Saint Xavier University

A review of the anatomy and current research on the female predisposition to noncontact anterior cruciate ligament injuries

Honors ProjectSubmitted

In Partial Fulfillment of theRequirements of HONOR 352/53and for Graduation with Honors

Spring 2015

By:Suzanne Broski

Mentor:Dr. Rudyard Sadleir

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Thesis written by

Suzanne Broski

Approved by

________________________________________________, Mentor

Accepted by

__________________________________________, Honors Program Director

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Abstract

Adolescent female athletes represent the population that is most likely to suffer from

noncontact anterior cruciate ligament (ACL) injuries, with a 4-6 times greater risk than their

male counterparts. Following an explanation of the normal anatomy and functioning of the ACL

and the mechanism of ACL injury, this paper explores possible causes of the gender disparity

surrounding ACL injury rates. Specifically, research studies regarding the differences between

male and female anatomical structure, biomechanics, neuromuscular functioning, genetics, and

hormones are considered. Despite attempts to conclude a singular cause of the increased risk of

noncontact ACL injury amongst female athletes, the gender disparity is likely multifactorial.

Therefore, prevention programs implemented to reduce the risk of female ACL injury should

target the modifiable risk factors which include biomechanical and neuromuscular functioning.

Ideally, these programs should be started as soon as a female begins participating in athletics in

order to avoid developing maladaptive techniques that may put her at risk of ACL injury.

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INTRODUCTION

The anterior cruciate ligament (ACL) is a ligament in the knee that serves a stabilizing

function. In the United States, approximately 1 of every 3,500 individuals injures their ACL

annually, and an estimated 125,000-200,000 ACL reconstructions are performed each year.

Worldwide, an estimated one million ACL injuries occur every year (Noyes & Barber-Westin,

2012). ACL injury is very commonly suffered during athletic participation as a result of indirect

force to the knee, such as sudden changes in direction or speed. The consequences of injury to

the ACL are severe, especially for competitive athletes; these athletes may potentially miss an

entire season or more of their sport, lose scholarship funding or wages, suffer psychologically,

and/or experience future osteoarthritis. In addition to painful physical costs, the financial costs of

ACL reconstruction and rehabilitation are staggering for the patient and the broader community.

The average cost for ACL surgical repair is $38,121, which is independent of the expenses

incurred in physician evaluation, radiography of the knee, or postoperative rehabilitation

(Peterson & Krabak, 2014).

Studies suggest that most noncontact ACL injuries, or injuries that are not a result of a

direct external force to the knee such as a football tackle, occur between ages 16 to 18. Females

have a 4-6 times greater risk of ACL injury than their male counterparts participating in the same

sport (Hewett, Shultz, & Griffin, 2007). As the number of female athletes continues to rise, the

statistics regarding female ACL tears are especially alarming. Noncontact ACL injuries have

been reported to be as high as one out of every 100 high school female athletes and one out of

every 10 collegiate female athletes (Hewett, Shultz, & Griffin, 2007).

ANATOMY OF THE ACL

To better understand the causes of ACL injury, it is important to establish a full picture of

the anatomy of the human knee. The knee joint is the middle joint of the hind limb and is the

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largest joint in the human body. It joins the femur (thigh bone) to the tibia (shin bone). The

patella, or knee cap, articulates with the femur and protects the knee joint in the front. The knee

has the ability to move with six degrees of freedom- three translations and three rotations. The

translations of the knee include proximal-distal (towards the body vs. away from the body),

anterior-posterior (front vs. back), and medial-lateral (side to side), and the rotations include

internal-external, varus-valgus (bow-legged vs. knock-kneed), and flexion-extension. The

muscles of the body pull on the bones of the skeleton to provide movement, and the primary

muscles that control the movement of the knee joint are the quadriceps, located on the anterior

surface of the thigh, and the hamstrings, located on the posterior surface of the thigh. In addition

to the bones and muscles of the knee, there are four ligaments that join the bones of the knee and

stabilize the joint. The lateral collateral ligament (LCL) and the medial collateral ligament

(MCL) run along the outside and inside margins of the joint, respectively. Named by their

alignment, the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL) cross

deep within the center of the joint in the shape of an X.

Figure 1. Knee Anatomy: This illustration depicts an anterior perspective of the right knee. The ACL is shown coursing anteriorly, medially, and distally from femur to tibia. Note the alignment in relation to the PCL (American Academy of Orthopedic Surgeons, 2009).

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The ACL, the primary focus of this paper, is a band of dense regular connective tissue

that connects the femur to the tibia. Specifically, the ligament originates at the medial side of the

lateral femoral condyle and crosses obliquely through the intercondylar fossa to the medial tibial

eminence where it is inserted. The tibial attachment is somewhat broader than the femoral

attachment, with the ACL beginning to “fan out” in the proximal one-third of the ligament. As

the ACL courses anteriorly, medially, and distally from femur to tibia, it turns in a slight lateral

spiral giving it a twisting appearance (Jackson, 1993).

The ACL primarily resists anterior displacement of the tibia with respect to the femur and

secondarily resists tibial rotation. However, the ACL does not function as a singular cord, but

rather as two major fiber bundles, named according to the insertion sites on the tibia: the

anteromedial (AM) bundle and the posterolateral (PL) bundle. The AM bundle acts as the

primary tissue restraining anterior tibial translation; in other words, the AM bundle prevents the

tibia from displacing anterior to the femur. The PL bundle stabilizes the knee at full extension to

prevent internal and external rotation of the tibia. The difference in functionality between the two

bundles results from varied tensions throughout the knee joint’s range of motion. With the knee

extended, the AM bundle is moderately loose, and the PL bundle is tight. When the knee is

flexed, the femoral attachment of the ACL has a horizontal orientation that causes the AM

bundle to tighten and the PL bundle to relax (Petersen & Zantop, 2006).

BIOMECHANICS OF THE ACL: the pathway to INJURY

The ACL limits the anterior tibial translation, axial tibial rotation, and varus or valgus

knee angulation. The ACL provides 87% of the total restraining force to anterior tibial

translation when the knee is flexed at a 30° angle and 85% at a 90° angle (Noyes & Barber

Westin, 2012). Without an intact ACL, stabilization of the knee is greatly reduced. Varying

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angles of knee flexion, knee constraint, and the magnitude and direction of the applied load can

produce different forces in the ACL. Studies done by Berns et al. (1992), Markolf et al. (1995),

and Fleming et al. (2001) all demonstrate that anterior shear force on the proximal end of the

tibia increases ACL loading. Additionally, the studies show that knee valgus angulation and

internal rotation of the tibia, in combination with anterior shear force at the proximal end of the

tibia, increase the strain on the ACL. The quadriceps muscles are the primary contributors of

anterior shear force on the proximal end of the tibia, while the hamstrings reduce ACL loading

during contraction. Finally, the studies show that a decreasing knee flexion angle increases ACL

loading (Hewett, Shultz, & Griffin, 2007).

An ACL injury can be categorized as a partial tear, a complete tear, or a bone avulsion.

Partial tears are characterized as such when only a portion of the ACL fibers are damaged. A

complete tear is defined by both the AM and PL bundles being completely severed. A bone

avulsion occurs when the bony attachment area of the ACL gets pulled away from the rest of the

bone. 70% of ACL tears are a result of noncontact forces such as cutting, pivoting, accelerating,

decelerating, or landing from a jump. The forces that cause noncontact ACL injuries are a

product of ground reaction forces and internal soft-tissue and muscles forces. The other 30% of

ACL injuries are termed contact injuries, which result from a direct blow to the knee joint.

Contact ACL injuries are commonly seen in football. Kiapour et al. (2014) identified a multi-

planar mechanism of non-contact ACL injuries. Using human cadaveric tissue, the study

indicated that a combination of anterior tibial translation, knee abduction (valgus moment), and

internal tibial rotation leads to greater strain on ACL, and therefore, increases the risk for injury.

The results also emphasized the significant role of anterior tibial translation and knee abduction

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as primary contributors and internal tibial rotation as a secondary contributor to the risk of ACL

injury.

THE GENDER DISPARITY IN ACL INJURY RATES

The female athlete has a 4-6 times greater risk of injuring her ACL in comparison to the

male athlete playing a sport with similar landing and cutting movements. The gender disparity

associated with ACL injury is speculated to be multifactorial in nature. Proposed causes of

gender differences in ACL injury rates include the effects of anatomical structure, biomechanics,

neuromuscular functioning, genetics, and hormones. While many studies have attempted to look

at these categories as separate entities, it is important to note the possible interplay between risk

factors.

Anatomical differences

Naturally, an area of initial interest regarding the gender disparity of the rate of ACL

injuries is the anatomical structure of the female ACL compared to the male ACL. The ACL

passes through the intercondylar femoral notch and moves within it during motion of the knee

joint. According to research by Shelbourne et al (1998) the rate of ACL injury is higher in people

with more narrow notches (defined as <15 mm), regardless of gender. However, they also found

that women, on average, have more narrow notches compared to men, which may account for the

greater incidence of ACL injuries. Further, the study showed that following ACL reconstruction

in which the new ACL size is standardized for both sexes, the higher rate of ACL tears in

patients with narrower notches is eliminated. As a result, Shelbourne et al concluded that the

width of the notch is not a causative factor of ACL tears, but a narrow notch does reflect a

smaller ACL housed within which may affect the susceptibility to injury. This study did not go

as far as to determine if a larger ACL (in terms of volume) represents a stronger ligament.

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Although Shelbourne et al. and other authors have found that, on average, females have smaller

intercondylar notches even when accounting for smaller stature, various other studies prove this

finding to be controversial. Lombardo et al. (2005) did not find a significant correlation between

intercondylar notch width and ACL injuries, while LaPrade et al. (1994) found that athletes with

a narrower notch were at increased risk of ACL injury but found no difference in notch width

between genders. The controversies between studies seem to result from differences in study

designs and methodologies, such as imaging and measurement techniques. Regardless, the

equivocal results concerning the width of the intercondylar notch width with regards to female

ACL injuries highlight that fact that anatomy is not the sole determinate in ACL tears.

Hypotheses relating to different anatomical structure are not limited to the knee joint

specifically. Researchers have also investigated the role of the entire lower extremity in ACL

injury. The femur meets the tibia at an angle termed the quadriceps femoris angle or “Q angle.”

Clinically, it is defined as the angle in the frontal plane that is formed by the intersection of the

line that connects the center of the patella to the anterior superior iliac spine and the line that

connects the center of the patella to the tibial tubercle. In comparison to men, women not only

have an increased pelvic width, but they also have a shorter femoral length; these anatomical

differences result in females having an increased Q angle. According to studies by Zelisko et al.

(1982) and Haycock and Gillette (1976), a larger Q angle results in greater force placed on the

medial aspect of the knee, and essentially greater risk of ACL injury. Yet, studies by Gray et al.

(1985) and Endsley et al. (2003) showed no correlation between Q angle measurements and

injury rate. Thus, the results reveal that static anatomical measurements may not be the best

predictors of ACL injury in females.

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According to Boden et al. (2000), the female athlete has increased joint laxity compared

to the male athlete. They reported that patients who suffered from ACL injuries demonstrated

excessive extension at the knee joint and increased ability to touch the palms of their hands to the

floor. Uhorchak et al. (2003) found that women with generalized joint laxity were at a 2.7 times

greater risk of ACL injury than females without joint laxity. Joint laxity has been shown to

increase knee hyperextension and knee valgus, which can put strain on the ACL. Boden et al.

also reported that ACL injured athletes displayed more lax hamstring muscles. While males

demonstrate decreased flexibility with age after puberty, females demonstrate increased

flexibility after puberty. Studies by Hewett et al. (2006) and Huston and Wojtys (1996) suggest

that a decrease in dynamic control of the knee in females could be partially caused by increased

hamstring flexibility during and after puberty. However, the co-contraction of the quadriceps and

the hamstring muscles needs to be considered to add more strength to this argument.

Biomechanical differences

Biomechanical risk factors of female ACL injuries have been areas of in-depth research

because they are modifiable, as opposed to anatomical risk factors. The knee, hip, and ankle have

been focused on during research in order to determine the contribution of each joint to ACL

injury. According to research done by Hutchinson and Ireland (1995), planting and cutting

(29%), straight knee landing (28%), and one step-stop landing with the knee hyperextended

(26%) are the most commonly observed movements involved in female noncontact ACL

injuries. The posture and lower extremity alignment of females during these movements may put

the athlete at increased risk of ACL injury. In general, women change direction in a more erect

position than men, which can result in decreased flexion in the knee and hip, increased valgus in

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the knee, and greater activation of the quadriceps muscles- all of which can add strain to the

ACL.

A study by Hewett et al. (2006) showed that knee abduction was more than 8° greater in

patients who had suffered ACL injuries as compared to the control group. It is likely that the

increased valgus moment contributed to the mechanism of ACL injury in those patients. Ford et

al. (2005) used motion analysis to demonstrate that during cutting movements, females display

greater knee abduction angles compared to male athletes. Looking at both of these studies

together, it can be inferred that the knee valgus position of female athletes during change of

direction increases the likelihood of ACL injury. However, in terms of knee flexion, research has

been inconclusive. A study by Malinzak et al. (2001) found that females had less knee flexion

compared to males during a side step movement, while Mclean et al (2005) did not report a

gender difference during the same movement. Withrow et al. (2006) reported less knee flexion in

females during landing from a 60 cm height but not from a 20 cm height. Fagenbaum and

Darling (2003), in contrast, found that females actually demonstrated greater knee flexion angles

compared to males during a drop jump landing. Without further research, the evidence indicates

that female knee flexion angles are similar to those of males during athletic maneuvers.

The hip biomechanics of a female during landing may contribute to ACL injury risk.

Lephart et al. (1997) reported that women land with greater hip internal rotation than men, and

Zazulak et al. (2007) found that while the gluteus maximus can minimize excessive hip rotation,

it is activated less in females than in males. Ford et al. (2005) found differences in hip

stabilization in the dominant versus nondominant sides of the female athlete. Women land with

greater external hip adduction and decreased hip flexion angles on the dominant side. Greater

external hip adduction may cause instability during dynamic movement, and in combination with

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asymmetry, can lead to valgus positioning of the knee while landing. As already mentioned,

valgus loading on the knee puts strain on the ACL.

Increased ankle eversion, represented by the medial side of the foot remaining planted on

the ground while the lateral side elevates off the ground, may contribute to the gender disparity

in ACL tears. Ford et al. (2005) found that during change of direction, female athletes had a

greater ankle eversion than male athletes. Consequences of excessive ankle eversion are valgus

knee stress, anterior tibial translation, and increased loading on the ACL (Nyland et al, 1997).

However, foot pronation and internal tibial rotation also couple with ankle eversion.

A relationship between core stability and ACL injury risk has been the speculated. The

core is “the strength and function of the abdominal, back extensor, and pelvic floor muscles that

contribute to stability of the lumbopelvic-hip region complex” (Biering-Sorensen, 1984). The

core is considered a person’s center of gravity and is where all movements originate. A stable

core allows for efficient movements of the lower extremity and helps to reduce forces on the

joints. Although the effects of core stability on ACL injury risk remain controversial, results

from an analysis of 104 varsity college athletes indicate a positive correlation between measures

of core stability and risk for lower extremity injury (Ireland, 2002). A study done on competitive

alpine skiers by Raschner et al. (2006) concluded that not only are female skiers more at risk for

ACL tears, but also that absolute core strength is a predicative variable in female alpine skiers.

More research is necessary to consider core strength a significant risk factor for female ACL

injuries, but this research topic is worth pursuing.

Neuromuscular differences

Neuromuscular control is defined as “the unconscious efferent response to an afferent

signal about dynamic joint stability” (Silvers & Mandelbaum, 2007). Control of the knee

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involves the interaction between feedforward and feedback systems. The feedforward system

anticipates forces on the knee and activates muscles in order to protect the knee joint. The

feedback system is reflexive in nature, and results from the force on the joint. On the onset of

puberty, males and females begin to display differences in neuromuscular development. Hewett

et al. (2007) found that the neuromuscular development of females lags behind the increase in

size and weight during puberty, whereas the neuromuscular development and physical

development of males generally coincide. Until neuromuscular control improves, a lack of

coordination in the adolescent female may contribute to the increased risk of ACL injury.

The stability of the knee depends on the strength and recruitment of the surrounding

musculature. The quadriceps and hamstrings muscles work together in an antagonist-agonist

relationship to protect the knee joint from excessive anterior tibial translation and knee

abduction. The quadriceps muscle serves as the antagonist to the ACL such that it increases the

anterior shear force on the tibia, while the hamstring muscle serves as the agonist by preventing

excessive anterior translation of the tibia. Failure of the ACL can result from weakened strength

or slowed activation of the hamstrings in comparison to the quadriceps. A study done by

Malinzak et al. (2015) used electromyography analysis to measure hamstring and quadriceps

activity in males and females when landing from a jump and performing cutting maneuvers. The

resulting data showed that females had greater quadriceps activity and less hamstring activity

compared to males. Also, females contracted their hamstring fibers 50 milliseconds slower than

males and with less intensity. A quadriceps dominant mechanism during landing or cutting puts

the female at increased risk for ACL injury because of the increased displacement of the tibia

and greater stress on the ACL. Myer et al. (2005) reported a low ratio of medial quadriceps to

lateral quadriceps recruitment and an increase in lateral hamstring firing in female athletes

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compared to males. The combined imbalance of musculature in the portions of the quadriceps

and hamstrings increases the risk of ACL injury because it results in knee valgus positioning and

greater anterior shear force on the knee (Rozzi et al, 1999).

The musculature of the hip also contributes to the dynamics of the knee. Zazulak et al.

(2015) reported that women have lesser gluteal muscle firing compared to men, and this decrease

in musculature activation in the hip produces more force in the lower extremity which causes

valgus collapse in the knee. Additionally, Griffin et al. (2000) reported that decreased activation

of the hip muscles reduces the maximum activation of the quadriceps and hamstrings, and

therefore, reduces the capacity of the load that can be placed on the lower extremity without

causing injury.

Genetic differences

Varying gene expression has been an area of speculation concerning the difference in the

rates of male and female ACL tears. With the advancement in genetic analysis, the correlation

between genetics and ACL injury has been increasingly researched. A recent study done by

Johnson et al. (2015) used biopsies of ruptured ACL tissue from seven males and seven females

between the ages of 12 and 22. Microarray analysis identified thirty-two genes with significant

differential expression between the male and female specimens, and three of these genes code for

specific proteins related to ACL’s structure. Two genes, ACAD and FMOD, are known to

regulate the extracellular matrix of the ligament, and these genes were upregulated in the female

tissue samples compared to the male tissue samples. The gene WISP2 functions in collagen

turnover and production and was downregulated in the female biopsies. Johnson et al. concluded

that the significant difference in the female gene expression of ACAD, FMOD, and WISP2 may

lead to weaker ACL’s in females compared to males. While this study is a noble starting point

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for research into the correlation between female genes and ACL injury, it is limited in that it uses

samples of already ruptured ACL tissue.

Hormonal differences

Hormonal differences between males and females that emerge during puberty have been

thought to contribute to the gender disparity in ACL injury rates. Specifically, the hormonal

changes during the 28 day female menstrual cycle have been an area of active research

concerning ACL injury. The menstrual cycle can be divided into three phases: the follicular

phase (days 1-9), the ovulatory phase (days 10-14), and the luteal phase (days 15-28). During the

early follicular phase, low concentrations of the sex hormones, progesterone and estrogen, are

present; however, a spike in estrogen occurs during the late follicular phase and continues into

the ovulatory phase. The luteal phase is marked by a rise in progesterone, in addition to a rise in

the hormone relaxin in the second half of the stage. With the discovery of hormone receptor sites

on the ACL, the variations in the concentrations of progesterone, estrogen, and relaxin have been

studied to determine the effect on the structure of the ACL.

The contribution of estrogen concentrations on ACL injury is a controversial area of

research. A study done by Wojtys et al. (2002) found that there is an increased incidence of

noncontact ACL injuries during the ovulatory phase of the menstrual cycle and a decreased

incidence during the follicular phase. In contrast, Slauterbeck et al. (1999) saw the greatest

number of ACL injuries just before menstruation, during the luteal phase. Myklebust et al.

(1998) reported an increase in ACL injury during the follicular phase during menstruation. The

variance in the findings may be contributed to menstrual phase definitions and sex steroid

measurements. Additionally, the studies rest on the assumptions that all females have a normal

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28 day cycle with consistent phase lengths. Study designs that take into account the unique

characteristics of each female’s menstrual cycle should be aimed for in future research.

Fluctuating hormone concentrations during the menstrual cycle may decrease ligament

strength, and therefore, contribute to female ACL injuries. Booth and Tipton (1969) found that

estradiol, a type of estrogen, significantly decreases ligament strength, and Samuel et al. (2007)

found that relaxin decreases soft tissue tension. In an in vitro study, Yu et al. (1999) found an

inverse relationship between the concentration of estrogen and ACL fibroblasts throughout the

menstrual cycle and proposed that females may be more prone to ACL injury because of the

direct effect of estrogen on collagen synthesis in the ligament. Slauterbeck et al. used a rabbit

model to study the effect of estrogen on failure load of the ACL. They found that with an

increased concentration of estrogen, the tensile properties and the failure load of the ACL

decreased. In contrast, Strickland et al. (2003) observed no differences in maximum force,

stiffness, failure load, or failure site in sheep with estrogen implants, and Seneviratne et al.

(2004) concluded that there were no clinically significant changes in the material properties of

sheep ACL in vivo due to estrogen fluctuation during the menstrual cycle. Joint laxity resulting

from hormonal changes has also been speculated as a cause of increased ACL injury rate. Wojtys

et al found an increase in knee joint laxity during the ovulatory phase of the menstrual cycle,

while Heitz et al. (1999) reported that ACL laxity increased across the cycle. However,

Karageanes et al. (2000) found no difference in laxity throughout the cycle. Currently, research

conducted on physically active women with normal and comparable menstrual cycles is lacking

and should be a direction for future study.

Although it seems as if hormones have an effect on ACL collagen production and knee

laxity to a certain extent, the neuromuscular and biomechanical repercussions of the biological

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changes need to be considered. Estrogen directly and indirectly affects the female neuromuscular

system. Sarwar et al. (1996) found that during the ovulatory phase of the cycle, quadriceps

strength increases and muscle relaxation slows, which may put a female at increased risk for

ACL injury. Estrogen also affects the central nervous system. Lebrun et al. (1995) found

differences in isokinetic strength, anaerobic and aerobic capacity, and endurance in females

throughout different phases of the menstrual cycle. Motor skills were found to decrease during

premenstrual phases in a study performed by Posthuma et al. (1987). Taking all research into

account, hormones are likely to be a contributor to the neuromuscular control of the knee joint,

but must be considered in light of the other risk factors.

PREVENTION

The purpose of researching the causes of noncontact ACL injuries in females is to be able

to implement prevention programs to reduce their incidences. Considering the many physical,

emotional, and financial consequences that are involved with ACL injuries, great efforts have

been taken to develop effective injury prevention strategies, especially for females. While

anatomical, genetic, and hormonal risk factors are important to research for the sake of

understanding how and why ACL injuries occur, these factors are not as easily modifiable as

biomechanical and neuromuscular risk factors. For this reason, the prevention of ACL injuries in

females has been primarily focused on biomechanical and neuromuscular training.

The abundance of available research relating to ACL injuries has resulted in a variety

prevention programs with different elements. However, each one seems to place emphasis on

landing and cutting with knee and hip flexion, avoiding knee valgus position, increasing the

strength of the hamstring, gluteus medius, and hip abductor muscles, and decelerating under

control (Silvers & Mandelbaum, 2007). Time of implementation (such as preseason or during

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season), frequency, and duration differ amongst the programs. Additionally, the targeted age

group, skill level, and sport contribute to the variety of prevention techniques.

According to an analysis of prevention programs by Renstrom et al. (2008), a successful

program incorporates “neuromuscular training/control, muscle strengthening, plyometrics, as

well as education and feedback regarding body mechanics and proper landing patterns in a

dynamic atmosphere.” The most successful programs were initiated 6 weeks prior to regular

season practice and competitions. The training should take 15-20 minutes and should be

performed at least 3 times per week. In order to reinforce proper landing and cutting techniques,

the program should be continued throughout the season, and may be incorporated as part of a

warm-up routine. The athlete should receive feedback from a teammate or coach who know and

understand the safe position that the athlete should maneuver in. Using a mirror or video

recording is also helpful so that the athlete can analyze their own form.

Plyometrics drills mimic the maneuvers of real sport situations in a controlled setting.

Cutting, jumping, and lateral movements are incorporated into high intensity agility drills that

challenge the athlete to maintain proper form throughout the duration. Emphasis should be

placed on beginning and ending a cut or jump with flexed knees and hips. Additionally, the

knees should not angle inward, and more weight should be distributed to the front of the foot

upon landing. Plyometric drills can train the athlete to perform explosive movements with proper

muscle recruitment and mechanics. As the program advances, drills should increase in difficulty

and disturbances should be incorporated in order to simulate the unpredictable nature of athletics

(Myer et al., 2008).

In order to consistently and naturally maintain proper form, strength training is a

necessary component of a prevention program. Because research has shown that hamstring

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recruitment is lacking in many females, hamstring strengthening should be a point of emphasis.

Strengthening of hip abductors and gluteus medius muscles may also reduce the likelihood of

landing in a knee valgus position. Coordination of movement can also be improved through core

strengthening exercises. Lastly, any asymmetry in strength should be resolved in order to

maintain balance (Myer et al., 2008).

Just as the form for shooting a basketball is a technique taught by a coach, athletic

maneuvers such as jumping and cutting should be similarly taught. The development of poor

habits at a young age like landing in an erect stance may exaggerate a female’s predisposition to

ACL injury when competition and intensity increase with age. Therefore, a prevention program

involving mainly plyometrics should be started before puberty in females in order to avoid poor

development of neuromuscular and biomechanical patterns. Ideally, the program should be able

to be easily taught and supervised by a coach. In order to increase the likelihood of participation

and adherence, it also should be enjoyable for youth girls. However, as a female enters into

puberty, the prevention program should intensify to account for strength deficits and/or

asymmetry.

CONCLUSION

The female athlete has a 4-6 times greater risk of suffering a noncontact ACL injury

while participating in the same sport as a male counterpart. Much research has produced

inconclusive evidence as to the reason for the gender disparity regarding injury. It is likely that

the disparity is multifactorial in nature, resulting from differences in anatomy, biomechanics,

neuromuscular control, genetics, and hormones amongst males and females. Because

biomechanical and neuromuscular patterns can be modified to a certain extent, ACL injury

prevention programs have targeted these risk factors. Ideally, prevention programs in females

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should be initiated as soon as athletic participation begins in order to prevent maladaptive

athletic maneuvers that may put the athlete at risk. Hopefully, with the increased awareness of

the female disposition to ACL injuries, young female athletes will be encouraged to participate

in prevention programs, and the gender gap in injury rates will narrow.

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