Acute anterior cruciate ligament (ACL) rupture is common in sports requiring frequent cutting, pivoting, sudden stops, or landing from a jump. The incidence of ACL ruptures has been estimated at 1 in 3,000 people per year within the general population in the United States alone. 1 Paulos reported that nearly 50,000 primary ACL surgeries were performed in 1982 in the U.S. 2 This figure has increased with the increase in participation in sport activities. ACL reconstruction is the sixth most commonly performed orthopedic procedure in the U.S. (Table 1), and approximately 175,000 primary ACL reconstruction surgeries were performed annually in the U.S. The estimated annual cost for ACL reconstruction surgeries alone was over $2 billion. 3–5
Acute ACL rupture is a devastating injury that can significantly affect patients' activity levels and quality of life. Complete ACL tears can lead to chronic knee problems, such as knee instability, meniscus and chondral surface damage, and osteoarthritis. Up to two thirds of patients with complete ACL tears may develop chronic knee instability and secondary damage to menisci and chondral surfaces, 6–12 significantly affecting knee function and often forcing patients to decrease their activity levels and change their life styles. 13 Rupture of the ACL led to moderate to severe overall disability reported by 31% of the patients for walking activities alone, by 44% of the patients for routine activities of daily living, and by 77% of the patients for sports activities. 9 In addition, damages to joint structures of the knee following an acute ACL rupture and meniscus damage have been identified as factors in knee osteoarthritis, which can severely impair patients' functional activities and independence. 7,9,14,15 Prevention of ACL injuries is important not only to reduce the incidence of an acute injury, but also to prevent secondary knee disorders that can reduce independence and adversely affect quality of life. We review the current literature on prevention of ACL injuries, especially the prevention of ACL injuries in female athletes, with an emphasis on the most recent studies of risk factors related to neuromuscular control of the lower extremity for noncontact ACL injuries.
NONCONTACT ACL INJURIES
Most ACL injuries are noncontact in nature: there is no direct blow to the knee when the injury occurs. In 103 high school and collegiate team sport athletes with ACL injuries, 80 of the 103 (78%) ACL ruptures were noncontact injuries. 9 McNair and Marshall 16 found that 70% of the patients had their injuries in noncontact situations. Boden et al. 17 interviewed 65 male patients with 72 ACL injuries and 25 female patients with 28 injuries. Patients with skiing-related ACL injuries were excluded. They reported that 71% of the ACL injury patients in their study had noncontact ACL injuries.
The study by Boden et al. 17 also revealed important common characteristics of noncontact ACL injuries: 25% of the injuries occurred in basketball, 21% in football, and 21% in soccer. They also found that 41% of the injuries occurred in recreational sports, 34% in varsity sports, and 23% in intramural sports. At the time of injury, 35% patients were decelerating, 31% were landing, 13% were accelerating, and 4% were falling backward. Twenty-two percent thought that their injuries were caused by the interface between their shoes and playing surfaces. Most of the patients (96%) could touch the floor with their fingers, knuckles, or palms in a forward bending position with straight legs, suggesting good hamstring flexibility. A further detailed review of all noncontact ACL injury videotapes collected by Boden revealed that over 70% of noncontact ACL injuries in basketball, volleyball, and gymnastics occurred in stop–jump tasks. These common characteristics of ACL injuries provide a basis for future quantitative studies on the mechanism and risk factors of the injuries.
ACL INJURIES IN FEMALE ATHLETES
Female athletes have a higher incidence of noncontact ACL injuries than male. The incidence of ACL injuries is higher in female athletes than in male athletes in basketball, soccer, volleyball, team handball, rugby, and track and field. 2,18–29 Pearl 25 reported that female basketball players injured their ACLs 7.8 times more often than their male counterparts. This finding was supported by Malone et al. 24 They collected historical data for a total of 402 male and 385 female basketball players from 29 institutions in three representative Division I collegiate basketball conferences. They found that 62 female players sustained ACL injuries with a prevalence of 16.1%, while 9 male players sustained ACL injuries with a prevalence of 2.2%. These results confirm that female collegiate basketball players are almost eight times as likely to sustain ACL injuries in comparison to their male counterparts. Lindenfeld et al. 27 documented the injuries in 300 indoor soccer games during a 7-week period at a local indoor soccer arena. A total of 31 knee injuries was documented with 10 ACL tears. Female players sustained 8 of the 10 ACL tears. They found that knee injury rates were 0.87 per 100 player-hours for female players vs. 0.29 per 100 player-hours for male players.
Arendt and Dick 29 confirmed the increased incidence of ACL injuries among female athletes in basketball and soccer. They investigated the ACL injury rate among collegiate athletes in basketball and soccer during a 5-year period. The authors considered that these two sports are highly comparable for men and women in the style of play, the rules, and intensity of preparation and competition. Historical data were collected for 461 men's and 278 women's soccer teams, 531 men's and 576 women's basketball teams, from 1989 to 1993. The 5-year average ACL injury rate was of 0.31 per 1000 athlete-exposures for female soccer players, in comparison to 0.13 per 1000 athlete-exposures for their male counterparts. The 5-year average ACL injury rate was 0.29 per 1000 athlete-exposures for women basketball players, in comparison to 0.07 per 1000 athlete-exposures for their male counterparts. An athlete-exposure is a unit of person-time, analogous to person-years, that denotes participation by athletes in games and practice. These results offer convincing evidence that the risk of ACL injuries for female athletes is at least three times higher than that for male athletes.
DeHaven and Lintner 22 reported that high school athletes had similar injury patterns as collegiate athletes, while Lindenfeld et al. 27 found that high school soccer players had a higher ligament injury rate than their collegiate counterparts. Chandy and Grana 30 investigated severe knee injuries among 24,485 male and 18,289 female high school athletes during a 3-year period. They reported that the incidence of season-ending knee injuries in female high school athletes was 0.88%, compared with 0.22% in male high school athletes. The projections of high school athletes' knee injury rates in the U.S. reported at the National Athletic Trainers' Association's clinical symposium 31 indicated that 4.1% of female high school athletes had knee-related injuries, in comparison to 2.2% for male high school athletes. These studies indicate that an elevated risk of ACL injury for girls also exists in high school populations.
POTENTIAL RISK FACTORS FOR ACL INJURIES
To prevent ACL injuries, especially the injuries in female athletes, risk factors must be identified. Several internal and external factors have been proposed as potential contributors to the increased incidence of noncontact ACL ruptures, especially for women. Intrinsic factors include a narrow intercondylar notch, a weak or small ACL, increased anterior–posterior laxity of the knee joint, malalignment of the lower extremity, pelvic position, navicular drop, and subtalar joint pronation. 29 Extrinsic factors include shoe–surface interaction, the playing surface, and altered neuromuscular control strategies and consequential movement patterns. 29,32 In addition, an athlete's level of conditioning, strength, coordination, and skill may also play a role in noncontact ACL injuries. 33–36
These potential risk factors were recently re-categorized as environmental, anatomic, hormonal, and biomechanical. 3,4 Environmental factors emphasize the type of playing surface and the type of shoes. Anatomic factors include lower extremity alignment, knee joint laxity, muscle strength, femoral notch, and ACL size. The effect of estrogen on the mechanical properties of the ACL is the main focus among hormonal risk factors. Increased joint loads associated with lower extremity motion patterns due to altered neuromuscular controls are the main concerns among the biomechanical factors.
STUDIES ON THE ENVIRONMENTAL FACTORS FOR ACL INJURIES
Recent studies provide evidence that playing surface may contribute to noncontact ACL injuries. Many ACL injuries in team handball seem to be due to a high level of friction between shoes and the playing surface, which is considered a major risk factor for noncontact ACL injuries. 37 In the National Football League, noncontact ACL injuries seem more frequent when the playing surfaces are dry. 38 A 3-year prospective study on the effects of the cleat design and configuration of football shoes suggested that the design of the cleats affected not only the torsional resistance but also the ACL injury rate in high school athletes. 39
Therefore, playing surface and shoes must be considered together when evaluating playing surface as a risk factor for ACL injuries. Also, we should notice the contradiction between the effects of shoe-playing surface interaction on performance and ACL injuries. High levels of friction between shoe and playing surface appear to increase the risk of ACL injuries, but enhance performance. Shoe-playing surface design may need to balance performance and safety factors. While shoe-playing surface interaction may be a risk factor for noncontact ACL injuries, there is no evidence at present that this interaction contributes to the increased incidence of noncontact ACL injuries in women.
STUDIES ON THE ANATOMIC FACTORS FOR ACL INJURIES
Once considered an important determinant of noncontact ACL injuries, 40–48 intercondylar notch width does not appear to be a satisfactory explanation for gender differences in ACL injury rate. Shelbourne et al. 48 investigated the relationship of the intercondylar notch width with the contralateral ACL injury rate and the rupture rate of the reconstructed ACLs. They recruited 714 patients, who underwent autogenous patella tendon graft ACL reconstruction as subjects. The patients were divided into two groups, a narrow intercondylar notch group (intercondylar notch width < 15 mm) and a wide intercondylar notch group (intercondylar notch width > 16 mm). The contralateral ACL rupture rate after initial unilateral ACL ruptures for the narrow intercondylar notch group was significantly higher than that for the wide intercondylar notch group. Also, the notch width in their female patients was narrower than in their male patients with the same standing height. However, their results did not show any gender difference in the contralateral ACL injury rate. Furthermore, there was no difference in the rupture rate of the reconstructed ACL grafts between the two groups with different intercondylar notch width, if the reconstructed ACL grafts were in the same size.
Muneta et al. 49 found a significant difference in the cross-sectional area of the ACL between men and women. Although there was essentially no correlation between intercondylar notch width and the cross-sectional area of the ACL, the mean cross-sectional area of the ACL was significantly larger in men. This result, combined with the results of the study by Shelbourne et al., 48 suggests that the size of the ACL might be a contributor to the increased ACL injury rate among women. However, no studies have been found which demonstrate an association between ACL size and injury rate.
Loudon et al. 50 studied selected static posture measurements of female athletes with ACL injuries. The selected posture measures included standing pelvic position, hip position, standing sagittal knee position, standing frontal knee position, hamstring length, prone subtalar joint position, and navicular drop. They reported that the standing sagittal knee position, navicular drop, and prone subtalar joint position discriminated between ACL injured and noninjured subjects. However, it is not clear whether these differences were risk factors or consequences of the injury.
Patella tendon–tibia shaft angle is another potential anatomic risk factor for noncontact ACL injuries that has not been addressed in literature. Patella tendon–tibia shaft angle (Fig. 1) directly affects the shear force applied on the tibia by the quadriceps muscles. The sine value of the patella tendon–tibia shaft angle indicates the ratio of the shear force to the resultant force applied by the quadriceps muscle group on the tibia through patella tendon. Nunley et al. 51 examined the patella tendon–tibia shaft angles of 10 male and 10 female college-aged recreational athletes. Each subject had seven x-ray films of the sagittal view of the knee, from 0° of knee flexion to 90°, at 15° intervals. A multiple regression analysis was conducted to express the patella tendon–tibia shaft angle as a function of knee flexion angle, and to compare this function between male and female subjects. This study shows that the patella tendon–tibia shaft angle varied from 27.8° to 12.2° for male subjects and from 34.8° to 13.3° for female subjects at 0° knee flexion. This angle varied from −0.1° to −11.3° for male subjects and from 5.4° to −2.1° for female subjects at 90° knee flexion. The results of this study further show that the averaged relationship between patella tendon–tibia shaft angle and knee flexion angle was a linear function of knee flexion angle and significantly affected by gender (P = 0.00). This relationship was expressed as EQUATION
for male subjects and EQUATION
for female subjects. The regression determinant of the best regression equation was 0.90. These results suggest that, on average, female subjects have a patella tendon–tibia shaft angle 3.6° greater than that of male subjects. Therefore, the between-subject variation in the patella tendon–tibia shaft angle may result in over 100% increase in the shear force applied on the tibia by the quadriceps muscle group at a given knee flexion angle. The results further show that the gender difference between female and male subjects may result in a 13% increase in the anterior shear force applied on the tibia by the quadriceps muscle group for female subjects at a given knee flexion angle. These significant differences in the anterior shear force applied on the tibia by the quadriceps muscle group, because of the between-subject variation and gender difference in patella tendon–tibia shaft angle, should qualify the patella tendon–tibia shaft angle as a potential risk factor for noncontact ACL injuries. Further studies are needed to investigate the anatomic factors that affect the patella tendon–tibia shaft angle, and the relationship between patella tendon–tibia shaft angle and noncontact ACL injuries.
Anatomic factors may play certain roles in the noncontact ACL injuries, but they have no contribution to the injury until the body is in movement. Therefore, the effects of anatomic factors on the risk of noncontact ACL injuries have to be evaluated dynamically during athletic tasks. There do not appear to be any studies addressing the interaction effects of anatomic factors and movement patterns on the rate of noncontact ACL injuries.
STUDIES ON HORMONAL FACTORS FOR ACL INJURIES
Although extensive efforts have been made to demonstrate the effects of female sex hormones on bone density and osteogenic cells, the evidence of effects of female sex hormones on connective tissue is limited. The results of the possible effects of female sex hormones on noncontact ACL injuries are not consistent. Liu et al. 52 identified estrogen and progesterone receptors from 17 human ACL specimens (13 from women and 4 from men). This opened the possibility that female sex hormones may have effects on ACL structure and composition. Next, Liu et al. 53 attempted to determine the effects of 17-β estradiol on fibroblast proliferation and collagen synthesis of human ACL. They found that collagen synthesis was reduced by over 40% with the normal physiologic levels of estrogen, and more than 50% with the pharmacologic levels of estrogen.
In an in vitro study, Yu et al. 54 simulated the effects of 17-β estradiol on cell proliferation and procollagen levels in human ACL fibroblasts during women's menstrual cycles. Samples of ACL from a healthy female subject were treated with 17-β estradiol for 1, 3, 7, 10, and 14 days to simulate the physiologic and supraphysiologic levels of estradiol presented in women's menstrual cycles. Cell proliferation and procollagen levels were used as indicators of collagen synthesis. There was a dose-dependent decrease in the proliferation of ACL fibroblasts with increasing estradiol concentrations. This dose-dependent effect of decreased fibroblast proliferation with increased estradiol concentrations became less apparent on days 7, 10, and 14. They also reported that procollagen synthesis decreased in a similar manner on days 1 and 3. They concluded that female athletes' menstruation causes early physiologic changes in fibroblast proliferation, and Type I procollagen synthesis in the ACL could predispose female athletes to ACL injuries. The authors suggested further studies with animal models to establish the effects of estrogen on the mechanical properties of the ACL and knee joint, to support the importance of this study in prevention of noncontact ACL injuries.
Slauterbeck et al. 55 investigated the effects of estrogen levels on the failure load of rabbits' ACL. Sixteen ovariectomized New Zealand white rabbits were divided into two age- and weight-matched groups. One group of rabbits served as the estrogen treatment group and received water-permeable Silastic implants filled with 0.25 mg of 17-β estradiol crystals. This produced a serum estrogen level of approximately 52 pg/mL, similar to that of a pregnant rabbit. The other group served as the control group and received water-permeable Silastic implants without estradiol. After 30 days, the rabbits were killed. Serum estrogen levels were measured at the time of ovariectomy and immediately before death. Knee specimens were prepared with only intact ACL attached to femur and tibia. The knee specimens were mounted on a material testing machine, with the anterior-lateral bundle of the ACL parallel to the direction of load application. Each specimen was loaded at a displacement rate of 0.5 mm/sec. The investigators reported a statistically significant 11% decrease in the ACL failure load of the estrogen treatment group, in comparison to the control group. Although the investigators did not record the locations of ACL ruptures during the experiment, their results provide certain theoretical background for studies of the association between female sex hormones and noncontact ACL injuries.
Although in vitro studies and in vivo studies with animal models provided basis for studies on female sex hormones as a risk factor for noncontact ACL injuries, the results of epidemiology studies are controversial. Moller-Nielsen and Hammar 56 studied the effects of the menstrual cycle and oral contraceptive use on traumatic knee injuries among women soccer players. Eighty-six female soccer players were recruited as subjects for a 12-month period. Female soccer players taking oral contraceptives had a significantly lower injury rate than female soccer players not taking oral contraceptives. In addition, female soccer players were more susceptible to traumatic injuries between days 1 and 14 of their menstrual cycle. Wojtys et al. 57 plotted the noncontact ACL injury rate in 40 female patients with their menstrual cycle to establish an association between women's ACL injury rate and the stage of menstrual cycle. Detailed injury mechanism and menstrual history data were collected, using a questionnaire. Patients were assigned to three groups, based on the relative date in their menstrual cycle when they were injured. The authors reported a trend (P = 0.09) toward an increased noncontact ACL injury rate between days 10 and 14 of the menstrual cycle, and a decreased noncontact ACL injury rate between days 1 and 9. 58,59 This partially conflicted with the results of the previous study. The study by Wojtys et al. used a common denominator to estimate ACL injury rates during three periods of the menstrual cycle. This raises the possibility that the differences among the observed injury rates during different stages of the menstrual cycle might be simply a difference in the sample sizes. Myklebust et al. 60 also investigated the association between female athletes' ACL injury rate and stage of menstrual cycle with 17 female handball players. In contrast to the results of previous studies, they reported a significantly decreased ACL injury rate between days 8 and 14. It should be noted that five subjects in the study by Wojtys et al. 59 and nine subjects in the study by Myklebust et al. 60 were taking oral contraceptives.
Although limited studies showed that female sex hormones may affect the mechanical properties of the ACL, the results of in vivo studies attempting to establish the association between sex hormones and ACL injuries are contradictory. Further studies with large sample sizes are needed to verify the relationship between ACL injury rate and female sex hormones. The ACL injury rates in different stages of the menstrual cycle should be estimated independently, and the effects of taking oral contraceptives on the results should be considered in future studies.
STUDIES ON BIOMECHANICAL AND MOTOR CONTROL FACTORS FOR ACL INJURIES
Altered neuromuscular control strategies, altered movement patterns, and body positioning are also believed to contribute to the increased incidence of noncontact ACL injuries for female athletes. 31,61 Considering that noncontact ACL injuries occur exclusively in dynamic situations, it is logical to study the association between motion patterns and the risk for ACL injuries. However, only a very limited number of studies have attempted to address the effects of movement patterns on the risk of noncontact ACL injuries. 62–64
McLean et al. 65 compared knee kinematics between female and male athletes during running and side cutting. Thirty high-performance athletes (14 women and 16 men) were recruited. Knee flexion, valgus–varus, and internal–external rotation angles were compared between genders and during different task. Although there appeared to be a constant difference in knee valgus angle between genders in side cutting task, there were no significant differences in the ranges of knee angular motions during the two tasks. They also reported a significantly increased intertrial variability of knee internal rotation pattern for their female subjects, compared with male subjects, during side cutting task. Gender differences in knee motion patterns did not contribute to the increased risk for ACL injuries in women. However, the subjects in this study were high-performance athletes, while the increased risk for noncontact ACL injuries in women were reported in collegiate and recreational sports. High-performance athletes may have learned how to control knee motions to avoid positioning their knees at risk.
Other recent studies provide evidence of a possible association between the risk for ACL injuries and lower extremity neuromuscular motor controls. Malinzak et al. 66 recently compared knee motion patterns between male and female recreational athletes in running, cutting, and jumping tasks. A recreational athlete was defined as a person who competes in a sport such as basketball, soccer, and volleyball at most three times per week, but does not follow a professionally designed training regimen. The study collected three-dimensional coordinates of selected body landmarks and dynamic electromyographic (EMG) data of the selected muscles. Knee joint flexion–extension, valgus–varus, and internal–external rotation angles were calculated. The normalized, integrated dynamic EMG (IEMG) of each selected muscle was obtained at 90° flexion. The knee angular kinematics and the normalized IEMG data were sampled from −50% to 100% of the stance phase in each task. Female subjects tended to have decreased knee flexion angle and increased knee valgus angle compared with male subjects in all selected athletic tasks (Fig. 2). The increased knee valgus angle of female subjects was consistent with McLean et al. 65 This study also showed that female athletes tended to have increased quadriceps muscle integrated EMG, indicating an increased quadriceps muscle activation and a decreased hamstring muscle integrated EMG, pointing towards decreased hamstring muscle activation in the selected athletic tasks (Fig. 3). Decreased knee flexion angle, increased knee valgus and internal rotation angles, increased quadriceps activation, and decreased hamstring activation increase the load on the ACL. 67–69 In addition, differences in knee motion patterns between male and female subjects started before landing (Fig. 2 and 3). 66 This suggests that the differences in knee motion patterns between male and female subjects might be due to different lower extremity motor controls.
Chappell et al. 70 compared knee kinetics in stop–jump tasks, followed by the kinematic study by Malinzak et al. 66 This study recruited 34 college-aged healthy recreational athletes (17 men and 17 women), as defined by Malinzak et al. 66 Gender differences in proximal tibia anterior shear force, knee flexion–extension and valgus–varus moments, and knee flexion angles were determined in three stop–jump tasks. The three stop–jump tasks were (1) stop–jump forward, (2) stop–jump vertically, and (3) stop–jump backward. Each stop–jump task consisted of an approach run of up to three steps, followed by a two-foot landing and then a two-foot takeoff. Females had significantly greater peak proximal tibia anterior shear force, greater knee extension moment, greater knee valgus moment, and smaller knee flexion angle at the peak proximal tibia anterior shear force than males during the landings of the three stop–jump tasks (Fig. 4). Anterior shear forces at the proximal tibia tend to load the ACL. 71–73 Also, increased knee valgus–varus moment tends to increase the load on the ACL, even when other conditions of knee joint kinematics and kinetics remain unchanged. 74,75 The increased knee extension and valgus moments and decreased knee flexion angle by female subjects provide further evidence that the observed differences in knee motion patterns between men and women may be associated with alternated lower extremity neuromuscular motor control. The literature and the recent kinetic studies suggest that the female athletes may have altered lower extremity motor control, which frequently brings them close to positions that place the ACL at risk during selected athletic tasks.
Chapell et al. 76 further compared knee kinetics and kinematics in different landing tasks. Female subjects had significantly greater peak proximal tibia anterior shear force than male subjects did in landing tasks with a horizontal movement component, but not in a simple vertical landing (Fig. 4). Female subjects also exhibited increased knee extension moment and knee valgus moment, and decreased knee flexion angle, than did male subjects when landing with a horizontal movement component (Fig. 4). These results suggest that the altered lower extremity neuromuscular motor controls of women may be mainly reflected in those landing tasks with a horizontal movement component. This indicates that landing tasks with horizontal movement components may be more dangerous for noncontact ACL injuries than simple vertical landings. Future training programs for the prevention of noncontact ACL injuries may need to focus on landing tasks with a horizontal movement component instead of simple vertical landings.
STUDIES ON FATIGUE AS A RISK FACTOR FOR ACL INJURIES
Fatigue is often suggested as a risk factor for noncontact ACL injuries. Nyland et al. 77 investigated the effects of fatigue on ground reaction forces, lower extremity kinematics, and muscle activation during running, rapid stop tasks, and cross-cutting. After fatigue, running and rapid stop performance showed trends of late quadriceps and hamstring muscle onset of activation, and early occurrence of maximal knee flexion. They suggested that the late quadriceps and hamstring muscle activation and early maximum knee flexion enhanced shock absorption at landing and knee stabilization in the presence of fatigue. The same group of investigators studied the effects of quadriceps and hamstring fatigue on the knee and ankle kinematics and kinetics during cross-cutting task. 78 Quadriceps fatigue results in increased ankle dorsiflexion moment, decreased peak posterior breaking force, decreased peak extension moments, and delayed peak knee flexion angle. Hamstring fatigue resulted in decreased peak impact knee flexion moment, increased internal tibia rotation, and decreased peak ankle dorsiflexion.
Rozzi et al. 79 and Wojtys et al. 80 investigated the effects of fatigue on neuromuscular function and anterior tibia translation. Both groups reported a significant increase in anterior tibia translation in an anterior tibia translation stress with quadriceps and hamstring muscle fatigue. Skinner investigated the effect of fatigue on the proprioception of the lower extremity. 81 Fatigue reduced their subjects' ability to reproduce joint positions.
Chappell et al. 82 recently investigated the effects of lower extremity muscle fatigue on the knee kinetics and kinematics in stop–jump tasks in 10 male and 10 female recreational athletes. The investigation used a fatigue exercise of repeated 30 m sprints and vertical jumps to simulate the fatigue from basketball, soccer, and volleyball games, where noncontact ACL injuries frequently occur. Subjects performed three stop–jump tasks before and after the fatigue exercise. They showed that both female and male subjects increased their peak proximal tibia anterior shear force during landing of each stop–jump task, and significantly increased their knee valgus moment and decreased knee flexion angle when fatigued (Fig. 5). However, female and male subjects had different mechanisms for the increased peak proximal tibia anterior shear force in the postfatigue exercise test. Both male and female subjects in this study showed decreased knee flexion angle in the postfatigue exercise test, but they also showed different changes in knee valgus–varus moment (Fig. 5). The male subjects showed decreased knee varus moment, while the female subjects showed increased knee valgus moment. Previous studies suggest that a decreased knee flexion angle tends to increase the stress on the ACL. 67–69 An increased knee valgus or varus moment tends to increase the stress on the ACL, as well. 74,75 These results suggest that the increased peak proximal tibia anterior shear force due to the lower extremity muscle fatigue may be mainly caused by a decreased knee flexion angle for male subjects, but may be a combined result of decreased knee flexion angle and increased knee valgus moment for female subjects. The results of this study added fatigue to the list of potential risk factors for noncontact ACL injuries.
Injuries to the ACL are common in sports, with devastating effects on the quality of life for patients. Most ACL injuries are noncontact in nature. Most noncontact ACL injuries occur in sports that require frequent cutting, pivoting, sudden stops, or landing from a jump. In comparable sports, female athletes have higher risk for noncontact ACL injuries than their male counterparts. Noncontact ACL injuries may be preventable, if modifiable risk factors can be identified. Several factors have been proposed and categorized as intrinsic and extrinsic factors, or as environmental, anatomic, hormonal, and biomechanical and motor control factors. Extensive studies of these proposed risk factors have been undertaken. No studies, however, have critically established the association between any proposed risk factors and noncontact ACL injuries. Accumulated evidence suggests that alternated lower extremity neuromuscular motor controls may contribute significantly to the risk for noncontact ACL injuries. Future studies should critically establish the association between these risk factors for noncontact ACL injuries, and focus on modifiable factors. Environmental factors should be studied with the consideration of performances. The effects of female sex hormones on mechanical properties of the ACL and knee joint should be conducted, considering knee motion patterns in athletic tasks. Anatomic and hormonal factors need to be studied dynamically, while considering biomechanical and motor control factors.
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Nicola Maffulli, M.D., Guest Editor
Keywords:© 2002 Lippincott Williams & Wilkins, Inc.
Anterior cruciate ligament; Injury; Injury prevention; Biomechanics; Motor control; Female athletes