.Significantefforts have been made to identify risk factors fornon-contactACL injury using a variety of methods in the last 2 decades.One method to identify movement characteristics in injury events is through the analysis of video records of ACL injury cases.Cochrane et al.12analyzed video records of 34 ACL injury cases in Australian football.They found that most of the injuries occurred during sidestepping or landing tasks when the knee flexion angle was less than 30°,and that 47%of the noncontact injuries had increased knee valgus motion and 42% had increased internal tibial rotation.

Krosshaug etal.9analyzed video records of 39 ACL injury cases in basketball.They estimated the time of injury being 17—50 ms after initial foot contact w ith the ground.Both males and females demonstrated small knee flexion angles at initial foot contact w ith the ground(＜15°)and 50 ms after (＜28°).This study also found that females had greater knee flexion angles than males did,and that females were more likely to have a valgus collapse than males did.Boden etal.14analyzed video records at a side view of 12 ACL injury cases and video records approximately at a front view of 17 ACL injury cases.They found that injured individuals had an increased rate of landing w ith flatfoot or rearfoot,increased knee abduction,and increased hip flexion compared to noninjured controls in sim ilar video records.Sheehan et al.13analyzed video records of 20 ACL injury cases occurred in single-legged landing tasks and 20 non-injured control cases. They found that the distances from center of mass to base of support and the angles between thigh and vertical axis were increased and that the angles between trunk and vertical axis were decreased in ACL injury cases compared to non-injured control cases.These studies were generally qualitative in nature.The video images used in these studies were not recorded forquantitative movementanalyses w ith little controlof image quality and no calibration was performed.Joint angles estimated from these two-dimensional(2D)video records were projections of angles between segment longitudinal axes on the view plane,which contained significant errors51—53and made the validity of the results questionable.

Recognizing the significant lim itations in 2D video analysis of injury events,Krosshaug and Bahr54developed a modelbased manual image-matching technique in attempt to reconstruct three-dimensional(3D)movements in injury events from single or multiple uncalibrated cameras.Koga et al.55analyzed movement characteristics of 10 ACL injury cases in female team handballand basketballusing the modelbased manual image-matching technique.They estimated that injuries occurred about40 ms after initial footcontactw ith the ground.Knee flexion and knee valgus increased during the fi rst 40 ms after the initial foot contact w ith the ground,and that the knee was externally rotated at initial foot contact w ith the ground,and internally rotated during the fi rst40 ms after the initial foot contact.The investigators concluded that the valgus motion coupled w ith internal tibial rotation under low knee flexion appeared to be important risk factors for ACL injury.55However the measurementerrors of the model-based manual image-matching technique were up to 11°in knee flexion angle,13°in knee internal/external rotation angle,and 5°in knee varus/valgus angle.54These significant measurement errors m inim ized the validity of this study.

Anothermethod to identify risk factors for ACL injury is to determ ine associations of injury risk factors w ith pre-injury movement characteristics through prospective cohort studies. In a prospective cohort study,56205 adolescent soccer, basketball,and volleyball players were screened for lower extrem ity biomechanics in a drop landing task,and subsequently followed for 13 months.Nine ACL injuries(seven in soccer and two in basketball)occurred.Compared to the noninjured players,the injured-players had increased knee abduction angles at initial contact,maximum knee abduction angles,maximum external knee abduction moments,peak vertical ground reaction forces,maximum external hip flexion moments,and side-to-side knee abduction momentdifferences during landing,and decreased maximum knee flexion angles and stance time.Statisticalanalysis demonstrated that the knee abduction moment was the most sensitive factor to predict ACL injury w ith 75%specificity and 78%sensitivity.This was the fi rstprospective cohortstudy in an attempt to screen jumplanding mechanics to identify biomechanical risk factors for ACL injury.However a small number of injuries,the late occurrence of the maximum knee valgus moment during the stance phase,a lack of horizontal deceleration in the testing task,and a lack of consideration of ACL loading mechanisms were identified as lim itations of this study.23Also a lack of cause-and-effect relationship between identified risk factors and the injury risk is another significant lim itation of this type of prospective cohort study.

Another study to prospectively identify risk factors for ACL injury was performed at three US military academies for 5 years.57A total of 6124 cadets were screened for lower extrem ity biomechanics in a simulated stop-jump task.Ninetyeight cadets had ACL injuries after the screening.Compared to the non-injured cadets,the injured cadets had increasedknee abduction at initial contact,increased maximum hip flexion,increased hip adduction at initial contact,and increased external hip rotation at initial contact during landing.This study found that male cadets had a sim ilar ACL injury rate as female cadets.The discrepancy in gender bias in ACL injury rates between the cadet population and other athletic populations indicates that the risk factors identified in the cadetpopulation may notbe generalizable to other athletic populations.A large sample size was obtained in this study, however sim ilar to the previous study,56a lack of consideration of ACL loading mechanisms and a lack of cause-andeffect relationship between identified risk factors and injury risk are significant lim itations of this study.

Sm ith et al.58conducted a large-scale prospective study to identify biomechanical risk factors.They used a semiquantitative method called Landing Error Scoring System (LESS)59as a lower extrem ity movement evaluation tool.A total of 2021 male and 1855 female college and high school athletes from various sports were screened for lower extrem ity movement patterns in a jump-landing-jump task and subsequently followed for 3 years.The LESS scores were compared between 28 ACL injured athletes and 64 matched controls.No significant difference in LESS score was found between the injured and non-injured groups.There are at least two possible explanations for the findings of this study:(1)the LESS could not differentiate lower extrem ity movement patterns between injured and non-injured groups,or(2)the movement patterns the LESS screened were not risk factors for ACL injury.

Goetschius et al.60predicted the probability of high knee abduction moments for the female athletes in the study by Sm ith et al.58The knee abduction moments were estimated from 2D knee valgus motion,knee flexion range of motion, body mass,tibia length,and quadriceps-to-hamstring strength ratio.61No significantdifference was observed in the predicted probabilities between 20 injured athletes and 45 controls.The results suggested thatmaximum knee abduction moment was nota risk factor for ACL injury in this population.

Risk factors for non-contact ACL injury are still largely unknown despite significant research efforts in last 2 decades. The identified risk factors were inconsistent among studies, and lacked connections with ACL loading mechanisms and cause-and-effect relationships w ith the risk of the injury.25,62These lim itations in the current literature on the risk factors for ACL injury are due to the inherent lim itations of the research methods.In the future,3D motion analysis methods thatcan be applied to accurately quantify motion during injury events are needed.A conclusion regarding injury mechanisms can only be drawn when the analysis are reliable.Studies w ith a better research design and consideration of ACL loading mechanisms are needed to identify ACL injury risk factors.

4.Prevention program s

Although the risk factors for ACL injuries are stillunclear, many injury prevention programs have been developed for soccer players as wellas athletes in other sports.Many studies have been conducted to evaluate the effectiveness of these prevention programs.These training programs can be categorized as balance training,plyometric training,long-duration neuromuscular training,or short-duration warm-up programs.

Caraffa et al.63investigated the effects of balance training on ACL injury rates in male soccer players.The prevention program included 20-m in five phases progressive balance training w ith different balance boards.The training was performed every day during pre-season and three times a week during the season for a total of three seasons.A total of 10 ACL injuries occurred to the 300 players in the intervention group,while a total of 70 ACL injuries occurred to the 300 players in the control group.The difference in ACL injury incidence between groups was statistically significant.However how the participants were assigned to the intervention or control group and how the proprioceptive training reduced ACL injury incidence were notclear.So¨derman etal.64studied the effects of balance board training on ACL injury rates in female soccer players.A total of 121 players in seven teams were random ized assigned to a training group and 100 players in six teams to a control group.The participants were instructed to perform a 10—15-min balance training on a balance board every day for 30 days and then three times a week for the restof the season.W ith a 37%drop-out rate,four ACL injuries occurred among 62 players in the intervention group,while one ACL injury occurred among 78 players in the control group during the season.Balance board training could notprevent ACL injury for female soccer players at the given level,which is contradictory to the previous study.63

Pfeiffer et al.65studied the effects of a plyometric training program on ACL injuries in high-school female soccer, basketball,and volleyball players.A totalof 577 players were included in the training group and 862 players were included in the controlgroup based on their w illingness to participate in the training program.The 20-m in training program consisted of exercises of jump landing techniques w ith a focus on a properalignmentof the hip,knee,and ankle.The training was performed tw ice a week throughout the 9-week season.The difference in the incidence ofnon-contactACL injury between training and control groups after training was not statistically different.Heidt et al.66studied the effects of preseason conditioning on ACL injury rate in high school female soccer players.A totalof300 players were recruited while 42 of them were random ly selected to a conditioning group and rest as control group.The conditioning included two treadm ill sessions and one plyometric session per week over a 7-week period w ith an aim to improve cardiovascular conditioning, plyometric work,agility,strength,and flexibility.One year after the conditioning,one ACL injury occurred in the conditioning group,while eight ACL injuries occurred in the control group.The difference in injury rates between groups was notstatistically significant.These two studies suggest that plyometric training alone is not likely to reduce ACL injuries.

Hewett et al.67investigated the effects of comprehensive neuromuscular training on non-contact ACL injury rates in high school soccer,volleyball,and basketball players.A total of 366 female athletes were included in the training group and463 female athletes were included in the control group based on their w illingness to participate in the program.An addition of 434 boys was included as another control group.The prevention program lasted 60—90 m in and included multiple components(jumping/plyometric,flexibility,and strengthening).The training was performed 3 days a week for6 weeks during preseason.After one season,no non-contact ACL injury occurred to the trained female athletes,while one noncontact ACL injury occurred to the untrained male athletes and five non-contact ACL injuries occurred to untrained female athletes.The investigators concluded that the training program significantly reduced the ACL injury rate.However, the results and conclusions of this study apparently depend on the statisticalmethods used for data analysis.68,69Besides,the need of significant extra time for this type of long-duration neuromuscular training might create obstacles in application.

Warm-up programs for ACL injury prevention have received great interests recently because of its short training duration and capability of being incorporated into regular training.Mandelbaum etal.70studied the effects of a warm-up program on ACL injury rates in female soccer players 14—18 years of age.Participants were assigned to a training or a control group based on their choices.The 20-m in program included running,stretching,strengthening,plyometric,and agility exercises.The ACL injury incidence was 0.05/athlete/ 1000 exposures in the intervention group compared to 0.47/ athlete/1000 exposures in the controlgroup in the fi rstyear of the study.The incidence was 0.13 injuries/athlete/1000 exposures in the intervention group compared to 0.51 injuries/ athlete/1000 exposures in the controlgroup in the second year of the study.The differences in ACL injury incidences between the intervention and control groups were statistically significant in both years.The investigators concluded that the ACL injury incidence in the intervention group was significantly reduced.This study was cross sectional in nature w ithout random group assignment.A lso the ACL injury incidences of the intervention and control groups before the experiment were unknown.Considering these significant lim itations,the results of this study only demonstrated a difference in ACL injury incidence between groups thatcould not be interpreted as a decrease in the incidence w ithin group due to the intervention.

Gilchristetal.71conducted a random ized controlled trial to study the effects of a previously mentioned warm-up program70on ACL injury rates in female collegiate soccer players.Thirty-eight teams were random ized to an intervention group and 37 teams were random ized to a control group. Twelve intervention teams and two control teams dropped out of the study.A total of 583 players in the intervention group and 852 players in the control group completed the study. A fter one season of intervention,the overall ACL injury rate was 0.20/1000 exposures in the training group compared to 0.34/1000 exposures in the control group w ithout statistical significance.Non-contact ACL injury rate was 0.06/1000 exposures in the training group compared to 0.19/1000 exposures in control group w ithout statistical significance.However,the training group had a significantly lower ACL injury rate during practice and significantly lower ACL injury rate for athletes w ith a history of ACL injury compared to the control group.The authors concluded that the warm-up program decreased ACL injury rate,especially for those w ith a history of ACL injury.A more than 30%drop-out rate in the intervention group was a limitation of this study.

Ha¨gglund et al.72conducted a random ized controlled trial to investigate the effects of a 15-m in neuromuscular warm-up program on ACL injury rate in adolescent female soccer players.The training group had 2471 players and the control group had 2085 players.The program included six exercises focused on knee alignment and core stability which was performed tw ice per week.After a competitive season,the training group demonstrated a significantly lower ACL injury rate compared to the control group.

Steffen et al.73conducted a cluster-randomized controlled study to investigate the effects of a 15-m in warm-up program on ACL injury rates in female youth players under the age of 17 years.The intervention group included 1073 players and the control group included 947 players.The intervention program included 10 exercises designed to improve core stability,balance,dynam ic stabilization,and eccentric hamstring strength.The training was performed for 15 consecutive sessions and then once a week for the restof an 8-month season. No statistically significant difference was found in the overall injury rate and ACL injury rate between the intervention and control groups.A low compliance rate was indicated as only 14 out of 58 training teams completed more than 20 training sessions.

The effects of ACL injury prevention programs on ACL injury rates in soccer are inconsistent.A lthough several studies reported that prevention programs reduced ACL injury rate among soccer players,significant lim itations in research design restricted the interpretations of their results. A common lim itation in these studies was the crosssectional research design w ithout pre-intervention injury incidence measures.In this situation,the results of those studies only showed that the intervention groups had significantly lower injury rate compared to control groups, and cannot be interpreted as the decrease in injury rate in intervention groups due to training.Therefore the effects of those training programs on the injury rate are essentially unknown.Another significant lim itation in current literature on ACL injury prevention programs was that the mechanism of injury prevention of those intervention programs was not clear.A lthough each intervention program had a focus of training,which risk factors the intervention program modifi ed were largely unknown,or the connections of the risk factors w ith the ACL loading mechanisms and injury rates were unclear.This lim itation is largely due to the lack of understanding of risk factors for ACL injury.Considering this lim itation,the inconsistent results of studies on ACL injury prevention program s should not be a surprise.Future intervention studies are encouraged to evaluate preintervention injury incidence as well as to measure ACL injury risk factors prior and after training to overcome these lim itations.

The clinical ineffectiveness of current ACL injury prevention programs could be attributed to low compliance.74Review of current ACL injury prevention programs showed that training programs in ACL injury prevention programs typically need 15—90 min,24,75which may be an explanation of the low compliance to ACL injury prevention programs in clinical applications as well as in studies.Efforts have been made in several recent studies to design new ACL injury prevention programs w ith m inimal additional training time to improve the compliance to prevention programs.76—78Future studies are needed to evaluate the training effects of these programs on ACL injury rates.

5.Conclusion

ACL injury is common in soccer,and has significant impact to the quality of life of injured individuals and significantly increases financial burden to society.Understanding ACL loading mechanisms and risk factors for the injury is critical for designing effective prevention programs. Recent studies provided convincing evidence that tibial anterior translation due to shear force at the proximal end of tibia is the primary ACL loading mechanism.Great posterior ground reaction forces on the lower extrem ity and small knee flexion angles are major contributors to the increased shear forces at the proximal end of the tibia and thus tibia anterior translation.No evidence has been found show ing that knee valgus moment is a primary ACL loading mechanism.The observed knee valgus motion in ACL injury cases are likely a post-injury event.The results of studies on ACL loading mechanisms are largely ignored in studies on risk factors for ACL injury.Many identified risk factors have little connections to ACL loading mechanisms.The results of studies on ACL injury prevention programs for soccer players are inconsistent.Current ACL injury prevention programs for soccer players are clinically ineffective due to low compliance.Future studies are urgently needed to identify risk factors for ACL injury in soccer that are connected to ACL loading mechanisms and have cause-and-effect relationships w ith injury rate,and to develop new prevention programs to improve compliance.

Acknow ledgment

This study was partially supported by Shandong Province Research Development(No.2012G0030039)and China Sports Administration Research(No.2012B012).

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Received 19 April 2014;revised 3 June 2014;accepted 7 June 2014 Available online 30 July 2014

*Corresponding author.

E-mail address:bing_yu@med.unc.edu(B.Yu)

Peer review under responsibility of Shanghai University of Sport.

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