Quadriceps fatigue alters human muscle performance during a novel weight bearing task

Abstract

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: The purpose of this study was to examine the effects of quadriceps muscle fatigue on patterns of muscle activation and performance in response to sudden, unexpected perturbations during a weight-bearing task.

Methods: Motion of the knee was measured as subjects were asked to track a visual target as accurately as possible while performing a resisted single leg squat task. Random perturbations were delivered in 20% of the trials by unexpectedly releasing the resistance during the flexion phase of the exercise. Absolute and constant errors were calculated to evaluate target tracking performance. Quadriceps and hamstring muscle activity was recorded during both perturbed and unperturbed trials. Twelve healthy women were tested before and after completing a repetitive submaximal eccentric quadriceps fatigue protocol. A second group of 12 women served as controls. Unexpected perturbations elicited long-latency responses characterized by facilitation of the quadriceps and inhibition of the hamstrings.

Results: Muscle fatigue increased the amplitude of the long-latency response in vastus lateralis by 4.3% maximum voluntary isometric contraction (P = 0.004). Changes in tracking error occurred in response to perturbations after fatigue in spite of significantly increased quadriceps muscle activity, especially during the extension phase of the exercise.

Conclusion: Quadriceps muscle fatigue alters the patterns of coordinated muscle activity and may render subjects less able to cope with unexpected perturbations during weight-bearing tasks.

Fig. 1

Testing device used to provide resistance during the single leg squat task.

Fig. 2

Representative example of the averaged EMG signal from vastus lateralis (VL) within the long latency response period from one set of fatigued (solid line) and non-fatigued (dotted line) trials in a single subject.

Fig. 3

A: Mean (SD) maximum voluntary eccentric knee extension torque before and after fatigue for the fatigue group (closed circles) and the control group (open circles). *Significant difference between conditions within the fatigue group. †Significant difference between groups. B: Representative example of eccentric knee extension torque from one subject in the fatigue group before, during, and after a series of eccentric contractions. The horizontal line indicates the target level of eccentric torque during the fatigue protocol based on the subject’s maximum voluntary eccentric contraction (MVEC) torque prior to fatigue.

Fig. 4

A: Mean (SD) normalized long latency response (LLR) amplitude for pre-fatigue (closed symbols) and post-fatigue (open symbols) conditions for the fatigue group (circles) and for the control group (squares). *Significant difference between conditions within the fatigue group. †Significant difference between groups in the post-fatigue condition. B: Mean (SD) amplitude of the LLR for subjects in the fatigue group expressed as a proportion of background muscle activity at the time of the perturbation for pre-fatigue (closed symbols) and post-fatigue (open symbols) conditions.

Fig. 5

Effect of quadriceps muscle fatigue on (A) absolute error and (B) constant error during the flexion phase (left panel) and extension phase (right panel) of the SLS task. Mean (SD) errors for pre-fatigue (closed symbols) and post-fatigue (open symbols) conditions for the fatigue group (circles) and for the control group (squares). The cycle interval within which the perturbation was delivered is indicated by the vertical dashed line in the flexion phase. *Significant difference between groups in the post-fatigue condition.

Fig. 6

Effect of quadriceps muscle fatigue on (A) vastus medialis (VM), (B) rectus femoris (RF), (C) vastus lateralis (VL), (D) medial hamstring (MH), and (E) lateral hamstring (LH) EMG activity during the flexion phase (left panel) and extension phase (right panel) of the SLS task. Mean (SD) normalized EMG for pre-fatigue (closed symbols) and post-fatigue (open symbols) conditions for the fatigue group (circles) and for the control group (squares). The cycle interval within which the perturbation was delivered is indicated by the vertical dashed line in the flexion phase. *Significant difference between groups in the post-fatigue condition.