With the growing number of female athletes, an increase is occurring in the number of sports-related injuries, which can cause physical, psychological, academic, and financial suffering. Female athletes are reported to be two to eight times more likely to sustain an anterior cruciate ligament (ACL) injury than male athletes. Further research on risk factors and preventative strategies for the female ACL is needed, because the cause of the disparity in injury rates remains equivocal and controversial. Individualized treatment for the injured knee is necessary and can include either conservative treatment or reconstructive surgery.
Epidemiology
Since the enactment of Title IX in 1972, the number of females involved in athletics has grown considerably. In the United States the percentage of girls participating in sports at the high school level increased almost tenfold from 1971 (3.7%) to 1998 (33%) . By 1998, females represented 40% of all high school and college athletic participants .
With the growing number of athletes, the number of sport-related injuries has also increased greatly. Seventy percent of anterior cruciate ligament (ACL) injuries occur during athletic participation . They are most common in sports that require rapid stopping, cutting, and changing direction, such as basketball, soccer, and team handball . In the United States, the overall ACL injury rate for the general population is approximately 100,000 tears per year, corresponding to 1 in 3000 individuals . Athletes who are injured may potentially miss an entire season or more of sports, lose scholarship funding, suffer psychologically and academically, or experience posttraumatic arthritis in the injured knee later in life .
The National Collegiate Athletic Association (NCAA) statistics have shown that, when compared with male athletes who participate in activities with similar rules and equipment, female athletes are two to eight times more likely to sustain an ACL injury . Environmental, anatomic, hormonal, neuromuscular, and biomechanical risk factors of the female athlete have been studied to predict injury and determine ways to prevent tears. Experts believe that multiple factors are most likely involved in predisposing the female athlete to ACL injuries, but further research is necessary to determine methods of preventing injury. Reducing the ACL injury rate would improve the physical, psychological, and financial health of female athletes worldwide.
The role of the anterior cruciate ligament
The knee joint consists of the patellofemoral and tibiofemoral joints. The patellofemoral joint is the articulation between the patella and anterior femoral plateaus, and the tibiofemoral joint is the articulation between the proximal tibia and the distal femur. Within the tibiofemoral joint, on the distal end of the femur, are the large medial and lateral condyles, which make up the proximal articular surface. The intercondylar notch separates these plateaus and is where the ACL passes laterally to the posterior cruciate ligament (PCL) . The articular surfaces of the proximal tibia that correspond to the femoral articular surfaces are two shallow concave medial and lateral plateaus . The distal articular surface of the femur articulates with the patella anteriorly and with the tibia posteriorly and inferiorly. Shallow grooves of the lateral and medial condyle separate the anterior and inferior articulations.
The tibiofemoral joint is dynamically stabilized by the quadriceps, hamstrings, and triceps surae muscles and is passively stabilized by the joint capsule, lateral and medial menisci, and four ligaments (lateral collateral ligament, medial collateral ligament, ACL, and PCL). The hamstrings and ACL resist forward movement of the medial and lateral plateaus, preventing anterior dislocation of the tibia relative to the femur. During deeper flexion angles, the quadriceps, hamstrings, and ACL resist forward movement of the plateaus. Backward movement of the plateaus is resisted by the PCL and quadriceps, preventing posterior dislocation of the tibia relative to the femur. The PCL and ACL also help prevent hyperextension of the tibiofemoral joint, medial and lateral displacement of the tibia relative to the femur, and internal rotation of the tibia relative to the femur .
The ACL can also be described as consisting of two bundles: the anteromedial bundle and the posterolateral bundle, which are named for the orientation of their tibial insertions . The anteromedial bundle is tight in flexion and resists anterior drawer (forward movement of the tibia) between 60° and 90° of flexion . The posterolateral bundle is tight in extension and resists anterior subluxation near full extension . A partial ACL tear can be caused by damage to either one of the two bundles . Anterior force when the knee is close to full extension can damage the posterolateral bundle, whereas anterior force when the knee is in greater flexion can damage the anteromedial bundle .
Noncontact ACL injuries, which account for 70% of all ACL injuries in both male and female athletes , commonly occur during planting and cutting, and landing maneuvers . Many different factors have been related to noncontact ACL injuries during these types of movements, including those that are more easily controlled (environmental factors) and those that are less easily controlled and innate to the female athlete (anatomic, hormonal, neuromuscular, and biomechanical factors).
The role of the anterior cruciate ligament
The knee joint consists of the patellofemoral and tibiofemoral joints. The patellofemoral joint is the articulation between the patella and anterior femoral plateaus, and the tibiofemoral joint is the articulation between the proximal tibia and the distal femur. Within the tibiofemoral joint, on the distal end of the femur, are the large medial and lateral condyles, which make up the proximal articular surface. The intercondylar notch separates these plateaus and is where the ACL passes laterally to the posterior cruciate ligament (PCL) . The articular surfaces of the proximal tibia that correspond to the femoral articular surfaces are two shallow concave medial and lateral plateaus . The distal articular surface of the femur articulates with the patella anteriorly and with the tibia posteriorly and inferiorly. Shallow grooves of the lateral and medial condyle separate the anterior and inferior articulations.
The tibiofemoral joint is dynamically stabilized by the quadriceps, hamstrings, and triceps surae muscles and is passively stabilized by the joint capsule, lateral and medial menisci, and four ligaments (lateral collateral ligament, medial collateral ligament, ACL, and PCL). The hamstrings and ACL resist forward movement of the medial and lateral plateaus, preventing anterior dislocation of the tibia relative to the femur. During deeper flexion angles, the quadriceps, hamstrings, and ACL resist forward movement of the plateaus. Backward movement of the plateaus is resisted by the PCL and quadriceps, preventing posterior dislocation of the tibia relative to the femur. The PCL and ACL also help prevent hyperextension of the tibiofemoral joint, medial and lateral displacement of the tibia relative to the femur, and internal rotation of the tibia relative to the femur .
The ACL can also be described as consisting of two bundles: the anteromedial bundle and the posterolateral bundle, which are named for the orientation of their tibial insertions . The anteromedial bundle is tight in flexion and resists anterior drawer (forward movement of the tibia) between 60° and 90° of flexion . The posterolateral bundle is tight in extension and resists anterior subluxation near full extension . A partial ACL tear can be caused by damage to either one of the two bundles . Anterior force when the knee is close to full extension can damage the posterolateral bundle, whereas anterior force when the knee is in greater flexion can damage the anteromedial bundle .
Noncontact ACL injuries, which account for 70% of all ACL injuries in both male and female athletes , commonly occur during planting and cutting, and landing maneuvers . Many different factors have been related to noncontact ACL injuries during these types of movements, including those that are more easily controlled (environmental factors) and those that are less easily controlled and innate to the female athlete (anatomic, hormonal, neuromuscular, and biomechanical factors).
Environmental factors
Shoe and surface type
With the proper ground–shoe surface interface during athletic events, female athletes may be able to reduce their risk for an ACL injury. Surfaces that increase friction between the player and the field of play can modify movement patterns in athletes, increasing their risk for ACL injuries. Drier playing surfaces, which increase the amount of friction between the surface and shoe, have been shown to have a greater ACL injury rate than wet surfaces . ACL tears experienced by Australian football players were most frequent during high-evaporation and low-rainfall periods . The effect of artificial playing surfaces on injury rate remains controversial. Synthetic turf has been shown to be a less-safe surface to play on than outdoor grass . By contrast, ACL injury rate decreased by 50% for a group of Texas football teams who played on “FieldTurf,” an artificial turf, rather than natural grass . Artificial indoor floors have also been shown to be more dangerous than natural wood floors .
Torg and Quendenfeld observed that the number and size of the cleats on a shoe correlated with the occurrence of knee and ankle injuries in American football players, with fewer injuries corresponding to smaller and fewer cleats. Lambson and colleagues examined the effects of four different cleat designs on knee injury risk for 3119 high school football players. The shoes with cleats placed at the peripheral margin of the sole and smaller, pointed cleats positioned interiorly were associated with higher torsional resistance and a significantly higher ACL injury rate than the other three designs combined.
A complete understanding of the complex interaction at the shoe–surface interface remains elusive . Livesay and colleagues studied peak torque and rotational stiffness for different combinations of cleat types and surface types. They found that highest peak torques were developed by a grass shoe–FieldTurf combination and a turf shoe–Astroturf combination. The grass shoe consisted of fewer cleats than the turf shoe, but each cleat was higher and wider. For both types of shoes, a grass surface provided the least amount of torque compared with the synthetic surfaces (FieldTurf, Astroturf, and Astroplay). The rotational stiffness was also greatest for the turf shoe–Astroturf combination. The authors concluded that a grass shoe-grass surface may be closest to ideal in preventing ACL injuries, but deciding which shoe–surface combination also depends on other factors, such as the sport, age of the players, and level of play.
Knee bracing
Prophylactic, functional, and postoperative or rehabilitative knee braces have been used to prevent ACL injuries or reinjuries. However, their effects on the knee injury rate remain controversial. Prophylactic bracing has been shown to protect the ACL during a controlled, direct lateral blow and does not add valgus pressure to the knee . It has also been shown to provide 20% to 30% greater resistance to a lateral blow that is sufficiently large enough to cause a medial joint-line opening . By contrast, a study of a college football team showed that the braced knees had a higher knee injury rate than nonbraced knees . Other studies have shown that prophylactic bracing has no effect on injury rate .
Findings on the benefits of rehabilitative and functional knee braces remain mixed. Studies have shown that functional knee bracing on nonoperative patients may provide psychological and subjective benefits, including a sense of stability . It can also increase coordination , reduce anterior tibial translation under low loading conditions , and reduce anterior–posterior laxity . A study using a functional knee brace that constrained knee extension showed an increase in knee flexion angle of athletes who performed stop-jump tasks . Fleming and colleagues showed that functional knee bracing protected the ACL during internal torques in the non–weight-bearing knee and during anterior–posterior shear loading in both the non–weight-bearing and weight-bearing knee.
Several studies have shown that the use of bracing for an extended period may have negative consequences, including reduced quadriceps strength. Houston and Goemans found that bracing decreased quadriceps muscle strength by 12% to 30%. Risberg and colleagues reported that postoperative patients who used bracing 1 to 2 years after the ACL tear had quadriceps strength values of less than 80% of the strength values of postoperative patients who only wore a brace for 3 months.
Several studies have shown that functional bracing does not protect the knee. Studies have shown that it does not decrease anterior tibial displacement relative to the femur or influence ACL strain values for external torques and varus and valgus moments . Studies have also shown no differences in strength, laxity, or function in the braced and nonbraced knee 2 years after surgery . In addition, Risberg and colleagues showed that bracing did not reduce the risk for further injuries to the meniscus or cartilage in the tibiofemoral joint.
The possible benefits of knee bracing remains controversial. Further studies that include a large sample size with a homogenous population are necessary to determine the effectiveness of knee bracing on ACL injury risk.
Anatomic factors
Bone lengths
The increase in torque on the knee joint that accompanies the growing tibia and femur in children can cause instability in the knee . Men have been shown to compensate for this through increasing their power, strength, and coordination more than women, which may help to explain the increased risk for ACL tears in women .
Ireland reported that the hip width to femoral length ratio is a better predictor of ACL injury risk than the absolute lengths and widths of the lower extremities . However, this ratio may not explain the difference in ACL injury rate between the genders, because the ratio has been shown to be roughly equal in men and women (0.73 and 0.77, respectively) .
Q angle and knee valgus
The quadriceps femoris angle, or Q angle , is the acute angle between the line that connects the anterior superior iliac spine to the midpoint of the patella and the line that connects the tibial tubercle to the same reference point on the patella . Studies have shown that Q angles are larger in women than in men and are larger in athletes who sustained a knee injury than in noninjured athletes . A larger Q angle increases the lateral pull on the quadriceps muscle that is connected to the patella, adding medial stress to the knee and increasing the risk for an ACL tear . A larger Q angle has been shown to predict 32.4% to 46% of variance in valgus–varus knee position , with valgus knee positioning predictive of future ACL injury risk . Despite these findings, the effects of a large Q angle and knee valgus remain controversial, with other studies reporting that static Q angle measures are not predictive of knee valgus or ACL injury risk during dynamic movements .
Femoral notch width and shape
In men and women, as height increases, total condylar width also increases. Studies have shown that, as height increases, notch width increases in men but not in women . Uhorchak and colleagues reported that women who had a narrow intercondylar notch (<13 mm) had a 16.8 times grater risk ratio for an ACL injury than those who had a larger notch. Other studies have shown that a small notch width is associated with an increased risk for an ACL injury and can increase the severity of the tear . Arendt reported that notch width is smaller in women sustaining bilateral ACL tears than those who had a unilateral tear. In addition, those who sustained a unilateral tear had a smaller notch width than those who had no tears . The effects of notch width size on ACL injury risk remains controversial, with Teitz and colleagues reporting no difference in notch width size in injured and noninjured knees.
The notch width index (NWI), which is the width of the intercondylar notch divided by the width of the distal femur at the level of the popliteal groove, has also been studied to determine whether it can predict ACL injury risk. A study by Souryal and colleagues reported no difference in NWI between noninjured subjects and those who experienced unilateral ACL injury. In addition, differences in NWI between the genders remain controversial. Griffin and colleagues reported that NWI is greater in women than in men, but several studies have reported no statistically significant gender-related differences in notch width indices and rate of ACL tears .
The shape of the femoral notch may also affect the ACL injury rate. Femoral notch shapes can be categorized as reverse U- or side C-shaped , H-shaped , or A-shaped . A decreased notch width and an A-shaped notch might put a female athlete at a higher risk for a noncontact ACL injury . However, evidence continues to conflict as to whether notch shape differs with gender and whether shape affects ACL injury rate.
Anterior cruciate ligament size and mechanical properties
Female ACLs are smaller than male ACLs when normalized for body weight . Several studies have shown a positive correlation between small ACLs and injury risk . A smaller ACL encounters a greater amount of stress when force is applied to a knee joint. However, whether this stress is high enough to cause injury to the ACL is unclear . In addition, women typically have a narrow notch width relative to the size of the ACL , which can cause impingement when the knee is in full extension, such as when landing from a jump or performing a cutting maneuver .
Mechanical property of the ACL may influence its ability to sustain increased loads. A cadaveric study by Chandrashekar and colleagues , which measured mechanical property by strain at failure, stress at failure, and modulus of elasticity, showed that female ACLs have a lower mechanical quality than male ACLs. Differences in material properties of female and male ACLs may prove important in determining why women have a higher risk for injury than men.
Laxity
Laxity, the combination of joint hypermobility and musculotendinous flexibility, is more prevalent in women than in men and may increase the risk for an ACL injury . After puberty, flexibility has been shown to increase in girls and decrease in boys, leading to greater generalized laxity in adult women than in adult men . Muscle stiffness is an important component in maintaining knee stability and injury prevention . When force is applied to the knee, the muscles surrounding the knee contract and the knee stiffens, dissipating the force that the ACL carries . Hamstring laxity can delay hamstring activation, decreasing the co-contraction between the quadriceps and hamstrings during foot strike and increasing the risk for an ACL injury . Joint laxity in the knee can potentially strain the ACL by increasing sagittal knee motion (hyperextension), coronal knee motion (valgus), and anterior tibial translation . Joint laxity in the foot is greater in women and may affect the ACL by increasing ligamentous laxity and navicular drop . Navicular drop has been shown to be a predictor of anterior tibial translation, which can strain the ACL .
Despite the greater laxity and flexibility in women , whether laxity predisposes a female athlete to an ACL injury remains controversial. Uhorchak and colleagues reported that women who have greater anterior–posterior knee laxity had a 2.7 times greater risk for an ACL injury. Boden and colleagues reported that the hamstring muscles were more lax in ACL-injured athletes when compared with noninjured athletes, whereas a study of individuals who had bilateral ACL ruptures reported that laxity was not a factor in ACL injury risk .
Body mass index
The effects of body mass index (BMI) on ACL injury rate remains controversial and inconsistent. Increased BMI can be associated with a decrease in relative strength. This can decrease the hip flexion angle and knee flexion velocity and increase the peak knee extension moment when landing from a jump, increasing the risk for an ACL injury . In a study of cadets at the U.S. Military Academy, women who had a higher-than-normal BMI had a higher risk for a noncontact ACL injury than those who had a lower BMI . However, in a study of men and women in basic combat training, Knapik and colleagues showed that BMI is not associated with overall injury risk in either sex.
Hormonal factors
An increased level of estrogen in women relative to men is believed to be an underlying cause of female ACL injuries . Estrogen receptors are present in human ACL fibroblasts , which produce the collagen that performs the major load-bearing function of the ACL . When estrogen is present, the synthesis of collagen by the fibroblasts is reduced , which can reduce the strength of the ACL and increase the risk for an ACL injury . Studies on nonhuman ACLs have shown that estradiol does not affect fibroblast proliferation and collagen synthesis . However, in vitro studies on human ACLs have shown that an increase in sex hormone concentrations may influence ACL metabolism and collagen synthesis in an interactive, dose-dependent, and time-dependent manner .
High levels of estrogen have also been shown to decrease the neuromuscular control of the knee and increase knee laxity . Posthuma and colleagues reported that motor skill ability decreases as estrogen levels increase during the premenstrual phases. Sarwar and colleagues reported that during the ovulatory phase, when estrogen levels peak, quadriceps strength increases and muscle relaxation decreases. Several studies have shown greater laxity during the luteal phase of the cycle . Other studies showed no difference in laxity during the different phases of the menstrual cycle or when estradiol levels are high .
The effects of fluctuating estrogen levels on ACL injury rate during the different phases of the menstrual cycle also remains controversial. Wojtys and colleagues reported the greatest incidence of ACL injury during the ovulatory phase, whereas several studies have shown a greater incidence of injury during the luteal phase and follicular phase . However, inconsistencies exist regarding menstrual phase definitions and sex steroid measurements, which likely impacts the variance in findings.
The effect of oral contraceptives on female ACL injury risk is unclear. Oral contraceptives lower the levels of estrogen in the female body . Several studies reported lower traumatic injury rates for athletes who take oral contraceptives , but a study of college varsity athletes sustaining noncontact ACL injuries showed that the use of oral contraceptives did not influence risk for injury . Until further research is conducted, no recommendations have been made to alter participation of female athletes using oral contraceptives .
Neuromuscular factors
Several studies have shown differences in neuromuscular development between men and women after the onset of puberty. Unlike girls, boys have neuromuscular spurts that match their growth spurts . The rapid increase in size and weight, with the absence of increased neuromuscular power and control, in girls at or near puberty may contribute to the higher incidence rate of ACL injuries .
Female knees also have shorter activation duration in the gastrocnemius and gluteus muscles, which initiate and maintain knee and lower extremity stiffness . In addition, females show leg dominance . Leg dominance is the imbalance among muscle strength, flexibility, and coordination between the lower extremities and can predict future ACL injury risk . Coactivation of the hamstrings and quadriceps muscles is important in protecting the knee against dynamic lower extremity valgus and excessive anterior drawer and abduction . Women have been shown to have quadriceps-dominant contraction during landing and cutting , which can increase the anterior displacement of the tibia relative to the femur and increase the risk for an ACL tear . Female athletes also have a low ratio of medial-to-lateral quadriceps recruitment and increased lateral hamstring firing . These neuromuscular mechanisms compress the lateral joint, opens the medial joint, and increases the anterior shear force, which could increase the risk for an ACL injury .
When landing from a jump, women have increased rectus femoris firing and decreased gluteal muscle firing compared with men . Using less hip musculature increases the force placed on the lower extremities, causing valgus collapse . Decreased hip muscle activation also reduces maximal possible quadriceps and hamstring activation, altering optimal load-bearing capacity, and increasing the risk for an ACL injury . Maneuvers performed with insufficient hip control in the transverse plane of motion have been shown to cause valgus collapse .
Mechanoreceptors in the ACL allow for a reflex response of hamstring activation when knee movements cause torque and elongation of the ACL, and can be indicators of anterior tibial translation on the femur . Haycock and Gillette found that uninjured women possessed lower single-leg sway measures than uninjured men, but after an ACL tear, women had increased single-leg sway. This finding may suggest trauma to the proprioceptive system in women who experience an ACL tear or that women who have decreased proprioception have a predisposition to ACL tears .
Fatigue can cause altered landing and cutting movements that can put the ACL at risk for a tear . Wjotys and colleagues reported that lower-extremity muscle fatigue increased anterior tibial translation by 32.5%. When performing stop-jump tasks while fatigued, a decrease in knee flexion angle, an increase in proximal tibial anterior shear force, and an increase in knee varus moment also occur . Although fatigue may increase ACL injury risk, no research currently shows that women fatigue faster or are less conditioned than men.
Biomechanical factors
Noncontact female ACL injuries are most commonly caused during planting and cutting (29%), straight knee landing (28%), or one-step stop landing with knee hyperextended (26%) . The posture and lower extremity alignment of women during these movements may predispose them to future ACL injuries. Women perform cutting maneuvers in a more erect position than men , which can cause decreased flexion in the knee and hip, increased valgus in the knee, and greater activation of the quadriceps muscles . Performing actions in a more crouched position may reduce the risk for ACL injuries in female athletes .
Foot and ankle
Foot pronation, measured with navicular drop, can be caused by joint laxity of the foot and can affect tibial translation and lower extremity alignment . Increased navicular drop moves the tibia forward and increases internal tibial rotation . Navicular drop has been shown to be greater in individuals who have sustained an ACL tear and greater in women than in men . However, further research is needed to determine whether navicular drop can predict ACL injury risk.
Variations in ankle joint angles have been shown to influence joint forces, moments, and muscular activation patterns in the knee . Female athletes have a greater maximum ankle eversion than male athletes when performing cutting maneuvers , which may cause an ACL rupture by increasing valgus knee stress and tibial rotation.
Knee
Several studies have shown that specific knee biomechanics during landing and pivoting increase the risk for ACL injuries. When landing from a jump, female athletes are weaker in knee extension and muscle strength and have greater adduction and abduction moments , which can alter the dynamic neuromuscular control of the lower extremity in the coronal plane . Knee abduction moments (valgus torques) and angles have been shown to be significant predictors of ACL injuries (73% sensitivity and 78% specificity) .
McNair and colleagues reported that ACL injury occurs between 20° of knee flexion and full extension, whereas Olsen and colleagues and Boden and colleagues reported injuries between 0° and 30° of flexion. At a knee flexion angle between 10° and 30°, the quadriceps muscles exert maximum anterior shear force, putting significant strain on the ACL . No consensus has been reached as to whether female athletes land and cut with greater knee flexion than male athletes . Yu and colleagues found that, during ground contact, knee flexion angles decrease after the age of 12 years in girls but not boys. Other studies have also shown that women have less knee flexion than men . However, Fagenbaum and Darling reported that women land with greater knee flexion angles than men. Furthermore, many studies have reported that knee flexion angles are the same for both men and women .
Hip
Hip biomechanics during landing may also predispose a woman to ACL injury risk. Hewett and colleagues reported that peak external hip flexion moments in the sagittal plane were greater in injured female athletes than in those who were uninjured. Lephart and colleagues reported that, during landing, female athletes have greater hip internal rotation than male athletes. The gluteus maximus can control excessive hip rotation, but is activated less in women than in men . In addition, women land with greater external hip adduction moments and smaller hip flexion angles on their dominant side . Greater hip adduction can decrease hip control and imbalances in side-to-side neuromuscular strength, flexibility, and coordination of the hip can increase knee valgus, which can put strain on the ACL .
Core stability
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. The core is where a person’s center of gravity is located and where all movements begin . A stable core can help stabilize the lower extremity, allowing it to function with optimal kinetic chain mechanics that reduce and stabilize forces . Theoretically, if the core is weak, efficient movements cannot be produced, even if the lower extremities are strong. These inefficient movements can cause injury in the knee .
The effects of reduced core stability on ACL injury rates remain controversial. One study showed that men performed better than women on core strength tests and a positive correlation was seen between external rotation and abduction strength. However, a correlation did not exist between a weaker core and higher ACL injury rates in either men or women . However, a different study showed that a weak core, demonstrated through hip abduction and external rotation weakness, increased the risk for lower extremity injury . In addition, anecdotal observations show that athletes who sustained an ACL tear have weakness in the core . More research using a large sample size and valid and reliable core stability measurements is necessary to elucidate the relationship between core stability and ACL injury risk.
Skill and level of exposure
The differences in skill level between the two genders have been studied to determine if women can decrease their risk for injury through training and experience. Skill level can be determined partially by an athlete’s level of conditioning and amount of prior experience . Harmon and Dick studied injury rates among the different NCAA division levels and assumed that athletes who were more skilled played at higher levels. When comparing the same gender and sport, they reported that ACL injury rates were the same among the different levels. However, Bjordal and Arnoy reported that the men in more skilled division levels of a soccer league had higher rates of ACL injuries.
Level of experience can reflect an athlete’s mastery and competency in a given sport. Level of experience can be measured by the amount of previous participation in organized sports. Report have shown that the amount of prior experience among NCAA female basketball and soccer players has increased over the past decade, but the ACL injury rate among both male and female athletes has remained statistically stable . Studies are limited, but a direct correlation between level of experience and ACL injury rate does not seem to exist among female athletes.
Athletes who are not well conditioned fatigue faster , resulting in decreased coordination, proprioception, and neuromuscular control , which can contribute to less joint stability and increased risk for an ACL injury . Harmon and Ireland reported no difference in the level of conditioning between noninjured male and female athletes or between male and female athletes who had ACL injuries. However, fatigue has been shown to affect proprioception, preactivation of muscle tension in anticipation of joint loading, and muscle-firing patterns in male and female athletes .
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