Muscle Fatigue Alters Neuromuscular Performance during Resisted Weight Bearing Exercise

Abstract

Neuromuscular control is essential to the maintenance of lower extremity stability during functional activities. Numerous studies suggest that muscle fatigue contributes to deterioration of neuromuscular performance and may increase the risk of injury. However, limited information is currently available regarding muscle synergistic patterns and triggered reflex responses during dynamic weight bearing activities in the presence of muscle fatigue. PURPOSE: To examine the effect of muscle fatigue on 1) voluntary and reflexive patterns of muscle activation and 2) performance accuracy while tracking a visual target during the weight bearing lateral step down exercise. METHODS: A specially designed apparatus was used to provide controlled resistance to knee motion during the lateral step down exercise. Motion of the knee was measured as subjects were asked to track a sinusoidal target waveform as accurately as possible during the exercise. Absolute, constant, variable, and endpoint errors were calculated to evaluate tracking performance. Random perturbations were delivered in 20% of the trials by unexpectedly releasing the resistance during the flexion phase of the exercise. Quadriceps and hamstring muscle activity was recorded during both perturbed and unperturbed trials. Tests were conducted in a group of 12 healthy females before and after completing a repetitive submaximal eccentric quadriceps fatigue protocol. A second group of 12 females served as controls. RESULTS: Muscle fatigue increased the level of quadriceps muscle activity throughout most of the exercise. In the flexion phase, average rectus femoris (RF) and vastus lateralis (VL) EMG increased by 2.5% (p=.008) and 3.7% (p=.002) MVIC, respectively. In the extension phase, RF activity increased by 5.5% MVIC (p < .001), while VL activity increased by 8.5% MVIC (p < .001). Muscle fatigue also resulted in a reversal in the direction of the constant errors during the extension phase of the exercise, causing subjects to lag behind the target. Unexpected perturbations elicited long latency responses characterized by facilitation of the quadriceps and inhibition of the hamstrings. Muscle fatigue increased the amplitude of this response in VL by 4.3% MVIC (p=.004). Despite the increased amplitude of the response, errors at the endpoint of the flexion phase increased by .67 cm (p=.034) after fatigue. CONCLUSION: These results indicate that muscle fatigue alters patterns of coordinated muscle activity, which may render subjects less able to cope with unexpected perturbations during activities that require a high degree of accuracy.