Participants Risk factor Association

(1)

LITHUANIAN UNIVERSITY OF HEALTH SCIENCES

MEDICAL ACADEMY FACULTY OF NURSING DEPARTMENT OF SPORTS MEDICINE

SAMER YOUNES

RISK FACTORS FOR ANTERIOR CRUCIATE LIGAMENT INJURIES IN FOOTBALL PLAYERS

A SYSTEMATIC LITERATURE REVIEW

Master´s Thesis

Supervisor:

Assoc. prof. Dr Alma Kajėnienė

(2)

TABLE OF CONTENTS

1. SUMMARY………...3

2. CONFLICT OF INTEREST………..5

3. ABBREVIATIONS LIST……….……….6

4. INTRODUCTION……….7

5. RESEARCH METHODOLOGY AND METHODS………9

5.1 Study eligibility………...9

5.2 Literature search strategy………11

5.3 Quality of reviewed articles………17

6. RESULTS AND THEIR DISCUSSION………..19

6.1 Quality and characteristics of the included studies……….19

6.2 Risk factors……….21

6.3 Gender………27

6.4 Age……….31

6.5 Professional and non-professional athletes……….31

6.6 Assessment of Quality………33

7. CONCLUSIONS………..36

8. PRACTICAL RECOMMENDATIONS………..37

9. LITERATURE LIST………38

(3)

1. SUMMARY

Master´s thesis by Samer Younes. This is a systematic literature review,

¨Risk factors for anterior cruciate ligament injuries in football players: Systematic literature review¨.

Aim:

To analyze the risk factors of sustaining ACL injuries in football players in scientific literature of last five years (2013-2018)

Objectives: 1. To address risk factors of sustaining ACL injury in football players; 2. To compare risk factors of sustaining an ACL injury in football players between gender; 3. To compare the risk factors of sustaining ACL injury in football players between age; 4. To compare the risk factors sustaining an ACL injury in football players between professional and non-professional athletes

Methodology:

Systematic literature review was performed including English articles from 2013 through 2018.

Targeted data bases; PubMed, Br J Sport Med, Am J Sports Med.

Keywords: ACL injury, football players, risk factors, age, gender, professional, non- professional. Articles eligibility was assessed according PICO criteria:

Population: Articles investigating anterior cruciate ligament injuries in football players competing at professional and non professional level, articles comparing gender and articles comparing ages. Intervention: Both observational studies and studies investigating biomechanics, kinematics and kinetics Comparison: With non-injured football players.

Outcome: Strength of the association between a risk factor and anterior cruciate ligament

injury. Strength of association between level of professionalism, gender and age. Articles not older than 5 years old. Articles in English.

Results:

The targeted databases delivered a total number of 1837 articles. After exclusion and full eligibility assessment, 25 articles were included and reviewed. 9 of these articles achieved good quality rating while 16 achieved moderate rating.

Conclusions:

1. The numerous associated risk factors for ACL injury in football players proves that it indeed is multifactorial in its origin. Most commonly mentioned risk factors were;

(4)

and level of play.

2. Females are at increased risk of sustaining ACL injury due to a more extended knee joint, greater knee valgus, greater hip adduction and les postural dynamic stability 3. Younger youth players are at higher risk due to a more compliant musculoskeletal

system. This is however somewhat inconclusive.

4. Higher level of play generally has higher risk of injury. Non-professionals are at risk due to poor neuromuscular control.

(5)

2. CONFLICT OF INTEREST

The author does not declare any conflict of interest.

(6)

3. ABBREVIATIONS LIST

ACL = Anterior cruciate ligament ADF = Ankle dorsiflexion flexibility

AJSM = American Journal of Sports Medicine ATT = Anterior tibial translation

BJSM = British Journal of Sports Medicine EMD = Electromechanical delay

HSS journal = Musculoskeletal Journal of Hospital for Special Surgery LED = Lower extremity dexterity

MCL = Medial collateral ligament MRI = Magnetic resonance imaging

NATION = National Athletic Treatment, Injury and Outcomes Network NCAA = National Collegiate Athletic Association

PCL = Posterior cruciate ligament

PRISMA = Preferred Reporting Items for Systematic Reviews and Meta-Analyses ROM = Range of motion

TMG = Tensiomyography

(7)

4. INTRODUCTION

Football is the biggest sport in the world with over 265 million practitioners all over the globe according to the latest Fédération Internationale de Football (FIFA) Big Count 2006. It is fast growing, especially among females and female youth players with a rise of 19% and 32%

respectively compared to the last FIFA Big Count in 2000 [1]. In 2014 new data was presented by FIFA showing further rise in number of female players and it is predicted to continue its rise in numbers after the implementation of a FIFA’s Women’s Development Program focusing on encouragement, development and increased opportunities for female players, and especially young females [2].

The cruciate ligament tear or rupture is a common and potentially career-ending injury for football players associated with a number of proven and hypothesized risk factors. ACL reconstruction surgery is one of the most commonly performed procedures [3]. Having in mind that on average, only 65% return to their pre-injury level and only 55% to competitive level along with studies reporting a secondary injury rate of players younger than 25 years of 23%

shows the magnitude of the problem [4-10]. Female elite football players have an average rehabilitation length of 8.5 months but can extend as long as 12 months depending on playing position. Even though males have a higher return to sport rate than females, it is nevertheless a major issue for males as well [11-12].

In the light of the stated facts that knee injuries are important and possibly devastating part of practicing football and ACL tears being one of the most common, threatening and is often associated with a range of other accompanied injuries to the knee joint, it is very important to address risk factors and to know the mechanism behind the injury to be able to prevent and properly treat.

Additionally, as stated before, football is rapidly increasing in popularity among females which makes it a very live issue with an increased need and demand for studies.

With this background and especially youths being invested in, it is more than fair to conduct this systematic review with focus on the specific risk factors; gender, age and comparing the level of athletics on the risk of sustaining ACL injury but also to find and address other potential risk factors for a better understanding and prevention.

The aim of this study was to analyze the risk factors of sustaining ACL injuries in football

(8)

The objectives of this research are:

1. To address risk factors of sustaining ACL injury in football players

2. To compare risk factors of sustaining an ACL injury in football players between gender 3. To compare the risk factors of sustaining ACL injury in football players between age 4. To compare the risk factors sustaining an ACL injury in football players between

professional and non-professional athletes

(9)

5. RESEARCH METHODOLOGY AND METHODS

5.1 Study eligibility

Searched databases PubMed

Searched string ”risk factors for ACL injuries in soccer”, ”ACL injury AND soccer AND age NOT return NOT prevention NOT reconstruction”, ”ACL injury AND soccer AND gender NOT return NOT prevention NOT reconstruction”, ”ACL injuries AND soccer NOT return to sport NOT prevention NOT reconstruction”

Time filter 5 years (from 2013-2018)

Language filter English

Target journals American Journal of Sports Medicine; British Journal of Sports Med Table 1. Inclusion criteria

P I C O

Population Intervention Comparison Outcome

Articles investigating anterior cruciate ligament injuries in football players competing at

professional and non professional level, articles comparing gender and articles comparing ages.

Both observational studies and studies investigating biomechanics,

kinematics and kinetics

With non-injured

football players, genders, different ages, level of play.

Strength of the

association between a risk factor and anterior cruciate ligament injury.

Strength of association between level of

professionalism, gender and age. Articles in English.

(10)

Table 2. Exclusion criteria

P I C O

Population Intervention Comparison Outcome

Articles investigating athletes or sports in general and for which data relative to football players only could not be extracted

Articles about impact/effectiveness of prevention

programs.

With other sport disciplines

Articles older than 5 years; risk factors for a second ACL injury; treatment for ACL injuries, return to sports after an ACL reconstruction, career length after an ACL injury; articles studying risk factors of knee injury not focusing on anterior cruciate ligament rupture or for which data specifically related to anterior cruciate ligament could not be extracted; articles focusing on treatment and management of anterior cruciate ligament in

football players from a clinical/surgical standpoint;

articles studying return to play after anterior cruciate ligament injury in football players.

Review articles. Articles not in English.

(11)

5.2 Literature search strategy

The literature search was performed by screening different electronic databases including PubMed, SAGE Journals - American Journal of Sports Medicine (AJSM) and British Journal of Sports Medicine (BJSM). The first search was performed on 4 June 2018 and the last search was performed 4 September 2018.

The search strategy was completed to some extent different in the selected electronic databases as the traditional word ”football” is referred to as ”soccer” in the USA. Accordingly, with regard taken to the fact that the AJSM and PubMed presents American studies, the word ”soccer” was applied when using these databases whereas the word ”football” was used in BJSM. The overall search strategy was conducted by using keywords including; ”risk factors for ACL injuries in soccer”, ”ACL injury AND soccer AND age NOT return NOT prevention NOT reconstruction”,

”ACL injury AND soccer AND gender NOT return NOT prevention NOT reconstruction”, ”ACL injuries AND soccer NOT return to sport NOT prevention NOT reconstruction”. The search strategy was conducted by using identical keywords in all targeted databases in order to identify whether the applied strategy resulted in the same range of articles.

(12)

Figure 1. PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) flow diagram of included and excluded studies.

Id en ti fi ca ti o n Scree n in g E li g ib il it y In cl u d ed

Articles identified through electronic database search

(n = 1837)

Additional articles identified through other sources

(n = 0)

Total articles (n = 1837)

Articles after duplicates removed

(n = 57)

Total articles included in the systematic review

(n = 25)

Articles excluded after abstracts examined (for reasons see Table 2 below)

(n = 631)

Duplicates removed (n = 16)

Full articles assessed for eligibility

(n = 57)

Articles excluded for reasons of not meeting the inclusion

criteria (n =32 ) Articles excluded after

assessment year and language criteria

(n = 1133)

(13)

Table 3. Search strategy for electronic database review.

PubMed American Journal of Sports Medicine

BJSM

Keywords searched: Keywords searched: Keywords searched:

”ACL injury AND soccer AND age NOT return NOT prevention NOT

reconstruction”

”ACL injury AND soccer AND age NOT return NOT prevention NOT

reconstruction”

”ACL injury AND football AND age NOT return NOT prevention NOT

reconstruction”

Total searches: (n = 47) Total searches: (n = 24) Total searches: (n = 395)

Included: (n = 10) Included: (n = 0) Included: (n = 0)

Excluded: (n = 37)

- Older then 5 years: (n = 27) - Not specifically focused on

risk factors in football: (n = 4)

- Not specifically focused on ACL injuries: (n = 3) - Not accessible: (n = 3)

Excluded: (n = 24)

- Older then 5 years: (n = 20) - ACL injuries and/or other

injuries in other sports: (n = 1)

- Not specifically focused on ACL injuries: (n = 3)

Excluded: (n = 395)

- Older then 5 years: (n = 245) - ACL injuries and/or other injuries in other sports: (n = 76)

”ACL injury AND soccer AND gender NOT return NOT prevention NOT

”ACL injury AND football AND gender NOT return NOT prevention NOT

(14)

BJSM

reconstruction” reconstruction” reconstruction”

Total searches: (n = 52) Total searches: (n = 10) Total searches: (n = 36)

Included: (n = 13) Included: (n = 0 ) Included: (n = 0)

Excluded: (n = 39)

- Older then 5 years: (n = 32) - Not specifically focused on

risk factors in football: (n = 2)

- Not specifically focused on ACL injuries: (n = 2) - Not accessible: (n = 2) - Not in English: (n = 1)

Excluded: (n = 10)

- Older then 5 years: (n = 10)

Excluded: (n = 36)

- Older then 5 years: (n = 14) - ACL injuries and/or other

injuries in other sports: (n = 9)

”ACL injuries AND soccer NOT return to sport NOT prevention NOT

reconstruction”

”ACL injuries AND soccer NOT return to sport NOT prevention NOT

reconstruction”

”ACL injuries AND football NOT return to sport NOT prevention NOT

reconstruction”

Total searches: (n = 336) Total searches: (n = 1 ) Total searches: (n = 373)

(15)

BJSM

Included: (n = 37)

Included: (n = 0) Included: (n = 0)

Excluded: (n = 299)

- Not specifically focused on risk factors in football: (n = 46)

- Not specifically focused on ACL injuries: (n = 15) - Not in English: (n = 1) - Not accessible: (n = 10)

Excluded: (n = 1)

- Older then 5 years: (n = 1)

Excluded: (n = 373)

Keywords searched:

”Risk factors for ACL injuries in soccer”

Total searches: (n = 563 )

Included: (n = 13)

(16)

BJSM

Excluded: (n = 550)

- Not specifically focused on risk factors in football: (n = 198)

(17)

5.3 Quality of reviewed articles

A quality assessment score was formed to score the included studies and measure their quality.

Each article was scored 0, 1 or 2 in 8 different parameters to achieve a maximum score of 16. This is a modified Altman DG et al study of measuring quality of randomized control trials.

The obtained quality score was converted into percent and rated poor (0-49%), moderate (50-89%) and good (>90%).

Quality assessment score: Maximum score (16, 100%)

1. What study design was used and, and how were participants allocated?

2 = mixed design, randomized and counterbalanced between conditions􀀁

1 = repeated measures/within subjects only OR mixed design but not randomized.

0 = between groups only

2. Was the assigned intervention concealed before allocation?

2 = adequate􀀁

1 = unclear􀀁

0 = inadequate/impossible􀀁

3. Were the outcome assessors blinded to treatment status?

2 = Effective action taken to blind assessors

1 = Small or moderate chance of unbinding of assessors 0 = Not mentioned or not possible

4. Were the inclusion and exclusion criteria clearly defined?

2 = clearly defined

1 = inadequately defined􀀁

0 = not defined􀀁

5. Were the intervention and control group comparable at entry?􀀁

2= good comparability of groups, or confounding adjusted for in analysis/within group design

1= confounding small; mentioned but not adjusted for􀀁

0= large potential for confounding, or not discussed􀀁

6. Were the interventions clearly defined?􀀁

2= clearly defined interventions were applied􀀁

1= clearly defined interventions were applied but the application was not

(18)

0= intervention and/or application were poorly or not defined􀀁

7. Were the outcome measures used clearly defined?􀀁

2 = clearly defined􀀁

1 = adequately defined/recorded􀀁

0 = not adequately defined/recorded􀀁

8. Was the follow up period sufficient to measure the effects of the intervention?

2 – active surveillance and appropriate duration 1 – active surveillance, but inadequate duration 0 – surveillance not active or not defined

(19)

6. RESULTS AND THEIR DISCUSSION

6.1 Quality and characteristics of the selected studies

Table 4. Characteristics of included studies

Reference Study design Quality of study*

Participants Risk factor Association

Sheu et al 2014 [35]

Controlled laboratory study

Moderate 20 females 20 males

Gender Associated

Sigward et al 2015 [21]

Cross-sectional study

Gender Associated

Hogg et al 2018 [36]

Cross-sectional study

Gender Associated

Waldén et al 2015 [13]

Systemic video analysis

Moderate 39 males -Gender -Poor

neuromuscular control

Associated Associated

Brophy et al 2015 [14]

Prospective cohort study

-Gender -Poor

Brophy et al 2014 [17]

Case series Moderate 23 females 32 males

-Gender -Poor

Khalid et al 2015 [16]

-Gender -Poor

Male association Associated

Lyle et al 2014 [20]

Good 14 females 15 males

-Gender -Poor

Partially associated Partially associated

(20)

Stanley et al 2016 [41]

Descriptive epidemiological study

Moderate 237 college females 86 college males

101 high school females

102 high school males

-Gender -Level of play

Associated No

association

Beynnon et al 2014 [42]

Cohort study Good Not reported (adjusted to person-day exposure)

-Gender -Level of play

Owusu- Akyaw et al 2018 [22]

Gender No

association

De Ste Croix 2018 [15]

Moderate 36 females 12- 17 years old

-Age -Poor

Hägglund et al 2016 [34]

Cohort study, randomized controlled

Good 4556 players aged 12-17 years

-Age

-Genetics

Not

statistically significant Associated Krutsch et

al 2016 [18]

Prospective controlled cohort study

Moderate 408 males Level of play Associated

Lee et al 2017 [43]

Controlled lab study

Moderate 15 high level players 15 low level players

Level of play Associated

Baumgart et al 2015 [23]

Randomized controlled trial

Good 31 in stretching group

31 in football group

Poor

Associated

(21)

Shultz et al 2015 [24]

Descriptive lab study

Poor

Associated

Potter et al 2014 [25]

Moderate 19 females Poor

Associated

Malloy et al 2015 [26]

Good 23 females Poor

Associated

Brown et al 2014 [27]

Good 16 females Leg

preference

Associated

Greska et al 2017 [28]

Good 20 females Leg

preference

No

association Alentorn-

Geli et al 2015 [30]

Good 40 ACL injured 38 control individuals

Poor

Associated

Alentorn- Geli et al 2015 [31]

Good 40 ACL injured males

38 control males

Poor

No

association

Kim et al 2014 [32]

Moderate 37 males Poor

Associated

Lulińska- Kuklik et al 2017 [33]

Moderate 134 males with ACL rupture 211 healthy males

Genetics Associated

* see assessment of quality score under section “5.3 Quality of reviewed articles”.

6.2 Risk factors

Risk factors for ACL injuries in football players were analyzed in 25 articles in total. Various

(22)

-Eleven articles investigated gender as a risk factor. Two of the articles achieved good quality rating and nine moderate rating. Number of participants ranged from 12 till 523 subjects. Eight of the articles concluded female gender as a risk factor while one found male gender to increase risk.

One article found no association between genders and one article could only find partial association.

-Two articles were reviewed on age. One achieved good quality score and one moderate rating.

The former had 4556 participants and found an association with lower age as a risk factor while the latter had 36 subject and found no association.

-Four studies were reviewed on level of play as a risk factor. One achieved good quality score and three moderate score. The number of subject ranged from 30 till 523. Two of the studies found that higher level increased the risk while one concluded that amateur players are at risk. One study found no correlation with level of play.

-Thirteen articles investigated poor neuromuscular control on the risk of ACL injury. Five of these were rated good score and eight achieved moderate rating. From 12 till 151 subjects participated in the studies. Eleven studies drew conclusions that poor neuromuscular control being a risk factor.

One study found partial association and one found no association.

-Two studies were included on genetic factors. One achieved good rating and one moderate. One had 4556 subjects while the other had 345 subjects in the study. Both found correlation with the investigated genetic risk factors.

-Leg preferance was examined in two studies (by the same author). Both with good quality ratings and 78 participants. One found it being a risk factor while the other found no correlation.

Waldén et al. [13] analyzed 39 videos of male professional football player’s ACL injuries between 2001 and 2011 to plot the mechanisms of the injuries. The author found common playing situations leading to dynamic knee valgus. The most commonly associated situations were pressing followed by re-gaining balance after kicking and landing after heading. The findings are coupled with bad postural and neuromuscular control.

Similar correlations were established by Brophy et al. [14] when analyzing fifty-five videos of professional and collegiate male and female soccer players. The authors found that soccer players were more prone to injure their ACL while the opponents having the ball and defending. The most common way was when tackling the opponent which can be explained by it being a reactive maneuver needing last second body adjustments in body position and technique and with females generally having poorer neuromuscular control with suboptimal biomechanics, females were put at greater risk.

(23)

Other authors have investigated into the risk associated with poor neuromuscular control. De Ste Croix et al. [15] evaluated the risk for ACL injuries in elite female youth soccer players by measuring neuromuscular control of knee pre- and post fatigue. It is thought that knee joint stability is effected by fatigue and measurements of the electromechanical delay (EMD) in gastrocnemius, semitendinosus and biceps femoris in youth girls were conducted. After performing eccentric knee flexion tasks the EMD measured by electromyography pre- and post fatigue concluded that the EMD was significantly longer post-fatigue putting the player at higher risk of injury. The football specific fatigue compromise the neuromuscular control needed to stabilize the joint.

Another article adds to the effect of neuromuscular fatigue and control. A.J. Khalid et al. [16]

investigated the effect of neuromuscular fatigue on neuromuscular control. The results showed that there were significantly higher knee joint loads when fatigued, significantly higher peak proximal tibia anterior/posterior shear force, vertical ground reaction force at initial contact and peak internal rotation moments, especially among men. The conclusion suggested that males are therefor at overall higher risk of injury.

Brophy et al. [17] investigated why young female athletes are at higher risk of non-contact ACL injuries than their male counterpart by comparing proprioception and core stability via dynamic postural stability testing. The results showed that males lasted longer on the platform (98 ± 14 s) than females (94 ± 13 s) (p = 0.04). Coronal plane and rotation stability differed significantly between genders (323 ± 126 vs. 365 ± 128, p = 0.04) and (318 ± 82 vs. 403 ± 153, p = 0.0002), respectively and concludes that less postural dynamic stability in the coronal plane and in rotational motions may be a contributing factor to ACL injuries.

Krutsch et al. [18] conducted a study during the introduction of a new professional soccer league in Germany to analyze the influence of the increased intensity on acute knee injuries. Comparing the players from the new professional league with players from amateur level, players in the professional league had a significantly higher (p = 0.04) incidence of ACL and PCL injuries, than players at the amateur level. More than 90 % of all ACL and PCL ruptures in both groups were sustained by players, who had played at least one level lower in the previous season. This proved the assumption that rapid increase in training and playing intensity is a risk factor for ACL injury.

These findings are in line with previous studies conducted and the author refers to the introduction of US Major League Soccer [19].

M.A. Lyle et al. [20] explored the dynamic control of interaction between lower limb and ground

(24)

by examining lower extremity dexterity (LED). It was tested by comparing LED-test performance and single limb-drop jump in 14 females and 14 males high school soccer players. The results of the experiment showed that females exhibited reduced LED-test performance (p =0.001) and higher limb stiffness during landing (p =0.008). Females also exhibited higher co-activation at the ankle (p =0.001) and knee (p =0.02) before landing raising the possibility that the higher leg stiffness observed in females during landing is an anticipatory behavior due in part to reduced lower extremity dexterity. The findings suggest that reduced lower extremity dexterity and compensatory stiffening may increase the risk of ACL injuries.

S.M. Sigward et al. [21] compared the frontal plane knee moments along with kinematics and kinetics during knee valgus moments at 45° and 110° on 25 male and 20 female soccer athletes in the ages of 16-23. The data was obtained through a force platform and cameras. The author then compared if there were any significant difference between the genders and found that females showed greater knee valgus moments than males. Excessive knee valgus is a previously studied and recognized mechanism behind many ACL tears which puts women at higher risk in accordance with this study which also suggest that the larger cutting angles and redirection demands put the football player at higher risk.

K.A. Owusu-Akyaw et al. [22] compared T-2 weighted Magnetic Resonance Imaging (MRI) scans for 15 females and 15 males who had sustained an ACL injury to predict the bone positions at time of injury. This was done by by creating three-dimensional models of the MRI scan, recreating the bone bruising overlap and measure knee flexion, knee valgus, internal tibial rotation and anterior tibial translation. However, there was no statistically significant differences between male and female patients in the position of injury with regard to knee flexion (P = 0.66), valgus (P = 0.87), internal tibial rotation (P = 0.26), or anterior tibial translation (P = 0.18).

Baumgart et al. [23] investigated in a randomized controlled trial the effect of playing soccer and static stretching on anterior tibial translation (ATT) of the knee joint. Thirty-one athletes were randomly assigned stretching exercises or put into a control group while thirty-one amateur soccer players completed a 90-minute soccer-specific training program. ATT was measured before and after intervention with a KT-100 knee arthrometer at forces 67 N, 89 N and maximal manual force.

There were no baseline differences between the groups. The outcome of the study revealed significant increase in ATT at all applied forces for both soccer playing and static stretching.

Maximal manual testing displayed a mean increase of ATT after static stretching of 2.1 ± 1.6 mm (P <0.0005) and after playing soccer of 1.0 ± 1.5 mm (P = 0.001). The increase in ATT influence

(25)

the mechanical factors of the ACL and the greater increase during static stretching may put the player at higher risk of sustaining such injury.

The issue of the increased knee laxity contributing to increased risk was also explored by Shultz et al. [24] Who investigated whether the increased knee laxity causes higher-risk landing biomechanics during prolonged fatigue exercise. 30 male and 29 female athletes participated in a 90 minutes intermittent exercise protocol designed to simulate the physiological and biomechanical demands of a football game. The study found that there was greater knee laxity during prolonged, intermittent exercise causing higher-risk landing biomechanics. The most pronounced changes and risk were in athletes with initially greater knee laxity.

In line with above findings Potter et al. [25] further explored how prolonged activity increase the ACL injury risk. The article examined hip and knee kinematics during cutting maneuvers in timed agility tests. 19 female college football players performed the test until they were not able to meet the set performance time and with the help of 3D coordinate data the investigators observed the recorded knee and hip angles. During the the sidestep and crossover cuts performed the investigators observed that prolonged activity predisposed the athlete to position their knee in a more extended and abducted posture. This position has been suggested to put more strain on the ACL and therefor increase the risk of such injury.

Malloy et al. [26] conducted a study to examine if ankle dorsiflexion flexibility (ADF) was associated with erroneous landing kinematics and kinetics increasing risk for ACL injury in female football players. 23 players participated in a pre-season screening assessment of ADF during drop vertical jump tasks. The results showed that females with less ADF displayed greater peak knee abduction moments (r=-0.442), greater peak knee abduction angles (r=0.355), and less peak knee flexion angles (r=-0.385) during the landing tasks concluding that ADF may indeed serve as a useful predictor of poor landing posture making it a risk factor for ACL injuries.

Brown et al. [27] examined the relationship between leg preference and knee mechanics during sidestepping. The study included 16 female college football players who where recorded performing a planned 45 degree sidestep maneuver with their preferred and non-preferred kicking leg. Analyzing the knee kinematics and kinetics of the performed maneuvers showed that there are differences between the preferred and the non-preferred leg in females, although they are small.

The author writes in his conclusions that the non-preferred leg experience increased knee power absorption, knee flexion velocity, abduction angle and internal rotation angle during weight acceptance of cutting maneuvers which may expose the ACL to increased risk of injury.

However, Greska et al. [28] performed a study evaluating how lower extremity neuromechanicals

(26)

was effected by leg dominance during unanticipated side-step cutting maneuvers in female college football players. 20 participants in the study performed side-step cutting tasks with their dominant and non-dominant legs and kinematic and kinetic data were once again collected to see joint angles and forces and muscle activity. The authors did not find any significant difference between the dominant and the non-dominant legs during the side-step cutting maneuvers suggesting that leg dominance does not increase the risk for ACL injury, overall showing conflicting results.

Conflicting results in these type of studies is not uncommon. Previous studies, such as, Brophy et al. [29] found that in non-contact ACL injuries, which is the most common mechanism, females were more likely to injure their supporting leg whiles males were more likely to injure their kicking leg.

Alentorn-Geli et al. [30] made a study investigating if the mechanical and contractile properties of skeletal muscles of the thigh could possibly be a risk factor for ACL injury. This was done by assessing the thigh muscles (vastus medialis, vastus lateralis, rectus femoris, semitendineus and biceps femoris) of the uninjured side with tensiomyography (TMG) of 40 male football players with confirmed ACL injury. The same was done for 38 sex- and sports level-matched players in a control group on both sides to compare. The results showed that the vast majority of all TMG parameters were higher in the uninjured side of the ACL-injured group compared to the control group. Most significantly higher was vastus lateralis-half relaxation time, rectus femoris- contraction time, rectus femoris-sustained time, rectus femoris- half relaxation time and biceps femoris-maximal displacement in comparison to the control group. The author also noted that quadriceps muscles (specially rectus femoris) showed more significant difference between the groups than the hamstring muscles. In conclusion the author recognizes that resistance to fatigue and muscle stiffness in hamstring muscles may be risk factors for ACL injury in male soccer players. And also, Co-contraction imbalance of quadriceps over hamstrings may increase the risk

of such injuries.

The same type of study was performed by the same authors [31] investigating if gastrocnemius muscles are neuromuscular risk factors for ACL injury in male soccer players. But in this study it was found that gastrocnemius muscles are not neuromuscular risk factor for ACL injuries.

However, Kim et al. [32] found that higher lateral gastrocnemius activity was associated with higher ACL injury risk by conducting a controlled laboratory study of 37 young, male middle school football players. The players were analyzed and kinematics, kinetics and electromyographic data was collected for each athlete during anticipated and unanticipated side-and cross-cutting maneuvers.

(27)

Apart from environmental and biomechanical causes for ACL injury there has been search for a genetic genesis of the problem. Lulińska-Kuklik et al. [33] examined the association between COL5A1 rs12722 and rs13946 polymorphisms, both individually and inferred haplotypes, with the ACL rupture risk in professional football players. 134 male professional football player with surgically diagnosed primary ACL rupture and 211 healthy male professional football player without any reported history of injury on ligaments or tendons were included in this study. The players were recruited from same soccer teams of similar age category and with similar level of exposure to ACL injury. After extraction of DNA from oral epithelial cells all the samples were genotyped. The results showed a statistically significant difference in the genotype frequency of the COL5A1 rs13946 polymorphism in dominant modes of inheritance for the C-C haplotype in ACL rupture group compared to control group (p=0.038) suggesting that variations of the COL5A1 gene may be a non-modifiable risk factor for ACL injury in football players.

Hägglundet al. [34] prospectively evaluated the risk factors for acute knee injuries and more specifically, ACL injuries in 4556 youth female football players. The author could find that ACL injured players had a higher mean age, higher mean BMI and more commonly reported familial predisposition for ACL injury. The familial predisposed players had a four times higher ACL injury rate.

6.3 Gender

There were 12 articles analyzing gender as a risk factor for ACL injury in football players. Out of these twelve, only one found that male gender increases the risk. The rest had female association.

The reasons for why female soccer players sustain more ACL injuries than men remain unclear, however our study show many possible factors for the higher frequency.

According to Sheu et al. [35] one of the reasons can be attributed to the significant differences in knee flexion angles on initial ground contact while performing change of direction cutting maneuvers of 90° and 135°. The study was performed on 20 males and 20 females high school soccer athletes. At 90°, males and females showed initial contact knee flexion angles (mean ± SD) of 39.0° ± 6.8° and 29.3° ± 6.2°, respectively (P < .0001), and mean maximum flexion angles of

(28)

56.4° ± 6.9° and 49.7° ± 7.0°, respectively (P = .0036). At 135°, males and females showed mean initial contact knee flexion angles of 36.8° ± 7.9° and 29.7° ± 7.8°, respectively (P = .0053), and mean maximum flexion angles of 60.7° ± 8.1° and 51.6° ± 9.4°, respectively (P = .0017).

These findings show that a more extended knee joint during cutting maneuvers puts more load and strain on the ACL which increases the risk of injury.

S.M. Sigward et al. [21] compared the frontal plane knee moments along with kinematics and kinetics during knee valgus moments at 45° and 110° on 25 male and 20 female soccer athletes in the ages of 16-23. The data was obtained through a force platform and cameras. The author then compared if there were any significant difference between the genders and found that females showed greater knee valgus moments than males. Excessive knee valgus is a recognized mechanism behind many ACL tears which puts women at higher risk in accordance with this study.

Hogg et al. [36] analyzed the passive hip joint range of motion (ROM) between genders in different sports as a potential risk factor for ACL injuries since it has been associated with greater dynamic knee valgus and found that women had greater ROMIR (38.1° ± 8.2° versus 28.6° ± 8.4°; F1,327 = 91.74, P < .001), ROMTOT (72.1° ± 10.6° versus 64.4° ± 10.1°; F1,327 = 33.47, P < .001), and ROMREL (1.5° ± 16.0° versus −7.6° ± 16.5°; F1,327 = 37.05, P < .001) than men but similar ROMER (34.0° ± 12.2° versus 35.8° ± 11.5°; F1,327 = 1.65, P = .20) to men.

The study concludes that the greater hip joint ROM can be a risk factor for the higher incidence of ACL injuries in females due to higher dynamic knee valgus but needs to be studied further.

Waldén et al. [13] analyzed 39 videos of male professional football players’ ACL injuries between 2001 and 2011 to plot the mechanism of the injuries. It was found that eighty-five percent of the injuries were non-contact or indirect contact injuries and the most common playing situations being pressing followed by re-gaining balance after kicking and landing after heading leading to situation of dynamic knee valgus. The interesting finding is that dynamic valgus collapse was rarely seen and the author compared it to other studies arguing that dynamic valgus collapse might be a sex-specific knee motion which predominantly occur in females [37]. Why this is remains to be answered.

Brophy et al. [17] investigated why young female athletes are at higher risk of non-contact ACL injuries than their male counterpart by comparing proprioception and core stability via dynamic postural stability testing. The results showed that males lasted longer on the platform (98 ± 14 s) than females (94 ± 13 s) (p = 0.04). Coronal plane and rotation stability differed significantly

(29)

between genders (323 ± 126 vs. 365 ± 128, p = 0.04) and (318 ± 82 vs. 403 ± 153, p = 0.0002), respectively and concludes that less postural dynamic stability in the coronal plane and in rotational motions may be a contributing factor to females being more prone to non-contact ACL injuries.

The author relates the coronal stability to external rotational strength of the hip as the underlying cause for the gender discrepancy in this plane. He refers to studies showing that female soccer players land with significantly greater hip adduction angles and external hip adduction moments compared to males [38]. Further the author compares to studies showing that female soccer players have weaker hip abductors than males and that they kick the ball with less activation of their hip abductors leading to greater collapse of hip [39-40].

Another study by Brophy et al. [14] analyzed fifty-five videos of professional and collegiate male and female soccer players and found that females were more prone to injure their ACL while the opponents having the ball and defending compared to men. The most common way was when tackling the opponent which can be explained by it being a reactive maneuver needing last second body adjustments in body position and technique and with females generally having poorer neuromuscular control with suboptimal biomechanics compared to men they may be put in greater risk for ACL injury.

A.J. Khalid et al. [16] investigated the effect of neuromuscular fatigue on neuromuscular control and if it is more pronounced in females. This was done by taking six male and six female soccer players and analyzing them during side-stepping tasks using 10 Motion Analysis Corporation cameras and a Kistler™ force platform in fatigue and non-fatigue conditions. The results showed that males have significantly higher knee joint loads when fatigued, significantly higher peak proximal tibia anterior/posterior shear force, vertical ground reaction force at initial contact and peak internal rotation moments than females do. The conclusion suggested that males are at overall higher risk of injury compared to females due to this fact and that training programs need to take this into consideration and customize them according to gender.

This study was the only one in this review to find correlations putting males at higher risk than females. The reason is not clear but may have anatomical explanations.

M.A. Lyle et al. [20] explored the dynamic control of interaction between lower limb and ground amongst men and women by examining lower extremity dexterity (LED), that is the ability to dynamically control endpoint force magnitude and direction as quantified by compressing an unstable spring with the lower limb at submaximal forces. It was tested by comparing LED-test performance and single limb-drop jump in 14 females and 14 males high school soccer players.

(30)

The results of the experiment showed that females exhibited reduced LED-test performance (p

=0.001) and higher limb stiffness during landing (p =0.008). Females also exhibited higher co- activation at the ankle (p =0.001) and knee (p =0.02) before landing raising the possibility that the higher leg stiffness observed in females during landing is an anticipatory behavior due in part to reduced lower extremity dexterity. The reduced lower extremity dexterity and compensatory stiffening may increase the risk of ACL injuries in females.

L.E. Stanley et al. [41] used the National Athletic Treatment, Injury and Outcomes Network (NATION) and National Collegiate Athletic Association (NCAA) Injury Surveillance Program (ISP) between the 2009-2010 and 2013-2014 academic years to compare knee injury rates between high school and college student in different sports and to compare between the two genders. The results for ACL injuries in football showed a higher incidence of ACL injuries on college level in females while there where a higher incidence on high school level among males.

The results showed a statistically significant higher rate of injuries in females over males on both high school and college level (College level; IRR, 4.08; 95% CI, 2.08-8.00 and High School level;

IRR, 1.96; 95% CI, 1.11-3.47) which additionally illustrates the higher discrepancy between the genders. This was also revealed by Beynnon et al. [42] which conducted studies between 2008 and 2012 about first time non-contact ACL injuries among college and high school athletes and also compared the gender with each other. The author could find that female soccer players had higher risk of sustaining ACL injury (RR, 2.10; 95% CI, 1.34-3.27).

K.A. Owusu-Akyaw et al. [22] compared T-2 weighted Magnetic Resonance Imaging (MRI) scans for 15 females and 15 males who had sustained an ACL injury to predict the bone positions at time of injury. This was done by by creating three-dimensional models of the MRI scan, recreating the bone bruising overlap and measure knee flexion, knee valgus, internal tibial rotation and anterior tibial translation. However, there was no statistically significant differences between male and female patients in the position of injury with regard to knee flexion (P = 0.66), valgus (P = 0.87), internal tibial rotation (P = 0.26), or anterior tibial translation (P = 0.18).

6.4 Age

We found two articles investigating into age as a risk factor for ACL injury in football players.

The results were conflicting and not enough to make any conclusions.

(31)

De Ste Croix et al. [15] evaluated the risk for ACL injuries in elite female youth soccer players by measuring neuromuscular control of knee pre- and post fatigue. It is thought that knee joint stability is effected by fatigue and measured the electromechanical delay (EMD) in gastrocnemius, semitendinosus and biceps femoris in U13, U15 and U17 girls. After performing eccentric knee flexion tasks measured by electromyography pre- and post fatigue the findings concluded that the EMD was significantly longer post-fatigue and for the younger athletes (U13 mostly) this was most significant, putting them at higher risk for injury partly due to a more compliant muscle- tendon system.

However, Hägglundet al. [34] prospectively evaluated the risk factors for acute knee injuries and more specifically ACL injuries, in female youth soccer players aged 12-17 years and found that age >14 years was a significant predictor for acute knee injury (HR 1.97; 95 % CI 1.30–2.97).

For ACL injuries more specifically, the findings were however not statistically significant for older age. The study was a sub-study of a previous cluster randomized controlled trial that evaluated the preventive efficacy of a neuromuscular training program in female youth football which had already been published.

6.5 Professional and non-professional athletes

4 articles were included exploring level of play as a risk factor for ACL injury in football players.

Beynnon et al. [42] conducted studies between 2008 and 2012 about first time non-contact ACL injuries among college and high school athletes. College athletes had a significantly higher injury risk than did high school athletes (adjusted relative risk [RR], 2.38; 95% CI, 1.55-3.54) and could conclude that an athlete’s risk of having a first time non-contact ACL injury is independently influenced by level of competition.

Krutsch et al. [18] took the opportunity to conduct a study during the introduction of a new professional soccer league in Germany to analyze the influence of the increased intensity on acute knee injuries. Comparing the players from the new professional league with players from amateur level, players in the professional league had a significantly higher (p = 0.04) incidence of ACL and PCL injuries, than players at the amateur level. More than 90 % of all ACL and PCL ruptures in both groups were sustained by players, who had played at least one level lower in the previous season. However, this is more likely to be due to the rapid change in intensity rather than the actual

(32)

level of playing.

M.J.C. Lee et al. [43] tested the anticipated and unanticipated response to sidestepping maneuvers between semi-professional and amateur soccer players, knowing that unplanned sidestepping may be a risk factor for ACL injury. The participants were tested in response to quasi-game-realistic vs non-game-realistic stimuli and the results showed that the higher level players placed their push- off foot closer to the pelvic midline compared with the lower level players and there were more errors during the sidestepping maneuvers in the amateur players who showed more postural inconsistency. This put less experienced soccer players at higher risk of sustaining ACL injury.

L.E. Stanley et al. [41] used the National Athletic Treatment, Injury and Outcomes Network (NATION) and National Collegiate Athletic Association (NCAA) Injury Surveillance Program between the 2009-2010 and 2013-2014 academic years to compare knee injury rates between high school and college student in different sports. The results for ACL injuries in football showed a higher incidence of ACL injuries on college level in females while there where a higher incidence on high school level among males.

(33)

6.6 Assessment of Quality

Different kinds of studies were included (randomized, mixed design, cohort, video analysis and controlled laboratory controlled studies). Even though the study types varied the groups were somewhat comparable at baseline.

The inclusion/exclusion criteria were described adequately in most studies whereas the possibility to blind the assessors were hard/impossible in most. However, the interventions, outcome measures and duration was generally well defined.

Thus the overall quality of the studies was moderate/good with improvements needed by ensuring randomization.

Nine studies achieved good quality rating while sixteen achieved moderate rating.

Table 5. Quality assessment

Study Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Score Quality

Rating Sheu et al

(2014)

1 0 0 2 2 2 2 2 11 Modera

te Sigward et al

(2015)

1 1 0 2 2 2 2 2 12 Modera

te Hogg et al

(2018)

1 0 0 2 0 2 2 2 9 Modera

te Waldén et al

(2015)

1 2 1 2 0 2 2 0 10 Modera

te Brophy et al

(2015)

1 1 2 2 2 2 2 2 14 Modera

te Brophy et al

(2014)

1 2 1 1 0 2 2 1 10 Modera

te Khalid et al

(2015)

1 0 2 2 2 2 2 2 13 Modera

te Lyle et al

(2014)

1 2 2 2 2 2 2 2 15 Good

Stanley et al (2016)

1 2 2 2 1 2 2 2 14 Modera

te

(34)

Beynnon et al (2014)

1 2 2 2 2 2 2 2 15 Good

Owusu- Akyaw et al (2018)

1 1 2 2 2 2 2 2 14 Modera

te

De Ste Croix et al (2018)

1 1 2 2 2 2 2 2 14 Modera

te Hägglund et

al (2016)

2 2 2 2 2 2 2 2 16 Good

Krutsch et al (2016)

1 2 2 1 1 2 2 1 12 Modera

te Lee et al

(2017)

1 2 1 2 2 2 2 2 14 Modera

te Baumgart et

al (2015)

2 2 2 2 2 2 2 2 16 Good

Shultz et al (2015)

1 2 1 2 2 2 2 2 14 Modera

te Potter et al

(2014)

1 1 1 1 1 1 1 2 9 Modera

te Malloy et al

(2015)

1 2 2 2 2 2 2 2 15 Good

Brown et al (2014)

1 2 2 2 2 2 2 2 15 Good

Greska et al (2017)

1 2 2 2 2 2 2 2 15 Good

Alentorn- Geli et al (2015)

1 2 2 2 2 2 2 2 15 Good

Alentorn- Geli et al (2015)

1 2 2 2 2 2 2 2 15 Good

Kim et al (2014)

1 1 2 1 1 2 2 2 12 Modera

te

Lulińska- 1 2 2 1 2 2 2 2 14 Modera

(35)

Kuklik et al (2017)

te

(36)

7. CONCLUSIONS

According to the scientific literature from 2013 to 2018 included in this study we can draw following conclusions:

1. The numerous associated risk factors for ACL injury in football players proves that it indeed is multifactorial in its origin. Most commonly mentioned risk factors were; gender, bad postural and neuromuscular control, age, leg preference, genetic factors and level of play.

2. Females are at increased risk of sustaining ACL injury due to a more extended knee joint, greater knee valgus, greater hip adduction and less postural dynamic stability

3. Younger youth players are at higher risk due to a more compliant musculoskeletal system.

This is however somewhat inconclusive.

4. Higher level of play generally has higher risk of injury. Non-professionals are at risk due to poor neuromuscular control.

(37)

8. PRACTICAL RECOMMENDATIONS

Due to it being a widespread and harmful problem for football players and not the least females, specific prevention programs are necessary for football players to avoid the potentially devastating consequences of an ACL injury. Programs enhancing neuromuscular control, endurance and strength need to be incorporated at an individual level to prevent the risk. Static stretching should be avoided before engaging in football sessions.

(38)

9. LITERATURE LIST

1. Fédération Internationale de Football Association (FIFA). FIFA Big Count 2006. [Internet].

FIFA Domunications Division, Information Services; 2007 May 31. Available from:

http://www.fifa.com/mm/document/fifafacts/bcoffsurv/bigcount.statspackage_7024.Pdf

2. Fédération Internationale de Football Association (FIFA). FIFA Women’s Football Survey 2014. [Internet]. CIES Football Observatory; 2014. Available form:

http://www.fifa.com/mm/document/footballdevelopment/women/02/52/26/49/womensfootballsu rvey2014_e_english.pdf

3. Christensen, James E. et al. Knee Anterior Cruciate Ligament Injuries Common Problems and Solutions. Clinics in Sports Medicine. 2018;37(2):265-280.

4. Waldén M, Hägglund M, Magnusson H, et al. ACL injuries in men's professional football: a 15-year prospective study on time trends and return-to-play rates reveals only 65% of players still play at the top level 3 years after ACL rupture. Br J Sports Med. 2016;50:744-750.

5. Ardern CL, Taylor NF, Feller JA, et al. Fifty-five per cent return to competitive sport following anterior cruciate ligament reconstruction surgery: an updated systematic review and meta-analysis including aspects of physical functioning and contextual factors. Br J Sports Med.

2014;48(21):1543-1552.

6. Engström, B., Forssblad, M., Johansson, C., & Tornkvist, H. Does a major knee injury definitely sideline an elite soccer player? Am J Sports Med. 1990;18(1):101–105.

7. Nicolini AP, de Carvalho RT, Matsuda MM, Sayum JF, Cohen M. Common injuries in athletes' knee: experience of a specialized center. Acta Ortop Bras. 2014;22(3):127-31.

8. Clausen MB, Zebis MK, Moller M, et al. High injury incidence in adolescent female soccer.

Am J Sports Med. 2014;42(10):2487-2494.

9. Ardern CL, Webster KE, Taylor NF, Feller JA. Return to sport following anterior cruciate ligament reconstruction surgery: a systematic review and meta-analysis of the state of play. Br J Sports Med. 2011;45:596–606.

(39)

10. Wiggins AJ, Grandhi RK, Schneider DK, Stanfield D, Webster KE, Myer GD. Risk of Secondary Injury in Younger Athletes After Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta-analysis. Am J Sports Med. 2016;44(7):1861-76.

11. Webster KE, Feller JA. Return to Level I Sports After Anterior Cruciate Ligament

Reconstruction: Evaluation of Age, Sex, and Readiness to Return Criteria. Orthop J Sports Med.

2018;6(8)

12. Bjordal JM, Arnły F, Hannestad B, Strand T. Epidemiology of anterior cruciate ligament injuries in soccer. Am J Sports Med. 1997;25(3):341-345.

13. Waldén M, Krosshaug T, Bjørneboe J, et al Three distinct mechanisms predominate in non- contact anterior cruciate ligament injuries in male professional football players: a systematic video analysis of 39 cases. Br J Sports Med. 2015;49:1452-1460.

14. Brophy, R. H., Stepan, J. G., Silvers, H. J., & Mandelbaum, B. R. Defending Puts the Anterior Cruciate Ligament at Risk During Soccer: A Gender-Based Analysis. Sports Health.

2015;7(3): 244–249.

15. De Ste Croix, M. B., Priestley, A. M., Lloyd, R. S. and Oliver, J. L. (2015), Female youth soccer and fatigue. Scand J Med Sci Sports. 2015;25:531-538.

16. Abdul Jabbar Khalid, Sujae Ian Harris, Loke Michael, Hamill Joseph & Xingda Qu. Effects of neuromuscular fatigue on perceptual-cognitive skills between genders in the contribution to the knee joint loading during side-stepping tasks, Journal of Sports Sciences. 2015;33(13):1322- 1331.

17. Brophy RH, Staples JR, Motley J, Blalock R, Steger-May K, Halstead M. Young Females Exhibit Decreased Coronal Plane Postural Stability Compared to Young Males. HSS J.

2015;12(1):26-31.

18. Krutsch, W., Zeman, F., Zellner, J. et al. Increase in ACL and PCL injuries after

implementation of a new professional football league. Knee Surg Sports Traumatol Arthrosc.

2016;24(7):2271-9.

(40)

19. Morgan BE, Oberlander MA (2001) An examination of injuries in major league soccer. The inaugural season. Am J Sports Med 29:426–430.

20. Lyle M.A., Valero-Cuevas F.J., Gregor R.J., Powers C.M. Control of dynamic foot-ground interactions in male and female soccer athletes: Females exhibit reduced dexterity and higher limb stiffness during landing. J Biomech. 2014;47(2):512-517.

21. Sigward SM, Cesar GM, Havens KL. Predictors of Frontal Plane Knee Moments During Side-Step Cutting to 45 and 110 Degrees in Men and Women: Implications for Anterior Cruciate Ligament Injury. Clin J Sport Med. 2015;25(6):529-34.

22. Owusu-Akyaw, K. A., Kim, S. Y., Spritzer, C. E., Collins, A. T., Englander Z.A., Utturkar, G. M. et al. Determination of the Position of the Knee at the Time of an Anterior Cruciate Ligament Rupture for Male Versus Female Patients by an Analysis of Bone Bruises. Am J Sports Med. 2018;46(7):1559–1565.

23. Baumgart C, Gokeler A, Donath L, et al. Effects of static stretching and playing soccer on knee laxity. Clin J Sport Med. 2015;25(6):541-5.

24. Shultz SJ, Schmitz RJ, Cone JR, et al. Changes in fatigue, multiplanar knee laxity, and landing biomechanics during intermittent exercise. J Athl Train. 2015;50(5):486-97.

25. Potter D, Reidinger K, Szymialowicz R, et al. Sidestep and crossover lower limb kinematics during a prolonged sport-like agility test. Int J Sports Phys Ther. 2014;9(5):617-27.

26. Malloy P, Morgan A, Meinerz C, Geiser C, Kipp K. The association of dorsiflexion flexibility on knee kinematics and kinetics during a drop vertical jump in healthy female athletes. Knee Surg Sports Traumatol Arthrosc. 2014;23(12):3550-5.

27. Brown SR, Wang H, Dickin DC, Weiss KJ. The relationship between leg preference and knee mechanics during sidestepping in collegiate female footballers. Sports Biomech. 2014 Nov;13(4):351-61.

(41)

28. Greska EK, Cortes N, Ringleb SI, Onate JA, Van Lunen BL. Biomechanical differences related to leg dominance were not found during a cutting task. Scand J Med Sci Sports. 2017 Nov;27(11):1328-1336.

29. Brophy R, Silvers HJ, Gonzales T, Mandelbaum BR. Gender influences: the role of leg dominance in ACL injury among soccer players. Br J Sports Med. 2010;44(10):694-697.

30. Alentorn-Geli, E., Alvarez-Diaz, P., Ramon, S. et al. Assessment of neuromuscular risk factors for anterior cruciate ligament injury through tensiomyography in male soccer players.

Knee Surg Sports Traumatol Arthrosc. 2015;23(9):2508-13.

31. Alentorn-Geli E, Alvarez-Diaz P, Ramon S, Marin M, Steinbacher G. et al. Assessment of gastrocnemius tensiomyographic neuromuscular characteristics as risk factors for anterior cruciate ligament injury in male soccer players. Knee Surg Sports Traumatol Arthrosc 2015 23:2502–2507.

32. Kim, J. H., Lee, K.-K., Kong, S. J., An, K. O., Jeong, J. H., & Lee, Y. S. Effect of

Anticipation on Lower Extremity Biomechanics During Side- and Cross-Cutting Maneuvers in Young Soccer Players. Am J Sports Med. 2014;42(8):1985–1992.

33. Lulińska-Kuklik E, Rahim M, Domańska-Senderowska D, et al. Interactions

between COL5A1 Gene and Risk of the Anterior Cruciate Ligament Rupture. J Hum Kinet.

2018;62:65-71.

34. Martin Hägglund and Markus Waldén, Risk factors for acute knee injury in female youth football. Knee Surg Sports Traumatol Arthrosc. 2016;24(3):737-746.

35. Sheu CL, Gray AM, Brown D, Smith BA. Sex Differences in Knee Flexion Angle During a Rapid Change of Direction While Running. Orthop J Sports Med.

2015;3(12):2325967115617932.

36. Jennifer A. Hogg, Randy J. Schmitz, Anh-Dung Nguyen, and Sandra J. Shultz. Passive Hip Range-of-Motion Values Across Sex and Sport. J Athl Train. 2018;53(6):560-567.

(42)

37. Quatman CE, Hewett TE The anterior cruciate ligament injury controversy: is “valgus collapse” a sex-specific mechanism? British Journal of Sports Medicine. 2009;43:328-335.

38. Ford KR, Shapiro R, Myer GD, et al. Longitudinal sex differences during landing in knee abduction in young athletes. Med Sci Sports Exerc. 2010;42:1923–31.

39. Broglio SP, Sosnoff JJ, Rosengren KS, et al. A comparison of balance performance:

computerized dynamic posturography and a random motion platform. Arch Phys Med Rehabil. 2009;90:145–50.

40. Borchers JR, Pedroza A, Kaeding C. Activity level and graft type as risk factors for anterior cruciate ligament graft failure: a case-control study. Am J Sports Med. 2009;37:2362–7.

41. Stanley, L. E., Kerr, Z. Y., Dompier, T. P., & Padua, D. A. Sex Differences in the Incidence of Anterior Cruciate Ligament, Medial Collateral Ligament, and Meniscal Injuries in Collegiate and High School Sports: 2009-2010 Through 2013-2014. Am J Sports Med. 2016;44(6):1565–

1572.

42. Beynnon, B. D., Vacek, P. M., Newell, M. K., Tourville, T. W., Smith, H. C., Shultz, S. J. et al. The Effects of Level of Competition, Sport, and Sex on the Incidence of First-Time

Noncontact Anterior Cruciate Ligament Injury. Am J Sports Med. 2014;42(8):1806–1812.

43. Lee, M. J., Lloyd, D. G., Lay, B. S., Bourke, P. D. and Alderson, J. A. Different visual stimuli affect body reorientation strategies during sidestepping. Scand J Med Sci Sports.

2017;27:492-500.