*Jon. H Hinds, inventor of the Power Wheel and owner of Monkey Bar Gymnasium, 600 Williamson St, Suite K-2, Madison, WI 53703 (www.monkeybargym.com).
Research evidence estimates that ACL injuries occur in <100 milliseconds, whereas reflexive muscular activation takes an estimated 128 milliseconds (62). This suggests that ACL injuries occur too fast to allow a reflexive muscular response to prevent the injury (47,52). Chappell et al. showed that untrained female recreational athletes prepare for jump landings with decreased hip and knee flexion at landing, increased quadriceps and decreased hamstrings activation (16). This landing posture and muscle activation imbalance may increase ACL strain and suggests that untrained female athletes may demonstrate predisposition to noncontact ACL injury because of inherent preprogrammed motor strategies.
Based on this premise, 1 factor in the success of recent ACL injury prevention programs is likely attributed to an improvement in the efficiency of the feedforward mechanism for dynamic lower extremity stability. The feedforward mechanism may be defined as involuntary or automatic anticipatory postural adjustments or activation occurring before a perturbation (4). Female athletes may adopt preprogrammed or preparatory muscle recruitment and movement patterns that reduce the probability of injuries caused by unexpected perturbations that occur during sport tasks (45,47). This preparatory muscle activity may improve reactive muscle activity via the muscle spindle by identifying unexpected perturbations more quickly, and potentially reduce the risk for knee ligament injury (52). Increased excitation of afferent pathways to muscle spindles occurs with perturbations, suggesting reactivity may be improved with training (52). These preparatory or preprogrammed patterns may be modifiable and reinforced with specific neuromuscular training designed to simulate sport-specific tasks with an emphasis on safer kinematics. These modifications to existing motor patterns are particularly important because more organized and efficient muscular preactivation may increase knee joint stiffness and dynamic stability to protect articular structures (19).
Myer et al. found improvement in sagittal and frontal-plane kinematics after plyometric training, observing increased hip and knee flexion at landing (86). Improvements in movement kinematics have also been reported in other studies using multicomponent training designs with decreases in frontal-plane motion and increased knee flexion angles (17,42,87). Neuromuscular training emphasizing landing technique with increased knee flexion angles may also improve effectiveness of hamstrings activation to limit anterior tibial shear. Increased knee flexion and hip flexion during landing tensions the hamstring muscles to provide a posterior force upon the knee to protect the ACL. This contention is further supported by the increased peak hip and knee flexion moment during landing (64). Based on differences observed after incorporating plyometrics, training programs that incorporate safe levels of varus and valgus stress may induce more “muscle dominant” neuromuscular adaptations (87). Confirming this theory, Hewett et al. began a prospective study implementing 6 weeks of plyometric training in female soccer, basketball, and volleyball high school athletes in 1999 (44). Female athletes that did not receive the training program had a 3.6 times higher incidence of knee injury vs. the trained group, suggesting significant benefits of plyometric neuromuscular training in female athletes (44). Hewett et al. completed a meta-analysis of ACL injury prevention programs that supplies more evidence for use of plyometrics. They found that 4 studies that used high-intensity plyometrics successfully reduced incidence of ACL injuries, while those that did not found no change in ACL injury incidence (43).
Technique during plyometrics and sport-specific tasks should be closely monitored by strength and conditioning professionals. Mizner et al. found that trunk and lower extremity strength were poor predictors of improvements in landing mechanics but that female athletes were able to make significant positive changes in landing mechanics in repeat trials after brief verbal instruction (77). Similarly, Padua et al. completed a systematic review of 6 studies assessing ACL injury prevention programs and found that each study demonstrating significant decreases in VGRF used verbal instructions and feedback for proper landing technique, auditory cues for minimizing landing forces, and performance under direct supervision on a regular basis (96). In contrast, those studies indicating no change in VGRF did not incorporate regular verbal or auditory feedback and performance under direct supervision on a regular basis (96). Prapavessis and McNair successfully demonstrated that subjects were capable of decreasing ground reaction force with jumping via altered technique after only 1 feedback session (106). Female athletes can also be trained to alter their knee range of motion (ROM) beneficially during landing when verbally cued (22,93,106,107). The emphasis on ROM, specifically increased hip and knee flexion during landing is particularly important because women landing in less knee and hip flexion demonstrate increased knee valgus angles, decreased energy absorption, and increased vastus lateral activity (96). These biomechanical changes potentially increase ACL injury risk as noted previously.
Several studies have also noted a significant reduction in ACL injury risk factors using balance and proprioceptive training in isolation and as part of a multicomponent program (15,51,99). Paterno et al. did confirm increases in single-leg total dynamic stability and in anterior-posterior balance with a 6-week neuromuscular training program for ACL injury prevention (99). Fitzgerald et al. also demonstrated improvements in function, crossover hop test scores, and knee stability using perturbation training in ACL-injured subjects, suggesting beneficial alteration of motor patterns (34). Hurd et al. found perturbation and balance training improved relative quadriceps to hamstring ratio on integrated EMG with improved hamstring and gastrocnemius activation timing after training, potentially decreasing ACL strain (52). Padua et al. also noted improvements in biomechanics and muscular stability with perturbation training, stating that perturbation training and exercises that require balancing on a single leg have increased antagonist coactivation of the knee flexor muscles, which may produce greater knee flexion (94). Although many of these studies used additional equipment including unstable surfaces, on-field challenges to balance may be performed in single-leg stance using partnered manual perturbations, eyes closed to decrease visual feedback, and sport-specific dynamic balance challenges, which have all been found to improve dynamic balance in healthy individuals (Figures 8A-C) (26). Additionally, gastrocnemius and soleus training may also be an important element in enhancing balance and limiting injury risk. Fatigue at the ankle plantar flexors was found to impair single-leg stance postural control and overall stability (110). The gastrocnemius and ankle musculature may also act as a synergistic and compensatory dynamic knee stabilizer in female athletes in closed kinetic chain situations as the quadriceps fatigue (90,94). Gastrocnemius training to improve strength and endurance may be easily accomplished on-field using single-leg calf raises with manual partnered, weighted, or elastic band resistance.
Cutting and change of direction maneuvers occur repeatedly during sport and have been identified as a frequent cause of ACL injury in female athletes (44). Imwalle et al. found significant increases in internal rotation at the hip and knee during a 90° cutting maneuver vs. 45° (54). In addition, Ebben found women sustain quadriceps activation longer during cutting and had lower hamstring-to-quadriceps activation ratios (28). This reinforces a need for neuromuscular movement training at various angles and planes, and hip, hamstring, and core strengthening to limit increased vulnerability to ACL injury during these movements. Agility training with an emphasis on safe mechanics and posture is imperative, particularly the inclusion of unanticipated change of direction, which may more closely simulate sport conditions. Unanticipated sidestep cutting was shown to increase varus-valgus and internal-external knee moments (113). Unanticipated sidestep cutting has been demonstrated to increase muscle activation before initial contact, suggesting the importance of feedforward motor planning discussed earlier. These motor planning strategies may be modifiable through supervised neuromuscular training that induces safe levels of knee instability to facilitate neuromuscular adaptation (5,6). Anterior cruciate ligament injury risk in women is increased during lateral reactive jumps based on decreased knee flexion and increased valgus angles known to increase ACL strain and frequently observed during noncontact ACL injury (113). Lateral jumps in women produced higher GRF, anterior tibial shear, and valgus and flexion moments (113). These forces were further increased when subjects performed the movements in a reactive manner vs. planned. Brown et al. also found significant differences in hip and knee postures in unanticipated vs. anticipated landings and felt that unanticipated training may be useful to promote central control adaptations during sport tasks (14). These research studies underscore the need for unanticipated reactive change of direction movements, particularly laterally, as a component in ACL injury prevention training for female athletes. A 6-week agility training program incorporating unanticipated directional changes demonstrated improved medial hamstring activation during pivoting and decreased vastus medialis oblique activation during ground contact (122). A 6-week training program involving modification of cutting technique via verbal and visual feedback also resulted in statistically significant reductions in knee valgus loading during sidestep cutting in planned and unplanned conditions (24). This reinforces the ability of supervised agility training to promote changes in both neuromuscular activation and biomechanical postural changes to potentially decrease ACL injury risk.
Pollard et al. found that ankle dorsiflexion ROM was negatively correlated with frontal-plane knee excursion and suggested that a restriction in the forward progression of the tibia during deceleration may produce compensatory hip internal rotation or foot pronation to control the body's center of mass (106). This makes gastrocnemius and soleus static stretching to improve flexibility an essential part of ACL injury prevention programs to allow improved dorsiflexion ROM (130). Maintaining adequate flexibility of the quadriceps and hamstrings is also critical to allow sufficient knee flexion and hip flexion, respectively, during sport maneuvers to limit ACL strain in these positions as noted previously. However, although static stretching warm-up may improve flexibility, dynamic warm-ups have demonstrated improved effects on power and agility test scores vs. static stretching warm-ups (66,75). Dynamic warm-up activities should be incorporated as the first phase of the warm-up program.
Although these warm-up programs may be universally implemented with female athletes, a formal prescreening program may help in identifying female athletes possessing some of the risk factors identified in this article, and potentially most at risk for ACL injury. Hand-held dynamometry is reliable in young, active women and may be effectively used to identify female athletes with weakness of the gluteus medius and maximus in hip abduction and extension or external rotation, respectively, suspected to increase injury susceptibility (58). Additionally, hamstring weakness may be identified using hand-held dynamometry and compared to quad strength, although hamstring-to-quadriceps activation ratios and timing of activation may be more essential in determining ACL injury risk than static strength measures. However, these measures are not easily obtainable without more expensive testing equipment that requires increased time demands, training, and skill.
Similarly, Padua et al. have developed the Landing Error Scoring System (LESS) as a clinical tool with confirmed validity and reliability to assess jump landing biomechanics that may assist in identifying female athletes with higher risk of ACL injury (97). However, one drawback the authors acknowledge is that the LESS's dependence on the use of a drop jump assessment may be insufficient to detect injury risk in athletes during a side or crosscutting maneuver, common in many sports such as soccer, lacrosse, etc. Similar limitations exist with screening using exclusively drop landings or jumps.
These limitations suggest a possible role for the use of multiple forms of testing in a variety of planes and movements to assess ACL injury risk. Lower extremity functional testing may be incorporated as a screening tool to assess neuromuscular control in horizontal tasks vs. vertically-oriented drop landing or jump tasks. Four lower extremity functional tests have been found reliable, including the following: single-leg hop for distance, triple hop for distance, 6-m timed hop, and crossover hop test (10,111). These tests may be useful to visually identify neuromuscular pathomechanics during these single-leg tasks that replicate sport-specific movements and predispose female athletes to ACL injury risk. These tests are also essential to identify asymmetry between legs or leg dominance as discussed previously, with differences of 20% or more considered to increase injury risk (84). Recently, Hickey et al. also demonstrated good reliability in the use of a modified agility T-test, incorporating 2 90° single-leg cuts to identify lower extremity asymmetry, and subsequently potential injury risk (49). The modified agility T-test may also allow visual examination of pathomechanics during cutting maneuvers that may not have presented during drop landing or jump tasks. The Star Excursion Balance Test has also been found to be a reliable tool to assess dynamic lower extremity balance and stability, and predicting lower extremity injury risk (59,102).
Universally and accurately defining and assessing core stability has proved elusive, and a scarcity of valid and reliable core stability assessment measures exist in the literature. Debate continues over which muscles comprise the core, whether endurance or strength is more critical to core stability and athletic performance, and the best means to assess these factors. Cowley et al. successfully demonstrated reliable assessment of core musculature power using front and side medicine ball tosses (21). A single-leg squat test has also been advocated not only to identify hip and knee pathomechanics as noted previously, but also as a screening for core stability (55,63,123). However, Weir et al. showed insufficient reliability of 6 clinical tests commonly used to assess core stability including the following: unilateral squat, lateral step-downs, bridging or prone plank, and observation of standing dynamic trunk control in the frontal, sagittal, and transverse planes (121). More research is needed to develop valid and reliable core stability assessment tools that can be confidently included as part of a more comprehensive screening process for ACL injury risk in women.
The surge in recent research and development of ACL injury prevention warm-up programs has produced a diversity of program offerings, making it essential to be able to identify biomechanical and neuromuscular risk factors in female athletes and develop programs to address specific deficits. Program supervisors must have a thorough understanding of the susceptibility at each region in the entire kinetic chain from the trunk to the foot. The influence of neuromuscular control and biomechanical faults at the trunk and lower extremities appear repeatedly throughout video analyses of ACL injury mechanisms and clinical research studies, and appear distinctly different in women vs. men. Based on available research evidence, it appears that multicomponent neuromuscular training programs consisting of training for the hip and hamstrings, core stabilization, plyometrics and neuromuscular training, balance and proprioception, agility training, and stretching are critical to limiting this ACL injury risk in women. This training may be introduced in warm-up format with essential verbal and visual feedback provided to participants. Programs producing measurable performance gains in athletic ability may foster improved adherence to training. Programs should be 8 or more weeks in duration to allow sufficient time for neuromuscular changes based on available literature. Further research studies to refine and critically examine the most critical elements and ideal training frequency and volume are essential to the continued future success of ACL injury prevention programs in female athletes. A multicomponent prescreening process incorporating various athletic movements in multiple planes may also prove useful to successfully identify female athletes with the greatest amount of pathology suspected to contribute to noncontact ACL injury risk and to establish baseline values to measure participant progress and training effects.
The author would like to thank David Pezzullo PT, SCS, ATC, for his critical review of this manuscript and Rachel Lampros, PT, for her exercise demonstration. No conflict of interest is declared, and no funding was received for this manuscript.
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