The bulk of anterior cruciate ligament (ACL) tears occur during sports participation, particularly sports such as soccer, basketball, and football where pivoting and cutting maneuvers are common occurrences (32,22). Among these injuries, the majority result from no forceful contact by another individual, but rather the ACL was damaged during deceleration, acceleration, plant-and-cut movements, sudden change of direction, landing from a jump, or other movements that can excessively load the knee. Such loading combined with high injury risk motions, such as knee valgus motion, where the knee moves medially similar to a “knock-knee” stance (Figures 1–4) can potentially strain the ACL, making it susceptible to possible damage (3,20,46).
Commonly, female athletes tend to exhibit knee valgus and decreased knee flexion during landing, resulting in anterior shear forces and stress on the ACL. Moreover, they tend to perform cutting maneuvers with their knees more extended (46). Consequently, these events contribute to the tendency of female athletes having higher ACL injury rates than male athletes in the same sport, and among female athletes, basketball and soccer players tend to have a higher incidence of injury (3). Anatomical factors, such as intercondylar notch width and knee joint laxity, along with developmental and hormonal factors (such as gender) ACL tensile strength, and menstrual status have been proposed as ACL injury risk factors. Unfortunately, these are factors that are difficult to modify (unlike movement patterns, muscle strength imbalances, and muscle activation patterns) all of which may be altered with proper training. Therefore, current ACL injury prevention interventions have primarily focused on modifiable biomechanical and neuromuscular control factors, such as, knee abduction, hamstring recruitment, excessive leg rotation, and decreased knee flexion (2,19).
INJURY RATES OF HIGH SCHOOL FEMALE BASKETBALL AND SOCCER ATHLETES
When considering the overall injury patterns of high school athletes in the United States, boys may have a higher injury rate particularly when sports not commonly played by both genders, like football and volleyball were included. For instance, in a study that compared severe injury rates in football, soccer, basketball, volleyball, wrestling, baseball, and softball, severe injury rates were higher in all the sports played by boys (r = 0.45) than those played by girls (r = 0.26). However, in the same study, when severe injury rates were compared among common sports, girls had significantly higher severe injury rates than boys when participating in the same sport such as basketball or in a similar sport such as softball (15). Female athletes tend to have higher injury rates than male athletes particularly in comparable sports such as basketball, soccer, baseball, and softball, where the rules of play and equipment are similar. Specifically, it has been shown that girls’ basketball and soccer teams have higher injury patterns than boys’ basketball and soccer teams (15,55). Furthermore, in a surveillance of North Carolina schools, girls’ soccer had the second highest injury incidence when comparing basketball, soccer, track, baseball, softball, wrestling, cheerleading, volleyball, and football (36).
The difference in injury rates between males and females are also present when comparing knee injuries. The knee is a commonly injured site among high school athletes, and knee injuries have been shown to make up the majority of injuries requiring surgery (15,35). Female basketball and soccer players have been inclined to have more knee injuries, knee surgery, and ACL surgery than male basketball and soccer players (55). The higher rates of knee injury in female basketball and soccer players compared with male basketball and soccer players are confirmed by Darrow et al. (15) who reported that girls had a higher amount of severe injuries to their knees than boys, and a large proportion of these knee injuries were caused by complete ligament tears. Although higher knee injury rates in boys than in girls have been observed (35) while examining high school athletes in the sports of football, soccer, basketball, baseball, wrestling, volleyball, and softball, these rates may have differed if the knee injury rates were examined among comparable sports only.
Despite the lack of a comparison of injury rates between males and females in similar sports, Ingram et al. (35) concluded that girls’ soccer and basketball still ranked among the sports with the highest knee injury rates, accompanied only by football and wrestling, which are primarily contact sports. Major knee injuries caused by noncontact mechanisms were twice as likely to occur in females as males, and they were the primary mechanism of injury in girls’ basketball and soccer, which were the top 2 high school girls’ sports with the most frequent knee injuries (35). Complete ligament tears in the knee were most common among female basketball and soccer players, with basketball players having a higher fraction of reinjures to the knee (56), and female athletes overall being more likely to have higher rate of reinjures than their male counterparts in comparable sports (55). Such disparities among female basketball and soccer players compared with other athletes are also evident among collegiate athletes. Collegiate women’s basketball and soccer players have been shown to have statistically higher rates of ACL injury than women’s lacrosse players (1) and men’s basketball and soccer players (23).
The higher prevalence of knee injury, particularly ACL injury, in female athletes compared with male athletes and female basketball and soccer players compared with other female athletes provides strong evidence for injury prevention training specifically within this population to minimize the occurrence of injury during activities involving little or no player contact. Thus, this article focuses on young female basketball and soccer players in an effort to increase the players’ strength and stability during common and frequent sport-related activities to reduce their risk of injury, particularly knee injuries.
LANDING AND CUTTING PATTERNS
NEUROMUSCULAR AND BIOMECHANICAL CONSIDERATIONS
Biomechanical and neuromuscular factors have been related to ACL injury, especially injuries occurring while pivoting, decelerating, cutting, and landing from a jump (10). It has been suggested that lower extremity alignment during movement influences the amount of stress on supporting structures of the knee, such as the ACL. Lower extremity misalignment exacerbates the stress on the ACL, leading to structural damage over time or if combined with an excessive load and force, it may result in complete structure failure at that time. High-risk lower extremity misalignment, which occurs when the tibia externally rotates and valgus motion occurs at the knee, has been noted in soccer players during cutting maneuvers.
Misalignments in basketball players during landing occur when the players hyperextend the knee with simultaneous internal rotation of the tibia (9). Overall, female athletes tend to cut and land with the knee more extended, thereby placing more stress on the knee. Neuromuscular factors such as muscle activation and resultant muscle strength and force are also suggested to contribute to ACL injury. Musculature around the knee provides stability for the knee, yet gender differences have been found in muscular strength, muscle recruitment, and muscle coactivation patterns, which all influence knee stability (46).
Female athletes tend to land and cut with less knee flexion than male athletes, which may contribute to their higher incidence of ACL injury (54). The ACL is the principal restraint to anterior tibial displacement, the forward sliding motion of the tibia relative to the femur occurring when the knee extends. Assuming 75% of the force exerted by a fully extended knee, the ACL endures more strain with less knee flexion (50). Ligament strain is referred to the deformation of the ligament as a result of continued loading. As ligaments are loaded initially, fibers in the ligaments elongate, and as loading continues, the tissue stiffens and more force is required for continued elongation. This further elongation is conveyed as strain and refers to the deformation of the tissue in reference to its original length. As strain increases, the ligament is subject to failure and potential subsequent rupture and tearing (51).
The injury risk associated with decreased knee flexion has been seen among female adolescent basketball, soccer, and volleyball athletes who participated in preseason biomechanical screenings and regular season observational follow-up for ACL injury incidence. Female athletes who had an ACL injury in the regular season had a maximum knee flexion during landing that was 10.5° less than those who did not experience an ACL injury, yet no difference in knee flexion were found at initial contact (30). Initial contact is the moment of first contact of the feet with the ground and does not constitute complete landing, which is when the player is fully pushing against the ground, resulting in a ground reaction force. Ground reaction force is the force the ground exerts on the body and is equal in magnitude to body weight.
When landing from a jump, a player dispels his or her momentum by pushing against the floor, resulting in a ground reaction force that is equal and opposite to the force exerted by his or her body. However, during deceleration, the ground reaction force exceeds the force exerted by body weight. The magnitude of the resultant ground reaction force is influenced by body posture during landing. Landing with a stiff upright posture causes a player to decelerate quickly, resulting in large mean and peak ground reaction forces, which may increase the risk of injury. Landing with knees, hip, and ankles flexed increases deceleration time, resulting in a smaller ground reaction force (60) (Figure 5).
Adolescent athletes participating in jumping sports, such as basketball, tend to have an average peak vertical ground reaction force that is 4.5 times their body weight (45), and those participating in gymnastics tend to land on heels, toes, or without flexing knees have been shown to have peak vertical ground reaction forces that are 3 to 8 times their body weight. Coordinated activity by skeletal muscles help control the motions of joints, and this coordinated action helps to absorb the compressing actions of the opposing forces of body weight and ground reaction forces (60). Skeletal muscle creates moments of force, the effect of force that causes twisting, rotational, or bending actions. These moments of force, also referred to as torque, control joint motions (61). In the previously mentioned study of female adolescent basketball, soccer, and volleyball athletes participating in preseason biomechanical screenings and regular season observational follow-ups for ACL injury incidence, the knee flexion moment did result in knee flexion for all athletes, but the athletes who eventually experienced an ACL injury flexed the knee less during the preseason screenings and regular season follow-ups compared with those who remained uninjured (30).
The tendency of athletes to flex the knee less during landing may be demonstrated even more in practice and games, where unlike a controlled laboratory environment, maneuvers are faster, immediate, and multidirectional, causing knee flexion to decline further. For instance, male and female high school basketball players have been shown to display significantly less knee flexion when performing jumps that were reactive rather than planned, and they tended to have less maximum knee flexion when jumping to the left compared with jumping vertically or to the right. Moreover, the effect of jump anticipation and jump direction on the degree of knee flexion was larger in females, who had less knee flexion during reactive jumps and when jumping to the left compared with males (57).
Cutting maneuvers can create large knee flexion moments on the knee especially at the point when the player pushes off the most, and these moments are possibly the result of an increase in flexion load (6). Knee flexion moments can also vary depending on the angle of the cut and whether the cut is abrupt or planned, resulting in varying degrees of knee flexion. Male soccer players have been shown to experience a 19% decrease in knee flexion when performing an unanticipated side-step cut at 60° (6). The degree of knee flexion can also be influenced by the presence of a defensive opponent. Increases in knee flexion have been observed while performing a cutting maneuver when a defensive opponent was present; however, despite this increase in knee flexion, female athletes were more liable to have less knee flexion during side-step cutting whether a simulated defensive opponent was present or not (44). Additionally, in a study of male and female collegiate recreational basketball, soccer, and volleyball players, knee flexion angle displayed by the females during cross-cutting was 8° smaller than that displayed by males (42). Female basketball and soccer players who land and cut with little knee flexion may have an increased risk of ACL injury considering that most stress on the ACL occurs when the knee is near full extension (50).
Knee valgus and varus alignments
In addition to resisting anterior displacement of the tibia relative to the femur, the ACL also functions as a resistor to valgus, varus, and tibia rotation. Valgus loading of the knee causes the leg to curve inward or adduct at the knee, and varus loading causes the leg to curve outward or abduct at the knee (62). It is suggested that valgus and varus knee alignments during landing and cutting increase the risk of ACL injury (25,54). The movements necessary to land from a jump or perform a cut may be responsible for the valgus and varus occurrences at the knee because these alignments are exhibited more during landing and cutting than during running (21). For instance, varus and valgus motions were significantly different during cutting maneuvers as opposed to a straight run in male soccer players (6).
Valgus or varus motions are also more pronounced when the task is abrupt and may occur more when players perform maneuvers unexpectedly (57). A particular side may also be more susceptible to valgus motions during landing and cutting, with female high school basketball and soccer players experiencing greater valgus moments on their dominant side (the leg the athlete would preferably use to kick a ball) during landing and cutting (14,17,29). Female athletes tend to experience more valgus and varus knee motions than male athletes during cutting and landing. At initial contact, female basketball and soccer players have been shown to have greater knee valgus when landing (18) and greater knee abduction angles when cutting (5) compared with their male counterparts. This tendency for valgus motion in female athletes is not limited to the point of initial foot contact or to landing and cutting tasks only.
Malinzak et al. (42) observed that female athletes consistently displayed valgus motion throughout each cutting task as well as during running. The tendency for female athletes to have more valgus motion than male athletes was consistent despite gender differences such as height. For instance, female basketball players tended to have significantly more valgus motion during landing compared with male players even when the values were normalized for height (17). These gender differences between males and females may develop during adolescence, with girls displaying more valgus motion as they age, considering Hewett et al. (29) found no difference in medial motion between prepubertal boys and girls. Medial motion refers to the movement of the knee toward the midline of the body; in other words, it indicates valgus motion, and in this study, prepubertal girls tended to demonstrate medial motion as much as prepubertal boys. However, postpubertal girls demonstrated more medial motion and larger valgus angles during landing compared with postpubertal boys and prepubertal girls.
Notwithstanding this differentiation, differences in valgus motion are not apparent between sports, and high school female basketball and soccer players may have similar risks of injury particularly when performing cuts (14). Valgus and varus alignments are a potential risk of ACL injury. Specifically, knee valgus moments and angles are significant predictors of ACL injury in adolescent female basketball and soccer players (30). Players who had an ACL injury during the regular season demonstrated greater knee abduction angles at initial contact and maximum displacement during landing in preseason screenings (30). Video analysis measuring foot position and lower extremity angles preceding ACL injury revealed that knee abduction moments increased progressively following initial foot contact with the ground, and females had more knee abduction than males (8).
Chaudhari and Andriachi (12), using a lower extremity model, evaluated knee alignments and resultant ACL thresholds, which they defined as the maximal amount of force the ACL can endure before the joint opens medially or laterally more than 8°. According to their model, the ACL injury threshold was the highest, 5.1 times body weight, during a neutral alignment. However, when alignments changed to either valgus or varus angles, the threshold decreased to 2.2 and 2.1 times body weight, respectively. In other words, with valgus or varus motions in the knee, the amount of force it takes to injure the ACL would be less, and with excessive valgus motion, the injury to the ACL may be preceded by a medial collapsing of the knee (39). Valgus motions of the knee have been shown to predict and precede ACL injury, and female basketball and soccer players tend to display these motions more than male players, particularly during landing and cutting; consequently, these players may have an increased risk of resultant ACL injury.
Increased hip adduction, decreased hip flexion, and internal rotation are also motions that may subject athletes to an increased risk of ACL injury (54). The degree of hip adduction has been shown to predict the degree of knee abduction or varus motion during cutting in high school female soccer players (34). Female basketball and soccer players also may have an increased hip adduction during landing causing knee valgus motion; however, isolated hip adduction moments may not be a risk factor for ACL injury itself (2). Female athletes tend to flex their hips less compared with male athletes when cutting, and this decrease in flexion is also seen when facing a defensive opponent (44). The degree of hip flexion during landing helps determine the magnitude of force at the knee because hip flexion combined with knee and ankle flexion help to decrease ground reaction forces (2,60).
Quadriceps and hamstrings strength
Knee stability occurs passively and dynamically. Bones, menisci, ligaments, and the joint capsule compose the passive system of knee joint stability (25). For example, the ACL primarily stabilizes the knee by regulating anterior and posterior translation, varus and valgus motions, and internal and external rotations (50). The dynamic system of stability consists of muscle contraction, occurring primarily by volitional muscle activation. Knee joint stability relies on a balance between both systems, but variations in strength between the quadriceps and hamstrings in female athletes place them at the risk of stressing their ACL. As the knee extends, an anterior force occurs at the knee, and the ACL is responsible for regulating this force by restraining the knee and resisting anterior displacement.
In addition to this passive system of knee stability, the hamstring functions as an antagonist to this anterior motion and contracts to help restrain the knee joint by pulling it in the posterior direction. If the quadriceps contracts strongly when the knee is between 0° of flexion (fully extended) and 45° of flexion and the hamstring contraction does not match the strong quadriceps contraction, the anterior force that is produced can considerably stress the ACL possibly resulting in an injury (25). Female basketball and soccer players, who subsequently experienced an ACL injury, when assessed during preseason for dynamic strength, had decreased hamstrings strength but no decreased quadriceps strength compared with male players (47). Quadriceps strength in these female athletes did not differ from that in male athletes, but the decrease in hamstrings strength resulted in the quadriceps being stronger than the hamstrings. This imbalance could have resulted in the quadriceps overpowering the hamstring and stressing the ACL, thereby contributing to the conditions that caused their injuries because female athletes who did not experience an ACL injury had decreased quadriceps strength but no decreased hamstrings strength compared with male athletes. Therefore, it is important to use training programs that will challenge and develop both the quadriceps and the hamstrings.
Muscle co-contraction: quadriceps and hamstrings balance in force
Balanced co-contraction of the quadriceps and hamstrings influences the degree of anterior tibia translation and decreases the anterior force stress on the knee. Li et al. (41) demonstrated the effects of isolated quadriceps loading and combined quadriceps and hamstring loading at angles of 0°, 15°, 30°, 60°, 90°, and 120° of knee flexion in cadaver knees. During isolated quadriceps loading, anterior tibial translation increased from full extension (0°) to 30° of knee flexion; however, as knee flexion increased, anterior tibial translation decreased. Forces on the ACL, during isolated quadriceps loading, increased from 0° to 15° of knee flexion and then decreased as the knee flexion angles increased. On the other hand, when quadriceps loading was accompanied with hamstring loading, anterior tibial translation was significantly reduced at all angles except at 0° and 15° knee flexion. Forces on the ACL were significantly reduced with quadriceps and hamstrings loading at 15°, 30°, and 60° of knee flexion. Forces at 15° of knee flexion, which were highest with isolated quadriceps loading, were reduced by 23%. Moreover, investigators found no significant difference between isolated quadriceps loading and combined quadriceps and hamstring loading when the knee was fully extended, suggesting that athletes should avoid this knee position as much as possible. Nevertheless, concurrent hamstring contraction during movements involving forceful quadriceps contraction is beneficial to reduce strain in the ACL.
Timing of muscle recruitment and muscle activation
Female athletes tend to activate their quadriceps more than their hamstrings, and their quadriceps tend to reach peak torque before their hamstrings. When muscle activation patterns were measured in collegiate recreational basketball, soccer, and volleyball athletes, via electromyography (EMG), quadricep’s EMG activity in females was consistently greater than that in males especially during running and side-cutting. Conversely, hamstring’s EMG activity in females was less than that in males particularly during running and cross-cutting (42). Huston and Wojtys (33) detected that female collegiate athletes reach peak torque in knee flexion slower than male collegiate athletes, but their time to peak torque in knee extension was similar. At higher speeds, female athletes still reached peak torque in hamstrings significantly slower than male athletes; however, their time to peak torque nearly equated the peak torque time seen in female nonathletes. Overall, investigators concluded that female athletes generated peak torque in the hamstring an average of 11 milliseconds after reaching peak torque in the quadriceps, whereas the other groups reached peak torque in the hamstring an average of 3–6 milliseconds before reaching peak torque in the quadriceps.
Activation of the hamstrings prevents knee hyperextension, but the ability of the hamstrings to prevent this motion relies on the amount of force it is capable of producing, which indicates the degree of muscle strength. It has been suggested that hamstring antagonist coactivation patterns are very low in athletes who do not routinely engage in training programs consisting of hamstring strengthening exercises, but significant changes in antagonist coactivation patterns in the hamstrings occur after a strength training program (4). Such evidence supports the need for female athletes to participate in training programs that are designed to equally train and strengthen the hamstrings and quadriceps to minimize imbalances between these muscles.
INJURY PREVENTION TRAINING PROGRAMS
A REVIEW OF ACL INJURY PREVENTION PROGRAMS
Neuromuscular training interventions with emphasis on core strengthening, joint stability, balance training, and jump training have been shown to modify and improve landing errors such as knee valgus and varus motions, and knee hyperextension (11,16,20). In addition, plyometric training, which includes jumping drills such as wall jumps and tuck jumps, has been implemented in ACL injury intervention programs in an attempt to modify inadequate muscle activation, decrease landing forces, decrease valgus moments and knee rotations, and to increase hamstring strength. Increases in preparatory muscle coactivation, decreases in landing forces, and increases in hamstring torque have been observed after plyometric training (13,31,48,52,58,63). Strength training has been implemented as a component of neuromuscular training programs (49), as well as an isolated program, and it has been shown to decrease knee valgus and hip adduction, increase muscle strength, and increase knee flexion angle (26,27).
Plyometric training involves multidirectional consecutive jumping, where the athlete jumps, lands, and immediately jumps again. Correct posture and body alignment are emphasized as well as jumping straight up rather than leaning to the side or to the front or back. Athletes are instructed to land softly with knees and hips flexed while immediately preparing to jump again (31,58). Programs tend to progress in the level of difficulty by requiring athletes to transition from jumping and landing with both feet to jumping and landing with one foot (53) and focus on building strength, power, and agility (31). Plyometric training has been shown to decrease landing forces and knee abduction and adduction moments (31) and increase hamstring strength (64).
Decreases in landing forces after plyometric training have been observed in college women involved in recreational sports (58) and in female high school volleyball players who, for example, decreased their peak landing force by 22% (31). Increases in performance have also resulted from plyometric training. Improvements in sprint speed and vertical jump height in NCAA female soccer players (13), increases in vertical height in high school volleyball players (31), and increases in average power and improved body position in female collegiate basketball players (63) have been noted. However, plyometric training alone may not reduce the risk of ACL injury, but it may be more effective when combined with other types of training. Female high school basketball, soccer, and volleyball athletes were assessed for ACL injury after a plyometric program, where athletes trained for 20 minutes before or after practice. Those who participated in the plyometric program incurred the same amount of ACL injuries as those athletes who did not participate in the program. Furthermore, all ACL injuries were incurred by basketball and soccer players, with basketball players experiencing more injuries than soccer players (53).
On the other hand, plyometric training combined with other types of training may decrease the risk of injury. For instance, female high school soccer players who participated in plyometric training, sports-specific cardiovascular conditioning, sport cord drills, strength training, flexibility training, and acceleration drills with emphasis on body position awareness and avoidance of high-risk movements had less ACL injuries than those who did not train in these components. Specifically, trained athletes had 1 ACL injury as opposed to 8 ACL injuries in untrained athletes (24).
Movement is a result of muscle force acting on the skeletal system, specifically joints, causing torque or moments, expressed as rotating, twisting, or bending actions. These actions depend on how strongly the muscle is stimulated and the degree of force production in the muscle (37). The maximal amount of force that a muscle generates is referred to as the muscle’s strength, and this strength is altered with training (37). The concept of strength training involves placing a demand on muscles to perform beyond their current levels, referred to as overloading. When muscles are consistently overloaded, the neuromuscular system adapts, resulting in an increase in muscle size, increased motor unit recruitment, and improved coordination of the agonists, all of which influence muscle strength.
Strength training exploits this principle of adaptation and progressively overloads the body to elicit further adaptations resultant in an increased strength and an improved athletic performance (37). Training increases strength in the muscles specifically trained, resulting in an increase in maximum voluntary contraction (27) demonstrated through increases in 1-repetition maximum tests (7,38) and improved athletic performance. Recreational female athletes, following a strength training program emphasizing quadriceps, hamstrings, gluteus maximus, and gluteus medius development, demonstrated an average increase in strength in these muscles ranging from 35% to 48% (27). A strength gain in the hamstrings is important when considering the role of hamstring co-contraction in reducing ACL strain (64). Reductions in ground reaction forces have also been noted after strength training. In the assessment of landing mechanics of female recreational athletes who participated in strength training and those who did not, athletes performed initial jumps and then were given immediate feedback on their jumping and landing biomechanics. Athletes were allowed to view their jumps along with demonstrations of proper jumping and landing technique before performing subsequent jumps. Those who participated in the strength training program experienced a greater decrease in peak vertical ground reaction force. Peak knee anterior shear force also decreased in this group, but increased in the group with no strength training. Although both groups experienced a decrease in knee valgus moment and an increase in knee flexion during landing, the magnitude of both was greater in athletes who did not participate in a strength training program (26). Others have indicated no changes in lower extremity motion patterns such as peak anterior tibial shear force and vertical ground reaction force with only strength training (27), suggesting that although strength training is beneficial for strength gains, it may not be very effective in translating the increases in strength to modifying some of the biomechanical injury risk factors such as decreased knee flexion and knee valgus that are associated with ACL injury. Such factors may be modified more effectively with more dynamic training that is more representative of common athletic maneuvers; however, resistance training is an essential component of overall training because it provides a solid foundation for athletic performance.
In an 8-week training program implemented with female basketball and soccer players, players were categorized into 2 groups consisting of either plyometric training or basic resistance training. Players trained in their respective programs 3 days a week for 30 minutes each day. Both groups significantly improved in quadriceps strength, but there was no significant difference in hamstring and hip abduction strength between baseline and postintervention. Kinematic data revealed that both programs promoted increases in knee flexion at initial contact, peak knee flexion, and time to peak knee flexion during landing. Both groups also demonstrated a decrease in knee flexion moment and hip flexion moment; however, there was no difference in vertical ground reaction force between the groups (40). Thus, this study further indicates the need for a more comprehensive training program.
Individually, each training program may be effective in altering some modifiable factors associated with an increased risk of ACL injury such as the degree of knee flexion, quadriceps and hamstring muscle strength and force imbalances, landing forces, and knee valgus motions. However, a training program that incorporates all or most of these individual programs, such as neuromuscular training programs, may prove more effective. Neuromuscular training tends to incorporate strength training, balance training, plyometric training, and proprioceptive training, in addition to core strengthening and dynamic joint stability training. Such training has been shown to increase knee flexion, decrease knee flexion moment and knee valgus moment, and decrease maximum knee valgus in female collegiate basketball and soccer players during landing (11). Promising results have also been observed in female high school basketball, soccer, and volleyball athletes who participated in a 6-week program with similar components and sessions occurring 3 days a week for 60–90 minutes per session (28). Athletes who trained in the program had significantly less ACL injuries than those who did not train in the program. Moreover, this reduction in ACL injury may persist over seasons.
Female soccer players aged 14–18 years were observed for 2 consecutive seasons while participating in a 20-minute prepractice program. Investigators found that those who trained had significantly lower occurrences of ACL injury than those who did not train. In the first season, the trained group experienced 2 ACL tears and the untrained group experienced 32 ACL tears, and in the second season, the trained group experienced 4 ACL tears compared with 35 ACL tears in the untrained group (43).
Neuromuscular training has also been shown to increase performance in female high school athletes. Basketball, soccer, and volleyball players have increased strength, improved jump distance and height, and decreased sprint times after a training program incorporating plyometric training, core strengthening and balance, resistance training, and speed training consisting of interval sprinting with nonresisted and partner-resisted sprinting (49). Additionally, female soccer players aged 13–18 years who trained 3 times a week before practice had faster sprint times (50). Considering the evidence, it would be beneficial to use a multicomponent program when attempting to increase knee stability when landing and cutting because such a program may prove more effective.
OFF-SEASON TRAINING PROGRAM
The following landing and cutting stability training program (Table 1) is intended to train athletes in such a way that they have sufficient neuromuscular activation, muscular strength, and technique to successfully land and cut with less stress on their knee joint, which may translate to reduction in injury risk. The training program incorporates strength, plyometric, lateral, agility, balance, and coordination as well as flexibility training, all of which are incorporated with emphasis on being aware of body position and moving with balance and stability (Table 2). Overall, the training program is aimed to teach athletes how to move properly, and the strength gained provides them with the capacity to do so. All training days should be preceded by a proper dynamic warm-up, which should include linear and lateral movement to prepare the athletes for subsequent activities, and training should conclude with proper stretching and flexibility exercises. This off-season training program may coincide with other team training and serve as an additional component to routine strength and conditioning and sports skills training.
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