alentorn-geli et al 2009. prevention of non-contact anterior cruciate ligament injuries in soccer...
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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: [email protected]
123
Knee Surg Sports Traumatol Arthrosc (2009) 17:705729
DOI 10.1007/s00167-009-0813-1
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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
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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
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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
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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
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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,
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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.
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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].
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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,
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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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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