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Prevention of non-contact anterior cruciate ligament injuriesin soccer players. Part 1: Mechanisms of injuryand underlying risk factors

Eduard Alentorn-Geli Gregory D. Myer Holly J. Silvers Gonzalo Samitier Daniel Romero Cristina Lazaro-Haro Ramon Cugat

Received: 5 October 2008 / Accepted: 18 April 2009 / Published online: 19 May 2009

Springer-Verlag 2009

Abstract Soccer is the most commonly played sport in

the world, with an estimated 265 million active soccer

players by 2006. Inherent to this sport is the higher risk of

injury to the anterior cruciate ligament (ACL) relative to

other sports. ACL injury causes the most time lost from

competition in soccer which has influenced a strong

research focus to determine the risk factors for injury. This

research emphasis has afforded a rapid influx of literature

defining potential modifiable and non-modifiable risk fac-

tors that increase the risk of injury. The purpose of the

current review is to sequence the most recent literature that

reports potential mechanisms and risk factors for non-

contact ACL injury in soccer players. Most ACL tears in

soccer players are non-contact in nature. Common playing

situations precluding a non-contact ACL injury include:

change of direction or cutting maneuvers combined with

deceleration, landing from a jump in or near full extension,

and pivoting with knee near full extension and a planted

foot. The most common non-contact ACL injury mecha-

nism include a deceleration task with high knee internal

extension torque (with or without perturbation) combined

with dynamic valgus rotation with the body weight shifted

over the injured leg and the plantar surface of the foot fixed

flat on the playing surface. Potential extrinsic non-contact

ACL injury risk factors include: dry weather and surface,

and artificial surface instead of natural grass. Commonly

purported intrinsic risk factors include: generalized and

specific knee joint laxity, small and narrow intercondylar

notch width (ratio of notch width to the diameter and cross

sectional area of the ACL), pre-ovulatory phase of men-

strual cycle in females not using oral contraceptives,

decreased relative (to quadriceps) hamstring strength and

recruitment, muscular fatigue by altering neuromuscular

control, decreased core strength and proprioception, low

trunk, hip, and knee flexion angles, and high dorsiflexion

of the ankle when performing sport tasks, lateral trunk

displacement and hip adduction combined with increased

knee abduction moments (dynamic knee valgus), and

increased hip internal rotation and tibial external rotation

with or without foot pronation. The identified mechanisms

and risk factors for non-contact ACL injuries have been

mainly studied in female soccer players; thus, further

research in male players is warranted. Non-contact ACL

injuries in soccer players likely has a multi-factorial eti-

ology. The identification of those athletes at increased risk

may be a salient first step before designing and imple-

menting specific pre-season and in-season training pro-

grams aimed to modify the identified risk factors and to

decrease ACL injury rates. Current evidence indicates that

E. Alentorn-Geli G. Samitier C. Lazaro-Haro R. CugatArtroscopia G.C., Hospital Quiron, Barcelona, Spain

G. D. Myer

Sports Medicine Biodynamics Center and Human Performance

Laboratory, Cincinnati Childrens Hospital Medical Center,

Cincinnati, OH, USA

G. D. Myer

Rocky Mountain University of Health Professions,

Provo, UT, USA

H. J. Silvers

Santa Monica Orthopaedic Sports Medicine/Research

Foundation, Santa Monica, CA, USA

D. Romero

Physical Therapy School, Blanquerna University,

Barcelona, Spain

E. Alentorn-Geli (&)Dr. Ramon Cugats Office, Hospital Quiron,

Plaza Alfonso Comn 5-7, 08023 Barcelona, Spain

e-mail: ealentorngeli@gmail.com

123

Knee Surg Sports Traumatol Arthrosc (2009) 17:705729

DOI 10.1007/s00167-009-0813-1

this crucial step to prevent ACL injury is the only option to

effectively prevent the sequelae of osteoarthritis associated

with this traumatic injury.

Keywords Prevention Non-contact ACL injury Soccer

Introduction

Soccer is the most commonly played sport in the world

[36], with an estimated 265 million active soccer players

participating in the sport as of 2006 [47]. The international

popularity continues to rise as indicated by the 23 million

increase in active soccer players compared to 8 years ago.

Despite the predominance of male players (90%), the

current trends suggest that the continued rise in participa-

tion is mainly from the increases in females, who choose to

participate in soccer [47]. Despite the suggestion that it is

considered a relatively safe sport for males, female soccer

players are at up to six times greater risk for sustaining an

anterior cruciate ligament (ACL) tear than their male

counterparts [3]. The increased participation and increased

risk of knee injury especially among females, has led to a

substantial increase in number of reported ACL injuries in

the sport. The reported incidence of ACL injury ranges

from 0.06 to 3.7 per 1,000 h of active soccer playing (game

and training) [15, 45], accounting for thousands of ACL

tears each year. It is also estimated that the occurrence of

ACL injuries on a soccer team expressed as a percentage of

all injuries on that team is 1.3% for males, and 3.7% for

females [156].

While there has been recent scientific efforts focused on

ACL injury treatment strategies, it is well established that

surgical reconstruction does not reduce the increased risk

for developing knee osteoarthritis after a traumatic knee

injury is sustained [42, 106, 120, 134]. In addition, ACL

injury is often concomitant with a meniscus tear, and

several authors found that this type of meniscus injury is

also an indicated risk factor for tibiofemoral osteoarthritis

[120, 136]. Beyond the short- and long-term physical

impairments, ACL injury also causes personal and pro-

fessional impairment for athletes, with a high economic

cost for both athletes and institutions [56, 57, 194].

Therefore, the prevention of non-contact ACL injuries is of

major relevance in sports traumatology.

The identified gender bias for non-contact ACL injuries

and associated detrimental effects in female soccer players

have served as the impetus for research efforts to define the

mechanisms of ACL injury and to delineate the most

relevant underlying risk factors that contribute to these

mechanisms. It is suggested that once these gender-related

mechanisms and associated risk factors are defined more

efficient neuromuscular training protocols can be instituted

to high-risk populations [130]. The purpose of this article is

twofold: first, to provide a current review of the literature

to define the most probable mechanisms of non-contact

ACL injuries and second, to delineate the role that envi-

ronmental, anatomical, hormonal, neuromuscular, and

biomechanical risk factors may contribute to the portrayed

mechanisms.

We employed Medline database for literature search

purposes. All articles under the topics ACL prevention,

ACL injury risk factors, non-contact ACL injuries,

mechanisms of ACL injuries, ACL injuries in soccer

players, and injuries in soccer from 1985 to 2008 were

considered of potential interest for this review. Articles

which included an intervention were excluded from this

review. In addition, each reference list from the identified

articles was cross-checked to verify that relevant articles

were not missed for the current review.

Mechanisms of non-contact ACL injuries in soccer

players

The study of mechanisms of non-contact ACL injuries in

soccer players is based on several methodological

approaches: interviews with injured players, video analysis,

clinical studies (where the clinical joint damage is studied

to understand the mechanism of the injury), in vivo studies

(measuring ligament strain or forces to understand liga-

ment loading patterns), cadaver studies, mathematical

modeling and simulation of injury situations, or measure-

ments/estimation from close to injury situations [93,

156].

We considered non-contact ACL tears to those injuries

with no physical contact with other players at the time of

injury. The rate for non-contact ACL injuries ranges from

70 to 84% of all ACL tears in both female and male ath-

letes [17, 45, 117, 137, 138]. Most ACL tears in soccer

occur in the absence of player-to-player (body-to-body)

contact [45]. Despite Arendt and Dick [3] found an equal

rate of contact versus non-contact mechanisms on ACL

injuries among male soccer players, it is overall accepted

that the vast majority of ACL tears occur through a non-

contact mechanism in both male and female athletes.

The most common playing scenarios precluding a non-

contact ACL injury include: change of direction or cutting

maneuvers combined with deceleration, landing from a

jump in or near full extension, pivoting with knee near full

extension and a planted foot [17, 45, 46]. Other described

mechanisms of ACL tears included knee hyperextension

and hyperflexion [53, 63, 176]. These playing situations

involve knee valgus, varus, internal rotation, and external

rotation moments, and anterior translation force [17, 110,

706 Knee Surg Sports Traumatol Arthrosc (2009) 17:705729

123

111, 141, 182, 194]. The anterior translation force, spe-

cifically at flexion angles around 2030, may be the mostdetrimental isolated force associated with ACL injury, and

is often identified as a contributing factor to ACL injury

mechanisms [10, 17, 110, 117, 194]. However, cadaveric

studies indicate that a combination of forces produces a

higher strain on the ACL than isolated motions and torques.

Thus, pure knee internal rotation, external rotation, valgus,

and varus moments do not strain ACL [10] to the magni-

tude of combined rotations such as an anteriorly directed

force added to valgus or internal rotation (Fig. 1) [10, 110].

Boden et al. utilized retrospective video analysis in

attempt to define the most common kinematic positions

related to ACL injury during competitive play. They

reported a lower extremity alignment associated with non-

contact ACL injury in which the tibia was externally

rotated, the knee was close to full extension, the foot was

planted during deceleration with valgus collapse at the

knee [17]. More recent reports have also indicated this

common mechanism of valgus collapse at the knee in

female athletes [94, 141]. Teitz reported very similar

deceleration positions in the majority of the ACL injuries

she examined; however, she also indicated that most often

the center of mass of the body was behind and away from

the base of support (area of foot to ground contact) [177].

Thus, there is mounting evidence that the most common

non-contact injury mechanism of injury in female athletes

occurs during a deceleration task with high knee internal

extension torque (with or without a visual perturbation)

combined with dynamic valgus rotation with the body

weight shifted over to the injured leg and the plantar sur-

face of the foot fixed flat on the playing surface [17, 94,

141, 177]. Interestingly, both male and female athletes may

demonstrate similar body alignment during competitive

play without succumbing to an ACL injury. Thus, it is

crucial to determine the underlying risk factors that con-

tribute to an increased propensity for this high-risk posi-

tion. Ultimately, it is the goal of clinicians and researchers

to determine the risk factors that preclude the actual ACL

injury.

Risk factors

Risk factors have been divided into extrinsic (those outside

the body) and intrinsic factors (those within the body)

[128]. However, other classification schemes do exist when

considering non-contact ACL injuries. In this article, risk

factors will be divided into environmental, anatomical,

hormonal, neuromuscular, and biomechanical, relative to

the guidelines established by the Hunt Valley meeting [62].

Environmental risk factors

Overview

Environmental factors include those aspects extrinsic to the

athlete such as sport, playing surface, weather character-

istics, the type of footwear, the shoe to surface interaction

(friction coefficient). There is a clear lack of randomized

controlled studies regarding environmental factors in soc-

cer players. The existing evidence on environmental factors

related to ACL injuries is mainly based on American

Football, Australian Football, or indoor sports like handball

[20, 21, 96, 143, 144, 162]. American Football, Australian

Football, and soccer are contact sports sharing some

common features regarding ground characteristics, shoe

choice, and many playing situations like cutting, landing,

or a change of direction with high acceleration and/or

deceleration components.

Weather

A relationship between meteorological conditions and the

incidence of ACL injuries was noted in Australian Football

by Scranton et al. [162]. The authors found a higher ACL

injury rate on natural grass during dry compared to wet

conditions. However, the report did not control for weather

conditions where injuries did not occur. Subsequently,

Orchard et al. [144] found that high water evaporation in the

month before the match and low rainfall in the year before

Fig. 1 ACL injury through a combination of knee valgus and anterior tibial translation force during a side-cut maneuver in soccer players

Knee Surg Sports Traumatol Arthrosc (2009) 17:705729 707

123

the match in Australian Football were significantly associ-

ated with a higher incidence of ACL injuries. It has been

postulated that the high rate of ACL tears during dry con-

ditions on natural grass could be explained by an increased

friction and torsional resistance from the shoe-surface

interface compared to wet conditions [21, 66]. Similarly,

Orchard et al. [145] found that cold weather was associated

with lower knee and ankle injury risk, including ACL tears,

in outdoor sports completed on both natural grass and arti-

ficial turf. Correspondingly, Torg et al. [179] demonstrated

that an increase in turf temperature, in combination with

cleat characteristics, affects shoesurface interface friction

and potentially places the athletes knee and ankle at risk of

injury. Studies regarding weather conditions may be limited

without their control for other potential confounding factors

like intrinsic biomechanical factors, neuromuscular condi-

tioning, or hydration status of athletes, among others.

Shoesurface interaction

Surface characteristics themselves, irrespective of whether

they are influenced by weather conditions, have an impact on

ACL injury rates. Orchard et al. found in Australian Football

that games and practices played on rye grass appeared to

have a lower incidence of ACL tears compared to Bermuda

grass. It was hypothesized that Bermuda grass, with a thicker

thatch layer, would increase shoe-surface traction secondary

to the fact that boot cleats would be better gripped by the

surface [143]. Also, grass cover and root density has been

associated with a greater shoesurface traction. Artificial

surface is generally associated with higher shoesurface

traction than natural grass [142], and thus with a higher risk

for ACL tears. In general, artificial turf has a higher peak

deceleration for high-energy impacts [101], and the greater

the surface hardness the greater the ground reaction force.

Bowers and Martin [20] demonstrated that the impact

absorption of artificial turf decreases as the age of the surface

increases. Additionally, Arnason et al. [6] found a higher

injury rate for soccer played on artificial turf compared to

natural grass and gravel, and a higher injury rate on natural

grass compared to gravel. Moreover, Hoff and Martin [77]

found a sixfold increase in the injuries reported in indoor

soccer compared to outdoor. Both artificial turf and indoor

flooring may have an increased coefficient of friction. An

increased shoesurface coefficient of friction or traction may

potentially improve performance, but may also increase the

risk for ACL injuries. Ford et al. [50] demonstrated that the

playing surface (grass vs. turf) significantly alters plantar

loading during cutting in male football players. In addition,

Burkhart et al. [25] reported in a prospective research study

that an athlete, who landed with an increased heel to flat-foot

loading mechanism was more likely to sustain to a non-

contact ACL injury during competitive play. In summary,

factors influencing shoesurface traction include: ground

hardness, ground coefficient of friction, ground dryness,

grass cover and root density, length of cleats on player boots

and relative speed of the game. These factors may contribute

to the inciting mechanism of ACL injury [142]. Unfortu-

nately, no definitive conclusions can be drawn regarding the

safest type of playing surface in soccer players. Recent

studies found no differences in the incidence, severity, nature

or cause of injuries in male and female soccer players

when comparing artificial turf versus natural grass [40, 54,

55, 173].

Footwear

Footwear is considered a potential risk factor for ACL tears,

since it modulates foot fixation during the game. It has been

shown that the number, length and cleat placement was

associated with the chance of ACL injuries [158]. Lambson

et al. prospectively evaluated ACL injury incidence in

American Football depending on the shoe design. The

authors found a higher risk of ACL tears for the edge

cleat design (longer irregular cleats placed at the peripheral

margin of the lateral sole with a number of smaller pointed

cleats positioned medially). This cleat placement may have

provided significantly higher torsional resistance compared

to other types of cleats [96]. However, Mitchell et al. [123]

reported that the foot mechanics and possibly the footshoe

interaction were not related to the propensity to demonstrate

high knee load kinematics that are related to increased risk

of ACL injury. While there is no current consensus relating

the environmental and shoesurface interaction to risk

factors that contribute to ACL injury, the initial evidence

reported above suggest that these factors may contribute to

the described mechanisms of ACL injury. However,

potential confounding factors (i.e., biomechanical, neuro-

muscular, hydration status, among others) need to be better

controlled in environmental risk factors studies. Also, some

conclusions are not completely generalizable to soccer

players as they were made for Australian Football (i.e., the

increased risk for wet conditions and for Bermuda grass

compared to rye grass).

Anatomical risk factors

Overview

There is no definitive evidence that any anatomical risk

factors are directly correlated with an increased rate of non-

contact ACL injury with respect to age and gender [62].

Moreover, the preventive potential of anatomical factors

is relatively small, since anatomy is difficult to modify

(Table 1). However, there are anatomical considerations

708 Knee Surg Sports Traumatol Arthrosc (2009) 17:705729

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that should be considered in order to ascertain an adequate

understanding of the implicated pathomechanics, which

may lead to an ACL tear. Body mass index, generalized

and specific knee joint laxity, and Q-angle anatomical

factors were investigated involving soccer players. In

contrast, evidence on intercondylar notch width, ACL size

and strength, pelvis and trunk anatomy, posterior tibial

slope, and foot pronation is not based on soccer players.

Relative mass

Some authors have observed increased body mass index as

a risk factor for ACL injuries, especially among female

adolescent soccer players [24, 71], college recreational

athletes [22, 62], and female army recruits [180]. It was

postulated that an increased body mass index would result

in a more extended lower extremity position with

decreased knee flexion upon landing [22, 62]. Unfortu-

nately, conflicting results do exist when completing a fur-

ther review of the literature, and other authors found no

impact of body mass index on ACL injuries in female

athletes, including soccer players [54, 55, 91, 95, 146].

Joint laxity

Generalized joint laxity is purported as a risk factor that

could potentially place the athlete at an increased risk of

ACL injury. Soderman et al. [170] investigated the risk of leg

Table 1 Summary of modifiable and non-modifiable intrinsic risk factors related to increased risk of ACL injury

Modifiable risk factors Non-modifiable risk factors Potential control or treatment technique

Anatomical BMI Monitor and control relative body mass

Femoral notch index (ACL size) N-M training targeted to decrease other risk factors

Knee recurvatum N-M training targeted to improve dynamic knee

flexion

General joint laxity N-M training targeted to improve joint stiffness

Family history (genetic predisposition) N-M training targeted to decrease other risk factors

Prior injury history Full physical rehabilitation following injury

Developmental and

hormonal

Sex, female N-M training prior to onset of risk factors

Pubertal and post-pubertal maturation

status

N-M training prior during pre-puberty

Preovulatory menstrual status Oral contraceptives in femalesa

ACL tensile strength N-M training targeted to decrease other risk factors

Neuromuscular shunt N-M training targeted to improve neuromuscular

control

Biomechanical Knee abduction N-M training targeted to improve coronal plane

loads

Anterior tibial shear N-M training targeted to improve dynamic knee

flexion

Lateral trunk motion N-M training targeted to improve trunk strength and

control

Tibial rotation N-M training targeted to control transverse motions

and influence sagittal plane deceleration

mechanics

Dynamic foot

pronation

Foot orthoses

Fatigue resistance Strength and conditioning training

Ground reaction forces N-M training targeted to improve force absorption

strategies

Neuromuscular Relative hamstring

recruitment

N-M training targeted to improve hamstring strength

and recruitment

Hip abduction strength N-M training targeted to improve hip strength and

recruitment

Trunk proprioception N-M training targeted to improve trunk strength and

control

N-M neuromuscular traininga Pilot evidence indicates it might be a potential control strategy

Knee Surg Sports Traumatol Arthrosc (2009) 17:705729 709

123

injuries among female soccer players presenting with gen-

eral joint laxity and knee hyperextension (among other risk

factors). The investigators demonstrated a significantly

increased risk of leg injuries (but not specific ACL injuries)

among athletes with generalized joint laxity and knee

hyperextension. Uhorchak [180] specifically reported a 2.8

times greater risk of non-contact ACL injury in the United

States Military Academy cadets with generalized joint laxity

compared to normal joint laxity subjects in a prospective 4-

year evaluation. There was also a greater risk of non-contact

ACL injury for females with a higher anteriorposterior knee

joint laxity but not for males. The authors also found a sig-

nificantly higher generalized joint laxity and anteriorpos-

terior knee joint laxity in females compared to males. It was

also retrospectively observed that ACL-injured subjects had

a significantly greater generalized joint laxity in comparison

to healthy age-matched controls [154]. The same authors

report a 78.7% proportion of genu recurvatum among ACL-

injured subjects versus the 37% in the control group. Specific

knee joint laxity has been related to increased valgusvarus

and internalexternal rotation knee laxity with an increased

functional valgus collapse [51, 72, 167] observed in young

female soccer and basketball players in comparison to their

male counterparts. Specific knee joint laxity is greater in

healthy females compared to males [149, 180], and knee

joint laxity measured as hyperextension and anteriorpos-

terior tibiofemoral translation has been recently related to a

higher risk of ACL injuries among female soccer and bas-

ketball players [133]. Therefore, it seems that knee joint

laxity could alter dynamic lower extremity motions and

loads a multiplanar fashion, which may place ligaments to a

higher risk of rupture. Ergun et al. compared 44 healthy male

soccer players (from local leagues) with 44 healthy controls

(age- and sex-matched sedentary medical students and hos-

pital staff with no history of regular sports activity) in the

sagittal plane for knee laxity and isokinetic muscle strength.

Soccer players demonstrated significantly less anterior

and anteriorposterior knee laxity and higher isokinetic

strength of the knee flexors and extensors compared to sed-

entary controls. Isokinetic strength difference was found to

be higher for the flexor muscle group of the knee [43]. More

studies are needed to elucidate the real role of generalized

joint laxity and specific knee joint laxity in the risk of ACL

tears, specifically controlling for neuromuscular factors.

Pelvis and trunk

The female athletes biomechanical profile is a complex

system, and the knee joint should not be considered as an

isolated component to evaluate risk factors for ACL injury.

As a consequence, the trunk, the pelvis, the hip, and the

ankle should be considered in their relationship to resultant

knee joint mechanics. Anterior pelvic tilt places the hip into

an internally rotated, anteverted, and flexed position, which

lengthens and weakens the hamstrings and changes moment

arms of the gluteal muscles [37]. Hamstring muscles are

important to prevent static and dynamic genu recurvatum

and to prevent anterior tibial displacement. Gluteal muscles

are important to assist hip flexion (gluteus maximus) and to

prevent a dynamic valgus collapse (gluteus medius).

Anterior pelvic tilt also increases genu valgus and subtalar

pronation [167]. Genu recurvatum, excessive navicular

drop, and excessive subtalar pronation are more commonly

found in ACL-injured subjects compared to non-ACL-

injured subjects, all factors that have also been related to

ACL preloading [107]. Nevertheless, the exact degree of

anterior pelvic tilt that directly correlates to ACL injury

remains controversial. It is debated whether the risk is

caused by the altered pelvic position itself, or by the func-

tional malalignment it creates [167]. In any case, clinicians

should be mindful that pelvic stability is a key factor for

lower extremity kinematics and kinetics [199, 200].

Torsional anatomic abnormalities are also related to

altered lower extremity biomechanics. Femoral torsion is

defined as the angle between the axis of the femoral neck

and a transverse line through the posterior aspect of fem-

oral condyles [122]. Femoral anteversion, an increase in

the mentioned angle, may cause an inefficiency of the

gluteus medius through a decrease in the internal moment

arm [167]. A weak gluteus medius may influence dynamic

valgus collapse because of the muscles inability to keep

the hip abducted, especially during weight-bearing activi-

ties such as landing, cutting, or changing direction. The

toe-in gait demonstrates the femoral torsion position and is

often associated with increased external tibial torsion [121,

122], which has been related to the functional valgus col-

lapse at the knee joint [141].

The influence of pelvis and trunk mechanics on non-

contact ACL injuries in soccer players needs to be better

characterized. Thus, studies examining the specific role of

the pelvis and trunk in the non-contact ACL injuries in

soccer players are warranted.

Q-angle

Another suggested anatomical factor that has been related

to an increased risk of ACL injury is the quadriceps angle

(Q-angle). The Q-angle is the angle formed by a line

directed from the anterior-superior iliac spine to central

patella and a second line directed from the central patella to

tibial tubercle. A high Q-angle may alter the lower limb

biomechanics [65, 124] and place the knee at a higher risk

to static and dynamic valgus stresses [23]. It was observed

that female basketball players with knee injuries had a

mean Q-angle greater than non-injured players [164].

However, other authors found that static Q-angle measures

710 Knee Surg Sports Traumatol Arthrosc (2009) 17:705729

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do not appear to be predictive of either knee valgus angles,

neuromuscular patterns or ACL injury risk during dynamic

movement [41, 60, 131]. Likewise, Pantano et al. [148]

demonstrated that peak knee valgus during a single leg

squat, and static knee valgus were not significantly greater

in young college athletes with higher Q-angle compared to

those with lower Q-angle. Subjects with a larger Q-angle,

however, had a significantly greater pelvic width to femoral

length ratios compared to subjects with a small Q-angle.

Pelvic width to femoral length ratios was related to both

static and dynamic knee valgus, but static knee valgus was

not related to dynamic knee valgus [148]. The authors

suggested that pelvic width to femoral length ratios, rather

than Q-angle was a better structural predictor of knee

valgus during dynamic movement.

Soderman et al. [170] specifically studied the influence

of the Q-angle in female soccer players of second and third

Swedish divisions. This study was not specifically assess-

ing ACL injuries, but the Q-angle was not associated with

an increased risk of leg injuries. Therefore, the exact role of

the Q-angle in the pathomechanics of ACL injuries needs

further investigation. At this point, there is not enough

evidence to suggest an increased Q-angle as a risk factor

for non-contact ACL injuries in soccer players.

Notch width, ACL size and strength

Gender differences have been associated with ACL struc-

tural properties. Chandrashekar et al. [26] found that ACLs

in women were smaller in length, cross-sectional area,

volume, and mass when compared with that of men. The

authors also demonstrated a lower fibril concentration and

lower percent area occupied by collagen fibrils in females

compared to males. In females, ACL stiffness and modulus

of elasticity were highly correlated to fibril concentration,

whereas in males ACL failure load and strength were

highly correlated to percent area occupied by collagen [64].

Interestingly, ultra structure of ACL has been related to its

mechanical properties. Women may have lower tensile

linear stiffness with less elongation at failure, and lower

energy absorption and load at failure than men [27, 156].

Unfortunately, cadaveric studies may not be generalizable

due to the high risk of potential bias. The behavior of in

vivo body system is more complex than a cadaveric knee.

These studies may be helpful to elucidate future hypothesis

on this issue, but caution must be taken with the current

conclusions.

A smaller intercondylar notch has been positively corre-

lated to injury risk [165, 180]. Controversy exists when

considering femoral intercondylar notch width as a risk

factor for ACL injury. It has been shown that a smaller notch

size is related to a higher risk of ACL rupture in studies with

high number of participants [99, 172]. In less powerful

investigations, femoral intercondylar notch width was not

related to ACL tears [160, 178]. Despite the lack of rela-

tionship between notch width and ACL size [127], a recent in

vivo study reported a significant correlation of the ACL

cross-sectional area to the notch surface area [39]. The

smaller the intercondylar notch the smaller the cross-sec-

tional area of the midsubstance ACL. The explanation for the

increased risk of ACL tear in small notch width subjects is

not fully understood, but it has been suggested that an

impingement of the ACL at the anterior and posterior roof of

the notch may occur during tibial external rotation and

abduction [39, 149]. In addition, sex differences in the

mechanical properties of ACL reported by Hashemi et al. and

Chandrashekar et al. adds more evidence to the small notch-

small ACL-increased risk of ACL rupture relationship.

Posterior tibial slope

Posterior tibial slope is not a clear anatomical risk factor

for ACL injuries. It was first shown that no relationship

was present between non-contact ACL injuries and the

caudal (posterior) slope of the tibia [118]. However, in a

recent publication, Stijak et al. [174] found that ACL-

injured patients had a significantly greater tibial slope of

the lateral tibial plateau and a non-significant lower tibial

slope of the medial tibial plateau compared to the control

group. Both studies were not conducted with soccer players

but with patients. Therefore, further research is needed to

determine whether the posterior tibial slope is a risk factor

for non-contact ACL injuries in soccer players. Studies

assessing posterior tibial slope must control for contralat-

eral tibial slope, intercondylar notch width, knee joint

laxity, lower extremity alignment, neuromuscular, and

biomechanical characteristics.

Foot pronation

Foot pronation and navicular drop have been considered a

risk factor for ACL injuries. Beckett et al. established a

direct relationship between subtalar joint hyperpronation

and ACL tears [8]. The authors compared 50 patients with

past medical history of ACL injury and 50 uninjured sub-

jects. The ACL-injured subjects had greater navicular drop

test scores than uninjured subjects. Later, Woodford-Rogers

et al. compared gymnasts, American Football, and basket-

ball players with history of ACL injury to matched uninjured

athletes. They observed a greater subtalar pronation in the

ACL-injured group [192] results that were also found by

other authors [2]. Also, Loudon et al. compared 20 ACL-

injured females and 20 age-matched controls in a retro-

spective study design. Seven variables were measured:

standing pelvic position, hip position, standing sagittal knee

position, standing frontal knee position, hamstring length,

Knee Surg Sports Traumatol Arthrosc (2009) 17:705729 711

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prone subtalar joint position, and navicular drop test. As

earlier reviewed, knee recurvatum, excessive navicular

drop, and excessive subtalar joint pronation were found to

be significant discriminators between the ACL-injured and

uninjured groups [107]. Unfortunately, foot pronation and

navicular drop are not free of controversy, since other

authors observed contradictory findings [86, 169]. In a

recent study conducted by Jenkins et al. [86], 105 soccer and

basketball players (53 women and 52 men) were recruited

and divided into an ACL-normal group and an ACL-injured

group. Two measures of foot structure (subtalar joint neutral

position and navicular drop test values) were recorded for

each subject. No statistically significant differences were

found in the foot structure measures between women and

men. The authors concluded that values derived from sub-

talar joint neutral position measurement and the navicular

drop test were not associated with ACL injury in collegiate

female and male soccer and basketball players. Addition-

ally, Mitchell et al. [73, 123] recently reported that dynamic

medial foot loading was not related to increased propensity

to demonstrate high ACL-injury risk biomechanics. Subta-

lar joint pronation creates a compensatory increase in the

internal tibial rotation, which has been found to be coupled

with internal tibial rotation at the knee during extension [9,

33]. Normally, subtalar joint pronation and tibial internal

rotation occur only during the contact phase of gait. If

pronation occurs beyond the contact phase, the tibia remains

internally rotated, impeding the occurrence of subtalar joint

supination and tibial external rotation, which normally

occurs as the limb moves through the midstance phase of

gait. This excessive internal tibial rotation transmits

abnormal forces upward in the kinetic chain [8]. Given a

forced movement with planted foot and internal rotation, the

preloaded ACL may be placed to a greater stress that may

evoke to a rupture. Subtalar joint pronation and internal

tibial rotation at the knee may produce an increased internal

femoral rotation and valgus angulation at the knee [153],

enhancing the risk of ACL injury.

Similar to those concerns indicated above related to

posterior tibial slope as a risk factor, studies assessing the

role of foot pronation specifically for soccer players are

needed before clear conclusions can be drawn in this

population.

Hormonal risk factors

Overview

While there has been a significant research focus on sex

hormone relationships to ACL injury, the literature provides

conflicting evidence, which has prevented a strong consen-

sus to be reached on whether ACL injury risk is associated

with specific sex hormone fluctuations. Almost all studies

assessing the hormonal risk factor for non-contact ACL

injuries involve athletes, although not all of them engaged

soccer players. The study of Martineau et al. [112] found that

oral contraceptive use decreased the ligamentous laxity in

female soccer players. However, there is not enough evi-

dence at this point to widely recommend oral contraceptive

use to prevent non-contact ACL injuries in soccer players.

Pilot evidence indicates that it might be a potential control

strategy in the future if strongest evidence is provided

(Table 1).

Sex hormones

It was shown that human ACL cells had both estrogen and

progesterone receptor sites [105]. Furthermore, it was sug-

gested that gender differences for ACL tears may be, in part,

explained by sex hormones. Specifically, hormonal risk

factors are believed to play an important role for non-contact

ACL injuries among female athletes. There are three phases

of the menstrual cycle: follicular (day 09), ovulatory (day

1014) and luteal (day 1528). Disparity of results exists

concerning the time of the menstrual cycle at which the

greatest number of injuries occur: follicular phase [4, 5, 135,

157, 168], around ovulation [1, 14, 187, 188], or the luteal

phase [125]. In a recent systematic review, seven studies

were pooled in an attempt to determine a potential relation-

ship of the menstrual cycle to ACL injury [75]. The seven

reviewed studies favored an effect of the first half, or pre-

ovulatory phase, of the menstrual cycle for increased ACL

injuries. The six studies that stratified the non-oral contra-

ceptive and oral contraceptive data also favored an effect of

the first half of the menstrual cycle for increased ACL inju-

ries. The authors concluded that the clinical relevance of this

finding is that female athletes may be more predisposed to

ACL injuries during the pre-ovulatory phase of the menstrual

cycle, which is consistent with the estrogen surge seen during

this phase of the cycle [75]. Not all hormonal studies com-

pared to a control group nor stratified for oral versus non-oral

contraceptive use. Both aspects are crucial to test the

hypothesis that sex hormones are a potential risk factor for

non-contact ACL injuries.

Effects on laxity

Sex hormones have also been related to an increased

anterior knee laxity. Zazulak et al. [201] conducted a sys-

tematic review on the effects of menstrual cycle on anterior

knee laxity. The authors included nine studies, and they

observed that six of them reported no significant effect of

the menstrual cycle on anterior knee laxity in women.

However, three studies observed significant associations

between the menstrual cycle and anterior knee laxity.

712 Knee Surg Sports Traumatol Arthrosc (2009) 17:705729

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These studies all reported an increased knee laxity during

the ovulatory or post-ovulatory phases of the cycle. A

meta-analysis, which included data from the nine reviewed

studies, corroborated the significant effect of cycle phase

on knee laxity. In the analyses, the knee laxity data mea-

sured at 1014 days was greater than at 1528 days, which

was greater than at 19 days. Hicks-Little et al. [76] also

found that the ovulation and luteal phases of the menstrual

cycle significantly increased anterior displacement about

the knee. Oral contraceptive use was found to decrease the

ligamentous laxity in female soccer players [112] and to

lower the traumatic injury rate [3, 125, 187]. In another

study, female athletes on oral contraceptives demonstrated

decreased impact forces and reduced medial and lateral

torques at the knee, increased hamstrings to quadriceps

strength ratios, increased stability on one leg and decreased

knee laxity relative to non-users [70]. For this sample, the

use of oral contraceptives appeared to increase the dynamic

stability of the knee joint. These results suggest that

hormonal stabilization increases dynamic stability of the

female athletes knee, and may reduce the risk of serious

knee injury in this high-risk athlete [70]. In contrast, others

have demonstrated a tendency to increase anterior tibial

displacement in oral contraceptive users compared to those

not using hormonal replacement therapy [76].

Effects on ACL tensile strength

Estrogen and progesterone have been found to affect the

collagen metabolism in both animal models and humans.

Essentially, estrogen (i.e., estradiol) decreased fibroblast

proliferation and type I pro-collagen synthesis whereas

progesterone levels attenuated estrogen inhibitory effect on

collagen metabolism of female ACLs, both in a dose- and

time-dependent manner [197, 198]. Controversy also exists

due to a disparity of results among animal models. Further

research is needed to better elucidate the concentration and

time dependency effects of estrogen exposure, as well as of

other sex hormones, with respect to the ACL tissue [166].

Sex hormones have also been reported to affect tensile

properties of ligaments [18, 92, 193], but other authors

found no significant differences in maximum force, stiff-

ness, energy to failure, or failure site of ACLs in sheep

[175]. Influence of sex hormones on mechanical properties

of ligaments has been only studied in animal models.

Further research is also needed to better establish the

influence of estrogen and other hormones on biomechani-

cal properties of ligaments.

Effects on neuromuscular function

Neuromuscular function seems to also be affected by sex

hormones. During the ovulatory phase, there was an

increase in quadriceps strength, a decrease in muscle

relaxation time, and an increase in muscle fatigability in

young healthy relatively sedentary females [159]. Sex

hormones also decrease motor coordination [152] and have

effects on isokinetic strength, anaerobic and aerobic

capacity, and high-intensity endurance in female athletes

[100]. Interestingly, Chaudhari et al. [31] investigated knee

and hip loading patterns at different phases in the menstrual

cycle. The authors compared performance on horizontal

jump, vertical jump, and drop from a 30-cm box on the left

leg between women (half of them taking oral contracep-

tive) and men. Men were tested once whereas women were

tested twice for each phase of the menstrual cycle (follic-

ular, ovulatory, luteal), and lower limb kinematics (foot

strike knee flexion) and peak externally applied moments

were calculated (hip adduction moment, hip internal

rotation moment, knee flexion moment, knee abduction

moment). No significant differences in moments or knee

angle were observed between phases in either female group

or between the two female groups (oral contraceptive users

and non-oral contraceptive users) or between either of

female groups and the male controls. The authors con-

cluded that variations of the menstrual cycle and the use of

an oral contraceptive do not directly effect knee or hip joint

loading during jumping and landing tasks [31]. Because

knee and hip joint loading was unaffected by cyclic vari-

ations in hormone levels, the observed difference in injury

rates was thought to be more likely attributable to persis-

tent differences in strength, neuromuscular coordination,

or ligament properties. As stated, sex hormones as a risk

factor for ACL injury is an attractive and promising area of

research. Nevertheless, there is still equivocal evidence on

many topics, and future research is again needed in this

area to better prevent, at least in part, many ACL injuries.

Neuromuscular risk factors

Overview

Neuromuscular control refers to unconscious activation

of the dynamic restraints surrounding a joint in response

to sensory stimuli [61]. The neuromuscular system gener-

ates movement and determines biomechanics of playing

actions. Unconscious muscle activation is crucial during

many actions in sport, and differences in neuromuscular

control may explain, in part, the increased ACL injury risk

exhibited by a certain cohort of soccer players [141]. Olsen

et al. [141] reported that team handball players were often

judged by the coaches to be out of balance, and in the

majority of cases, some form of perturbation (often contact

with another player) appeared to have altered the players

coordination or intended movement at the time of injury.

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Landing, cutting, and pivoting maneuvers in some females

have been shown to differ from males [51, 52, 115].

Essentially, female soccer players perform playing actions

with increased adduction and internal rotation of the femur,

reduced hip and knee flexion angles, increased dynamic

knee valgus, increased quadriceps activity (with a con-

comitant decrease in hamstring activity), and decreased

muscle stiffness around the knee joint [69].

Relative strength and recruitment

Dynamic stabilization via the neuromuscular control sys-

tem helps to protect the knee joint during dynamic sport-

related tasks [13, 104, 183185]. However, muscle actions

must be coordinated and co-activated in order to protect the

knee joint [185]. Hence, antagonistagonist relationships

are crucial for joint stability. For the knee joint, co-acti-

vation of hamstrings and quadriceps may be critical to

prevent or to reduce knee motion and loads that increase

the risk of ACL injury. Hamstring recruitment reduces

ACL loads from quadriceps [155, 185], and may help to

provide dynamic knee stability by resisting anterior and

lateral tibial translation and transverse tibial rotations

[104].

In vivo studies where a strain gauge was placed into

an intact ACL at the time of surgery demonstrated that

rehabilitation exercises that produced an isolated con-

traction of the quadriceps muscle near extension strained

the ACL more than exercises with co-contraction of both

quadriceps and hamstrings [48]. Specifically, the quad-

riceps muscle cause peak strain to the ACL around 30of knee flexion [10]. Women may have an imbalance

between muscular strength, flexibility, and coordination

within their lower extremities [90, 132]. Deficits in rel-

ative hamstring strength may contribute to increased risk

of ACL injury in soccer players. Colby et al. [32]

investigated quadriceps and hamstring muscles activation

patterns and determined the knee flexion angle during

the eccentric motion of sidestep cutting, cross-cutting,

stopping, and landing in healthy collegiate and recrea-

tional male and female athletes. The results indicated

that there is high-level quadriceps muscle activation

beginning just before foot strike and peaking in

mid-eccentric motion. Hamstring muscle activation was

submaximal at and after initial contact. The maximum

quadriceps muscle activation for all maneuvers was

161% of the maximum voluntary contraction, while

minimum hamstring muscle activity was 14%. Foot

strike occurred at an average of 22 of knee flexionfor all maneuvers. This low level of hamstring muscle

activity and low angle of knee flexion at foot strike

during eccentric contraction, coupled with relatively

unopposed forces generated by the quadriceps muscles at

the knee, could produce significant anterior displacement

of the tibia, which may play a role in ACL injury [32].

Chappell et al. [28] found that female soccer, basketball,

and volleyball players prepared for landing with

increased quadriceps activation and decreased hamstring

activation, which may result in increased ACL loading

during the landing of the stop-jump task and the risk for

non-contact ACL injury. Also, Padua et al. [147] found

an increased quadriceps and soleus activation during

hopping as well as a decreased hamstrings to quadriceps

activation ratio in women compared to men (both were

active subjects with previous recreational experience in

soccer, basketball, and volleyball). Hewett et al. dem-

onstrated that a plyometric training reduced the peak

landing forces and increased hamstring torques at landing

in female volleyball players [74]. The decrease in land-

ing forces implies that less force is transmitted to the

knee articulations and passive structures; therefore, more

energy is being absorbed by active muscular restraints.

In contrast, weak hamstrings contribute to a greater

ground reaction forces that place the ACL at a higher

risk of rupture [71]. In addition, adduction and abduction

moments at the knee significantly decreased after plyo-

metric training and were the sole significant predictors of

peak landing force. A decreased adduction and abduction

moment would decrease the risk of femoral condylar lift-

off from the tibial plateau [74]. On the other hand, peak

landing flexion (reflecting net quadriceps muscle activity)

and extension moments (reflecting net hamstrings muscle

activity) at the knee did not change after training and

were not significant predictors of peak landing force.

The plyometric training also increased the hamstring-to-

quadriceps muscle ratio by increasing the hamstring

muscle peak torque. As reviewed, soccer players dem-

onstrated significantly less anterior and anteriorposterior

knee laxity and higher isokinetic strength of the knee

flexors and extensors compared to sedentary controls

[43], what adds more evidence on the dynamic stabilizer

function of muscles. Moreover, Myer et al. [129]

recently found that female soccer and basketball players

sustaining ACL injuries had a combination of similar

quadriceps strength with decreased hamstring strength

compared to males. In direct contrast, female athletes

who did not go on to ACL injury had decreased quad-

riceps strength and similar hamstring strength compared

to matched male athletes. Female soccer and basketball

players who demonstrate increased relative quadriceps

strength and decreased relative hamstring strength may

be at increased risk for ACL injury. Hence, preseason

and continued in-season conditioning focused on ham-

string strengthening may be indicated for female soccer

players, who fall into this high risk unbalanced profile

[129].

714 Knee Surg Sports Traumatol Arthrosc (2009) 17:705729

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Relative joint stiffness and stability

Hamstring muscles are important to decrease anterior shear

forces and greatly reduce load on the primary restraint to

anterior tibial motion, the ACL [7, 126]. It was found that

increasing hamstring muscle force during the knee flexion

phase of a simulated jump landing significantly reduced the

peak relative strain in the ACL in vitro [185]. Through

knee joint compression, hamstrings limit anterior tibial

translation by allowing the concave medial tibial plateau to

limit anterior drawer [82] and by allowing more of the

valgus load to be carried by articular contact forces, pro-

tecting the ligaments [71]. Moreover, hamstring compres-

sion could protect against torsional loading, which has been

found to be greater for females compared to males [104,

189]. In fact, a vigorous quadriceps contraction has been

shown to induce ACL rupture in cadavers [38]. Women

demonstrate decreased hamstrings-to-quadriceps peak tor-

que ratios and increased knee abduction (valgus) moments

compared to males [71]. Hamstring muscles are activated

by ACL receptors when the ligament is placed under stress,

what adds more evidence to the hamstrings provide

agonistic support to the ACL. It was also suggested that

hamstrings are activated by an alternative reflex arc

unrelated to ACL receptors [171]. This ACL receptor-

dependent muscle activation suggests that decreased pro-

prioception could have an impact on knee stability. The

hamstring activation depending on the ability of the ACL

to sense a torque and elongation may justify the inclusion

of proprioception training in preventive and rehabilitation

programs [171].

Muscles crossing a joint provide stability to that joint. In

other words, muscle stiffness, or the resistance to dynamic

stretch may protect ligaments from rupture when a load is

applied. As reviewed, quadriceps and hamstring muscles

provide anteriorposterior joint stiffness. Others suggest

that sagittal plane knee joint stiffness is also relevant for

ACL injury prevention. Studies demonstrate that female

athletes show less muscular stiffness than their male

counterparts [58, 59, 67, 79, 88, 161, 186, 189]. Males

activate their lower extremity muscles significantly earlier

[67], and have longer activation duration in muscles that

initiated and maintained knee (gastrocnemius) and lower

extremity stiffness (gluteus) than women [88]. Decreased

muscular stiffness in females was shown for both anterior

tibial translation [58, 59, 81, 88, 186] and rotational forces

[58, 59, 161, 189]. In a recent study, Schmitz et al. [161]

investigated the varus/valgus and internal/external tor-

sional knee joint stiffness in both males and females. Knee

kinematics of 20 university students were measured while

applying 010 Nm of varus and valgus torques with the

subject non-weight-bearing, and 05 Nm of internal and

external torques in both non-weight-bearing and weight-

bearing conditions, with the use of a custom joint testing

device. When low magnitudes of torque were applied to the

knee, women had significantly lower stiffness values than

did men. With the exception of applied external torque

with the joint weight-bearing and varus torque with the

joint non-weight-bearing, women demonstrated an increase

in joint stiffness as the magnitude of torque increased from

lower to higher magnitudes. In contrast, for the men, joint

stiffness values remained unchanged as the magnitude of

applied torque increased. The authors concluded that

women exhibited lower knee stiffness in response to low

magnitudes of applied torque compared to men and dem-

onstrated an increase of joint stiffness as the magnitude of

applied torque increased [161].

Muscular fatigue

Since muscles contribute to joint stability, muscular fatigue

might be a risk factor for ligament injuries. Fatigued

muscles are able to absorb less energy before reaching the

degree of stretch that causes injuries [108]. Better condi-

tioned soccer players may have improved neuromuscular

control later in games relative to de-conditioned athletes.

This improved neuromuscular control may help athlete to

better absorb energy, leaving less energy to be absorbed by

other structures such as ligaments [158]. Under fatigued

conditions, it was shown that males and females decrease

knee flexion angle and increase proximal tibial anterior

shear force and knee varus moments when performing

stop-jump tasks [29]. Nyland et al. [140] investigated the

effect of quadriceps and hamstrings fatigue from eccentric

work on the activation onset of vastus medialis, rectus

femoris, vastus lateralis, the medial hamstrings, biceps

femoris, and gastrocnemius muscles in healthy female

athletes compared with controls directly after performing

crossover cut training. The authors demonstrated that

quadriceps fatigue from eccentric work produced earlier

gastrocnemius and delayed quadriceps femoris activation

during crossover cutting in female athlete compared to

controls, but activation onset did not differ compared to

hamstring fatigue. Neither hamstring nor quadriceps

femoris fatigue produced differences in medial hamstring

or biceps femoris activation onset compared to controls.

The authors concluded that the gastrocnemius muscles act

as a synergistic and compensatory dynamic knee stabilizer

in a closed kinetic chain situations as the quadriceps

femoris muscles fatigue [140]. Conversely, Fleming et al.

[49] demonstrated that the gastrocnemius muscle is an

antagonist of the ACL. Six subjects with normal ACLs

participated in the study. Subjects underwent spinal anes-

thesia to ensure that their leg musculature was relaxed.

Transcutaneous electrical muscle stimulation was used to

induce contractions of the gastrocnemius, quadriceps and

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hamstrings muscles, while the strains in the anteromedial

bundle of the ACL were measured using a differential

variable reluctance transducer. The ACL strain values

produced by contraction of the gastrocnemius muscle were

dependent on the magnitude of the ankle torque and knee

flexion angle. Co-contraction of the gastrocnemius and

quadriceps muscles produced ACL strain values that were

greater than those produced by isolated activation of either

muscle group when the knee was at 15 and 30. Co-con-traction of the gastrocnemius and hamstrings muscles

produced strains that were higher than those produced by

the isolated contraction of the hamstrings muscles. At 15

and 30 of knee flexion, the co-contraction strain valueswere less than those produced by stimulation of the gas-

trocnemius muscle alone [49]. Landry et al. [97, 98]

demonstrated that elite female soccer players exhibit an

increased gastrocnemius activity during unanticipated run,

side-cut, and cross-cut maneuvers. Female soccer players

demonstrated a higher gastrocnemius activity and a medi-

olateral gastrocnemius activation imbalance during early

stance to midstance of the side-cut and during both run and

cross-cut maneuvers that was not present in the male

players. Additionally, for unanticipated side-cut maneu-

vers, female athletes demonstrated greater rectus femoris

muscle activity throughout stance, and the only hamstring

difference identified was a mediolateral activation imbal-

ance in male athletes only [98]. Moreover, for unantici-

pated run and cross-cut maneuvers, rectus femoris activity

and vastus medialis and lateralis activity for the straight run

only were also greater in female than in male athletes [97].

Other notable difference captured for both maneuvers

included female players having reduced hamstring activity

compared to male players. Padua et al. [147] also observed

greater soleus activation during hopping in healthy women

compared to men.

Nyland et al. [139] further investigated the effects of

hamstring fatigue on transverse plane knee control during

a running crossover cut directional change (functional

pivot shift). The authors found that an eccentric work-

induced hamstring fatigue created decreased dynamic

transverse plane knee control as evidenced by increased

knee internal rotation during impact-force absorption, an

earlier peak ankle plantar-flexor moment onset, and a

decreased knee internal rotation with propulsion during

hamstring fatigue. It was suggested that this pattern may

represent compensatory attempts at dynamic knee stabil-

ization from the posterior lower leg musculature during

the reportedly ligamentous stressful functional pivot shift

phase of the crossover cut [139]. In turn, other authors

found an increased anterior tibial translation with mus-

cular fatigue in healthy knees [119, 190]. However,

Wojtys et al. [190] found that the recruitment order of the

lower extremity muscles in response to anterior tibial

translation did not change with fatigue. Melnyk and

Gollhofer [119] assessed reflex latencies and neuromus-

cular hamstring activity using surface electromyography.

Muscle fatigue produced a significant longer latency for

the monosynaptic reflex latencies, whereas no differences

in the latencies of the medium latency component were

found. Fatigue significantly reduced EMG amplitudes of

the short and medium latency components. The authors

suggested that a reduced motor activity rather than the

extended latency of the first hamstring response is the

reason for possible failure. McLean et al. [113] also

investigated the impact of fatigue on ACL injury risk. Ten

males and ten females were compared performing a land

from a jump. Females landed with more initial ankle

plantar flexion and peak-stance ankle supination, knee

abduction, and knee internal rotation compared with men.

They also had larger knee adduction, abduction, and

internal rotation, and smaller ankle dorsiflexion moments.

Fatigue increased initial and peak knee abduction and

internal rotation motions and peak knee internal rotation,

adduction, and abduction moments, with the latter being

more pronounced in females. Therefore, McLean et al.

concluded that fatigue-induced modifications in lower-

limb control may increase the risk of non-contact ACL

injury during landings. Gender dimorphic abduction

loading in the presence of fatigue also may explain the

increased injury risk in women [113].

Decision-making (i.e., anticipated and unanticipated

actions), in addition to fatigue, has been shown in isola-

tion to directly impact ACL injury risk [11, 78, 151]. For

example, Besier et al. [11] examined a sidestep cut at two

different angles under both anticipated and unanticipated

conditions and found increased varusvalgus and internal

external knee moments during unanticipated movements.

The authors suggested that the increased coronal plane

torques increased the potential for ACL injuries during

unanticipated movements. Lower extremity muscle acti-

vation during cutting is significantly different between

anticipated and unanticipated conditions [11]. Recently,

the effects of a combination of fatigue and decision-

making on landing postures were investigated. Borotikar

et al. [19] studied the combined effects of fatigue and

decision-making on lower limb kinematics during sports

relevant landings. Fatigue caused significant increases in

initial contact hip extension and internal rotation, and in

peak stance knee abduction and internal rotation and

ankle supination angles. Fatigue-induced increases in

initial contact hip rotations and in peak knee abduction

angle were also significantly more pronounced during

unanticipated compared to anticipated landings. It was

suggested that the integrative effects of fatigue and

decision-making may represent a worst case scenario in

terms of ACL injury risk during dynamic single leg

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landings, by perpetuating substantial degradation and

overload of central control mechanisms [19]. Addition-

ally, Olsen et al. [141] reported that ACL injuries

occurred when team handball players were out of balance,

or some form of perturbation (often contact with another

player) altered the players coordination. Laboratory

studies do not correlate with field studies at all. Fauno

and Wulff Jakobsen [45] reported that ACL injuries in

second half is not statistically different from first half, so

fatigue was not seen as a risk factor by the authors. Thus,

it appears that fatigue may contribute to other risk factors,

but may not in itself be an isolated risk factor for ACL

injury.

Biomechanical risk factors

Overview

Biomechanics of playing actions are necessary to under-

stand the pathomechanics of ACL injuries and to offer

effective prevention programs. It was postulated that hip

low forward flexion, hip adduction, hip internal rotation,

knee valgus, knee extension, and knee external rotation

may place the ACL to a high risk of rupture. It was called

the position of no return [84]. Biomechanical studies

have been conducted in cadavers, in vivo through strain

gauges placed at the time of surgery, and from analytical

modeling. Biomechanical risk factors for ACL injuries

have been described in all three planes. Abundant data

exists considering biomechanical risk factors in athletes.

Specific biomechanical studies involving soccer players do

also exist, especially in females.

Sagittal plane

Sagittal plane biomechanics have yielded many studies on

trunk, hip, knee, and ankle flexion angles when performing

sport tasks. The more joints are flexed during landing, the

more the energy is absorbed and the less the impact is

transferred to the knee. Also, the ACL and hamstring

anatomy explain why knee flexion is protective of ACL

damage. Every movement with influences over knee flex-

ion can contribute to ACL injury. From proximal to distal,

Blackburn and Padua [16] demonstrated that increased

trunk flexion during landing also increased hip and knee

flexion angles. The authors found that trunk flexion altered

neither transverse nor frontal plane kinematics during the

landing task. A less erected posture during landing has

been associated with a reduced ACL injury risk [61, 68,

89].

Hewett et al. [73] reported a significant increased peak

external hip flexion moment in ACL injured compared to

uninjured females soccer, basketball, and volleyball play-

ers, but was not observed to be a significant predictor of

ACL injury. These data suggest ACL-injured athletes had

an increased internal hip extensor moment due to an

increased gluteus maximus activity. Conversely, Decker

et al. [35] suggested that a decreased hip musculature

activity may produce a higher ground reaction force,

because muscles would be used to absorb energy from a

certain task. Landry et al. [97, 98] studies showed that elite

female soccer players exhibited a reduced external hip

flexion moment and hip flexion angle during unanticipated

side-cut, run, and cross-cut maneuvers. Similarly, Zazulak

et al. [202] found a decreased gluteus maximus activity in

females during single-legged landings.

Female soccer players demonstrate decreased hip and

knee flexion angles at landing compared to male soccer

players after the age of 13-year-old [196]. Young female

soccer, basketball, and volleyball players also showed

decreased hip and knee flexion angles compared to males

during the landing preparation of a vertical stop-jump task

[28]. Resultant initial contact lower extremity motion pat-

terns during landing of the stop-jump task may be pre-

programed just prior to landing. Therefore, female subjects

prepared for landing with a decreased hip and knee flexion

angle which may result in increased ACL loading during

the landing of the stop-jump task and the risk for non-

contact ACL injury [28]. It was postulated that a decreased

hip and knee flexion angles at landing places the ACL at a

greater risk of injury, because a greater peak landing force

is transmitted to the knee [74]. Yu et al. [195] showed that

hip and knee flexionextension angular velocity, rather

than angle or joint position, was correlated to the peak

posterior and vertical ground reaction forces at landing

from a stop-jump task. On average, females landed with

greater impact forces and had smaller hip and knee flexion

angles at the initial foot contact with the ground and

maximum knee flexion angle at the end of the landing. The

greater the hip and knee flexion angular velocity at the

initial foot contact during the landing of a stop-jump task,

the lesser the posterior and vertical ground reaction forces.

Also, the greater the peak proximal tibia anterior shear

force and peak knee extension moment during landing, the

greater the posterior and vertical ground reaction force.

Therefore, decreased hip and knee flexion angles at landing

as a risk factor for ACL injury may not be resultant from

increased ground reaction force [195]. Instead, the

increased risk of ACL injury from decreased knee flexion

angle could be explained by differences in ACL elevation

angle, the angle of insertion of the hamstrings, and by

differences in patellar tendontibial shaft angle. Near knee

extension, the ACL has a greater elevation angles, so the

ligament is more perpendicular to a tibial plateau line,

whereas the ACL is essentially parallel to the tibial plateau

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with knee flexion past 90 [103]. This change in orientationinfluences the load placed on the ACL and its ability to

sustain elastic deformation without injury [16]. The struc-

tural properties of the ACL are maximized under tensile

(longitudinal) loading conditions and minimized under

non-axial (shear) loading conditions [191]. As the knee

progresses into extension, the ACL elevation angle is

maximized. Under this configuration, the anterior tibial

shear force generated by the quadriceps/patellar tendon and

imparted to ACL is increasingly shear in nature. Con-

versely, as the ACL elevation angle decreases with knee

flexion, the shear component of the resultant ACL force

decreases while the tensile component increases recipro-

cally [16]. Additionally, as the knee progresses into flexion,

the angle of insertion of the hamstrings with respect to the

tibial longitudinal axis increases such that at knee flexion

angles greater than 100, the resultant hamstring force isdirected parallel to the tibial plateau [16, 204]. On the other

hand, at lower degrees of knee flexion, the angle of

insertion of the hamstrings with respect to the tibial lon-

gitudinal axis decreases such that the resultant hamstring

force is directed parallel to the ACL, which is placed

perpendicular to the tibial plateau, thus limiting the ham-

strings potential to counteract anterior tibial strain to the

ACL. Also, an extended lower limb at landing may strain

the ACL due to a greater patellar tendontibial shaft angle

that increases the anteriorly directed component of the

force produced by the quadriceps muscle [194]. As the

knee progresses into flexion, the patellar tendon insertion

angle with respect to the tibial longitudinal axis decreases

[204]. This change in patellar tendon orientation has a

profound influence on tibial shear force, as the anteriorly

directed component of the quadricepspatellar tendon force

is derived as a multiple of the sine of the insertion angle

[16]. Hence, at lower knee flexion angles, the quadriceps

exerts a higher anteriorly directed force that is poorly

counteracted by both the ACL and the hamstrings. Addi-

tionally, the maximum quadriceps force is estimated to be

produced around 60 of knee flexion [203]. Therefore, itmight be argued that an extended position around 20 ofknee flexion may produce less impact absorption through

the musculotendinous system increasing the force trans-

mitted to the passive structures of the knee. A large hip and

knee flexion angles at the initial foot contact with the

ground do not necessarily reduce the impact force during

landing, but active hip and knee flexion motions do [195].

It was also demonstrated that females had increased

quadriceps activation before landing from the same task

compared with male subjects [195]. Yu et al. also found

that female athletes exhibited an increased hamstring

activation before landing but a trend of decreased ham-

string activation after landing compared with male sub-

jects, whereas Krosshaug et al. [94] showed an increased

knee flexion angle both at initial contact and 50 ms after

initial ground contact in female basketball players com-

pared to males. Landing preparation with increased quad-

riceps activation may increase ACL loading during

landing, since muscle activation is a major determinant of

muscle contraction force [87]. A greater quadriceps acti-

vation [109] and increased knee extension moment [30]

during landing of a variety of athletic tasks in female

athletes compared to their male counterparts was also

observed by other authors. Hewett et al. [73] found similar

knee flexion angles at initial contact between ACL injured

and uninjured female athletes in a prospective study. The

peak knee flexion moment was also similar; however,

maximum knee flexion angle was lower in the ACL-injured

group compared to uninjured controls. Several authors

report that isolated sagittal plane forces are not high

enough to tear the ACL during sports [114, 150]. In

addition, there is no consensus on whether females land or

cut with less [80, 109], same [52, 114, 116], or greater [44,

94] knee flexion angles compared to males.

Ankle joint is a key component in any movement in

closed kinetic chain. Therefore, ankle sagittal plane

movements have also an involvement in the knee joint. Self

and Paine [163] showed that landing technique with the

largest ankle plantar-flexion position at ground contact

demonstrated the most shock absorption and reduction of

the peak vertical ground reaction force. It was recently

demonstrated that landing with the rear foot (dorsiflexed

ankle) was associated with less hip and knee flexion at peak

vertical ground reaction force than forefoot landing (plan-

tarflexed ankle) [34]. Also, the maximum knee flexion

angle with forefoot landing technique was significantly

higher than the rear foot technique. In contrast, hip and

knee flexion angles at initial foot contact were significantly

lower with the forefoot than rear foot technique. Given that

the maximum force transferred to the knee would be at the

peak vertical ground reaction force, a forefoot landing

might be preferred over rear foot technique. No significant

differences for foot-landing technique were observed

between males and females [34]. However, Burkhart et al.

[25] reported in a prospective research study that an athlete

who landed with an increased heel to flat-foot loading

mechanism was more likely to sustain to a non-contact

ACL injury during competitive play.

Coronal plane

Coronal plane biomechanics are also involved in the gen-

esis of non-contact ACL injuries. Houck et al. [78] inves-

tigated coronal plane trunk/hip kinematics and hip and

knee moments (measures of neuromuscular control) during

unanticipated compared to anticipated straight and side

step cut tasks. Hip angles, but not lateral trunk flexion,

718 Knee Surg Sports Traumatol Arthrosc (2009) 17:705729

123

were altered during unanticipated conditions. Lateral trunk

flexion remained near 810 for both anticipated andunanticipated tasks, what was explained by a compensatory

decrease in the left lateral tilt orientation of the pelvis

(taken at the same instant in stance as lateral trunk orien-

tation) during the sidestep unanticipated task by 23compared to both the anticipated and unanticipated straight

step task, and anticipated sidestep task. Lateral trunk ori-

entation was not a result of lateral trunk flexion, suggesting

even during unanticipated tasks the pelvis and thorax

rotated as a single segment. The authors interpreted that hip

abduction angles and foot placement, not lateral trunk

flexion influence trunk orientation [78]. Zazulak et al. [199]

investigated the effects of isolated trunk displacement after

a perturbation in a laboratory study as a predictor of knee

ligament injury. Twenty-five athletes (11 female and 14

male) sustained knee injuries over a 3-year period. Trunk

displacement in any plane was greater in athletes with

knee, ligament, and ACL injuries than in uninjured ath-

letes. Lateral displacement was the strongest predictor of

ligament injury. Trunk displacements, proprioception, and

history of low back pain predicted knee ligament injury

with 91% sensitivity and 68% specificity. This model

predicted knee, ligament, and ACL injury risk in female

athletes with 84, 89, and 91% accuracy, but only history of

low back pain was a significant predictor of knee ligament

injury risk in male athletes [199]. Therefore, core stability

may be an important component of ACL injury prevention

programs.

Hip angles during landing can be important determi-

nants of impact force at the knee [71]. Female soccer,

basketball, and volleyball players may have an increased

external adduction moment about the hip at landing.

Increased adduction moment about the hip may place an

increased valgus stress over the knee [51, 73], but the hip

adduction itself was not demonstrated to be a risk factor

for ACL injury [73]. Hip abduction angles and knee

moments were significantly affected by the type of task

and anticipation. Hip abduction angles decreased by 4.0

7.68, when comparing the unanticipated sidestep task tothe anticipated straight step, unanticipated straight step,

and anticipated sidestep tasks [78]. The hip abduction

angles were associated with foot placement and lateral

trunk orientation. The unanticipated sidestep task reduced

the distance between contact point and the trunk center of

mass, probably to perform faster, influencing the whole

kinetic chain proximally. During landing preparation of a

stop-jump task, young female soccer, basketball, and

volleyball players demonstrated a decreased hip abduction

compared to their male counterparts [28]. Jacobs et al.

[85] observed that healthy women had lower hip abductor

peak torque than healthy men. Hip abduction peak torque

correlated moderately with hip flexion peak joint

displacement in women, and most importantly hip flexion

and adduction peak joint displacement increased after a

30-s bout of isometric hip abduction exercise. It was also

observed that squat strength positively correlated with hip

abduction strength in female soccer players but not for

men [181]. The authors suggested hip abduction strength

assessment as a potential method to identify those sub-

jects at risk of ACL injury at landing. Imwalle et al. [83]

evaluated lower extremity motions in females performing

unanticipated cutting task that directed their running

angles to 45 and 90. They reported that hip internalrotation and knee internal rotation were increased during

the 90 cut compared to the 45 unanticipated cutangle. Mean hip flexion was also greater in the 90 cut.However, the only significant predictor of knee abduction

during both tasks was hip adduction. The authors

suggested that findings indicate that the mechanisms

underlying increased knee abduction measures in female

athletes during cutting tasks were primarily coronal plane

motions at the hip [83].

Coronal plane knee biomechanics are also related to

ACL injury. Hewett et al. [73] conducted a prospective

study where 205 female athletes participating in the high-

risk sports of soccer, basketball, and volleyball were

measured for neuromuscular control using three-dimen-

sional kinematics (joint angles) and joint loads using

kinetics (joint moments) during a jump-landing task. Nine

athletes had a confirmed ACL injury. Knee abduction

angle at landing was 8 significantly greater in ACL-injured athletes compared to uninjured athletes. In addi-

tion, ACL-injured subjects had 2.5 times greater knee

abduction moment and 20% higher ground reaction force

compared to uninjured subjects. Also, women have

exhibited greater valgus moments than men during the

landing phase of each stop-jump task [30]. Additionally,

increased motion, force, and moments occurred more

quickly in the injured compared to uninjured athletes [73].

For cutting maneuvers, Ford et al. [52] demonstrated that

females exhibited greater knee abduction (valgus) angles

compared with males.

There is a scarcity of studies on coronal plane ankle

biomechanics, but the very few studies demonstrate similar

results. Ford et al. [52] demonstrated that female basketball

players had greater maximum ankle eversion than did male

athletes during the stance phase of cutting. Similarly,

Landry et al. [97] found an increased ankle eversion angle

throughout stance in elite female soccer players compared

with male players for unanticipated run and cross-cut

maneuvers. It was already reviewed that ankle kinematics

influence knee joint. Excessive ankle eversion may

increase internal tibial rotation, knee valgus stress, anterior

tibial translation, and loading on the ACL during extension

[9, 33, 71, 153].

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123

Transverse plan

Transverse plane biomechanics have been focused on hip

and knee joints. Hip biomechanical findings mainly refer to

a greater hip internal rotation maximum angular displace-

ment [102] and a lower gluteal EMG activity [202] at

landing in female soccer, basketball, and volleyball players

compared to males. When performing unanticipated side-

cut maneuvers, female soccer players exhibited more hip

external rotation compared with the male athletes [98].

When performing unanticipated cutting tasks at 90 unan-ticipated cut angles, females increased hip internal rotation

and knee internal rotation relative to the 45 unanticipatedcut angle. However, the changes in transverse planes

motion were not related to concomitant coronal plane

motions at the knee [83].

Transverse plane knee biomechanics depend on the type

of task and decision-making. Besier et al. [12] showed

that varus/valgus and internal/external rotation moments

applied to the knee during sidestepping and crossover

cutting were considerably larger than those measured

during normal running. The same group found that cutting

maneuvers performed without adequate planning may

increase the risk of non-contact knee ligament injury due to

an increased internalexternal rotation moments applied to

the knee [11]. The authors attributed these results to the

small amount o