Significant positive correlations were revealed between hamstring flexibility and functional performance (hop/height distance) (r = 0.65; p = 0.00) and between physical activity level (Tegner score) and functional performance (r = 0.42; p = 0.03). A stepwise multiple regression analysis revealed that 42% of the hop per height distance can be explained by hamstring flexibility (r2 = 0.42; F = 19.75; Significant F = 0.00), but physical activity level did not significantly increase the predictability of functional performance in the hop test (Significant T = 0.09).
Contemporary theories regarding dynamic joint stabilization emphasize the significance of preactivated muscle patterns in anticipation of joint loads, but the results are mixed.7,11,13,16,18,21,37,51 Gauffin and Tropp16 and McNair and Marshall37 used a landing task similar to the one used in the current study and did not identify significant side-to-side or between-group differences in subjects with ACL injuries. However, Branch et al5 observed significant increases (38%) in the area of lateral hamstring activity when subjects with an ACL injury did a cutting maneuver. The current study also identified significant increases in preparatory lateral hamstring muscle activity in subjects with ACL injuries. Branch et al5 concluded that because episodes of functional instability frequently involve anterior tibial translation and internal rotation, increased activation of the lateral hamstring muscle would provide the most protection for maintaining joint stability. Although there are contradictory reports involving preparatory muscle activation, the evidence supporting preprogrammed, compensatory muscle activation strategies in subjects with ACL injuries is growing.5,11,18,27
The increased preparatory hamstring activity may best be explained by the feed-forward process of motor control.31 Feed-forward processing emphasizes preactivated muscle patterns in anticipation of movements and joint loads.11,13,18 After ACL injury, sensory feedback may be used to build a new internal model depicting the expected conditions during functional activities.28 Using this advance information about a task permits subjects with ACL injuries to preprogram muscle activation patterns and compensate for mechanical instability through dynamic restraint. With feed-forward processing, knee stability can be maintained during high speed, functional movements.
Debate exists regarding the capacity of reactive muscle contractions to help with functional joint stability. Timing (latency) and amount of reactive muscular contractions have the potential to assist with dynamic restraint in subjects with ACL injuries.2,24,45,58
Previous studies suggest that subjects with ACL injuries may use the feedback process of motor control to minimize anterior tibial translation. This reactive strategy includes increasing hamstring activity and decreasing quadriceps activity.3,5,6,8,12,16,31,40 However, the results of the current did not reveal significant differences and a quad avoidance strategy was not observed. It is possible that increased preparatory muscle activity in the ACL-injured group eliminated the need for modifications to the reactive muscular contractions. Pretensioning the hamstring muscle with a preparatory contraction may restrict excessive tibial translation and therefore reactive muscle activity remained unchanged.
The timing of reactive muscle contractions is another critical factor if the feedback motor control process is expected to contribute to dynamic restraint.2,24,45,58 It is suggested that if reactive dynamic stabilization is to be effective at protecting joint structures, a very fast motor response (30–70 ms) is necessary.2,24,58 Beard and Refshauge2 reported a significantly longer delay in the reactive hamstring contractions of limbs with ACL injuries (99 ms) than uninvolved limbs (53 ms) and a control group (43 ms). Wojtys and Huston58 confirmed these results and observed that subjects with ACL injuries had slightly longer delays depending on the time from injury. However, results of the current study concur with those of Jennings and Seedhom,24 who were unable to reproduce these deficits, reporting onset times of 41 ms in the uninvolved limb and 32 ms in the ACL-injured limb. There are numerous factors that may have contributed to these discrepancies including different test procedures, instrumentation, and subject samples. Another possible explanation is that joint perturbation tests do not elicit reflexive muscular activation originating from the ACL or other capsuloligamentous structures, but a monosynaptic stretch reflex response from muscle spindles in the medial and lateral hamstrings.2,24 The peripheral site where sensory feedback originates and the neural pathway followed contribute to the response time of muscles. The onset times recorded in this study coincide with the monosynaptic stretch reflex and seem to be unaffected by ACL injury.15,25,28
Muscle stiffness was calculated to determine the resistance of the hamstring muscles to stretch. The results identified group differences but numerous factors are involved with regulation of stiffness including joint position, preparatory and reactive muscle activation strategies, and flexibility.34,37,45,54,55 The complex interaction between these components mediates muscle stiffness and is incorporated into the neuromuscular control strategy best suited for dynamic restraint and functional performance.1,37
In vivo measurements of muscle stiffness originally were done to determine how performance was affected by the mechanical properties of the muscle1,37,38,56 Bach et al1 suggested that the neuromuscular control apparatus modifies muscle stiffness, depending on the requirements of a task, to optimize these properties. For example, one-legged hopping is better done in resonance with the natural frequency of muscles elastic properties.1,39 This strategy uses the mechanical properties of muscle to conserve energy and increase the efficiency of movement.1,39,56 Therefore, individual muscle stiffness seems to be an important determinant in the selection and performance of functional movement strategies. Wilson et al54 and Rudolph et al46 suggested that lower stiffness might be advantageous during functional activities because stored elastic energy is used to absorb loads more efficiently. Subjects with ACL injuries in the current study may use lower muscle stiffness to absorb ground reaction forces in tenomuscular rather than capsuloligamentous structures. McNair et al,38 however, identified a positive correlation between increased hamstring muscle stiffness and increased functional performance in subjects with ACL injuries. A second landing study done by McNair and Marshall37 revealed that subjects with ACL injuries with greater hamstring muscle activity had lower ground reaction forces and lower hamstring muscle stiffness. Subjects with ACL injuries in the current study also had greater preparatory hamstring muscle activity, less hamstring muscle stiffness, and were relatively functional when compared with subjects in the control group. This supports research by Rudolph et al46 and suggests there is an important relationship or interaction among muscle activation timing, stiffness, and function in subjects with ACL injuries. However, additional research with subjects with ACL injuries is needed because it is unclear whether low muscle stiffness is a congenital, predisposing factor to injury or a compensatory adaptation benefiting the dynamic restraint mechanism.
Previous isokinetic assessments have revealed peak torque deficits in the quadriceps and hamstring muscles of subjects with ACL injuries.14,23,29,33,35,43,47 However, only quadriceps peak torque values are consistently less in the ACL-injured limb, but this was not observed in the current study.29,33,47 Wojtys and Huston58 assessed peak torque and body weight ratio in a group of subjects with ACL injuries who were considered to be functioning well, and found normal or increased hamstring muscle strength.58 This data are consistent with the current results. No previous literature has been identified confirming the improved torque at 0.2 seconds for knee flexion of the ACL-injured group. These results support rapid force production as a characteristic beneficial to dynamic restraint in subjects with ACL injuries. The capacity of the hamstring muscles to generate greater torque in a shorter time would create a posterior force vector resisting excessive anterior translation of the tibia.33,43,51,58 This quality may be inherent or the result of muscle hypertrophy and recruitment strategies developed during rehabilitation.52 Subjects with ACL injuries who possess these characteristics may be able to preserve functional stability through dynamic mechanisms, and therefore elect not to have surgical reconstruction.
The premature development of tension from inflexible hamstrings may be beneficial for restricting excessive anterior tibial translation and provide dynamic restraint in the ACL-injured knee. Results of the current study agree with the results of Harner et al20 who assessed hamstring flexibility and found that females with ACL injuries had significantly less flexible hamstring muscles when compared with a control group. The mechanism responsible for hamstring inflexibility involves excitatory and inhibitory protective reflexes initiated by muscle spindles and golgi tendon organs. Protective reflexes originating from these mechanoreceptors are the primary determinants of maximum muscle length.19 Neuromuscular adaptations to these reflexive pathways can decrease hamstring muscle flexibility and result in the premature development of hamstring muscle tension as the knee is extended. This mechanism could resist excessive anterior translation, increase dynamic restraint, and assist with functional stability in the ACL-injured knee.20
Performance measures such as the hop for distance are designed to objectively assess functional status with a standardized test. Functional performance for the control group (74.27% ± 8.30%) was similar to previously established normative data (70.48% ± 10.88%) for healthy females and the subjects with ACL injuries were not significantly different (68.58% ± 12.52%) scoring in the normal limits of healthy females.26 Juris et al26 also tested subjects with ACL reconstructions who had almost identical scores (67.52% ± 14.22%) as the ACL-injured group in the current study. These results are an indication that the ACL-injured group was able to maintain a normal level of functional performance despite the mechanical instability. The neuromuscular characteristics assessed in the current study may be responsible for enhancing dynamic restraint capabilities, thereby maintaining functional stability and performance in the ACL-injured group.
Functional impairment was assessed with the Lysholm knee rating scale and confirmed that the control group (score, 95) did not experience functional impairment because of a knee injury.36 The ACL-injured group scored significantly lower (score, 78), which was considered fair.36 Although the ACL-injured group experienced more symptoms of knee injury, these symptoms did not seem to effect activity level or functional performance.
Significant positive correlations were revealed between hamstring flexibility and functional performance, and between activity level and functional performance. The relationship between hamstring flexibility and functional performance may result from the use of stored elastic energy during the single leg hop test.56 All subjects were observed doing a small counter movement before executing the hop test. This movement can enhance performance by pretensioning the elastic component of the hamstring muscle and eliciting a stretch reflex response.56 Subjects with more flexible hamstring muscles may be capable of storing and using the elastic energy more effectively as evidenced by increased hop distance.1,55,56 The correlation between activity level and functional performance may imply that the physically active subjects were familiar with the specific demands of the task and the most successful motor control strategies needed to execute the single leg hop test.
The current study explored dynamic restraint mechanisms by assessing the neuromuscular characteristics of females with ACL injuries and healthy female subjects. Females with ACL injuries showed greater preparatory activity in the lateral hamstring muscles, less hamstring stiffness and flexibility, but no differences in reactive muscle activity. The ACL-injured group also had greater peak torque and torque development for knee flexion. Positive correlations were seen among hamstring flexibility, activity level, and functional performance with 42% of the hop and height distance explained by hamstring flexibility.
Increased preparatory activity suggests that females with ACL injuries use feed-forward processing to pretension muscle thereby enhancing dynamic restraint capabilities. Decreased muscle stiffness in the ACL-injured group most likely involved a complex regulatory strategy to maximize energy absorption. The results of strength testing suggest that subjects with ACL injuries possess adaptations to the hamstring muscle group that permit them to produce greater force in a shorter time, therefore increasing dynamic restraint. Our research also suggests that hamstring inflexibility may contribute to dynamic restraint through adaptive shortening and the protective reflexes that mediate hamstring tension. The compensatory neuromuscular characteristics assessed in this study may be responsible for enhancing the dynamic restraint capabilities and assist with functional stability in females with ACL injuries. These characteristics should be explored to reveal their potential for preventing knee injuries through dynamic restraint.
We thank Dr. Elaine Rubinstein for time and expertise with the statistical analyses and Dr. Anthony Petrella for muscle stiffness testing.
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