An Exploration of the Effects of Cycle Time and Force Variations on Muscle Fatigue during Submaximal Intermittent Isometric Exertions

Abstract

This study investigated the effects of two intermittent task parameters on muscle fatigue, namely cycle time and force level variation. Of particular interest was whether a constant low-level underlying force would contribute to fatigue. Twelve students participated in four experimental conditions, each lasting 8 minutes. Participants repeatedly performed intermittent isometric exertions of biceps brachii at two cycle times (5 or 10 seconds), and two force levels, varying either between 0% and 50%max isometric force or between 10% and 40%max force. These parameters and a duty cycle of 0.5 ensured a mean force exertion of 25%max for all conditions. Muscle fatigue was inferred from local Ratings of Perceived Exertion (RPE), maximum force and EMG(RMS). Significant (p<0.05) increases in RPE and EMG(RMS), and reductions in maximum force overtime indicate fatigue development during all conditions. Conditions with force levels varying between 10% and 40%max force showed significantly greater RPE and EMG(RMS) recordings, but only marginal reductions in maximum force (p=0.350), compared to conditions with force varying between 0% and 50%max force. Cycle time did not significantly influence fatigue development since no significant changes occurred for RPE, maximum force or EMG(RMS). These results show accelerated fatigue development under conditions with a constant underlying force, despite the lower maximum exertion level, and irrespective of cycle time. Work involving intermittent static force exertions should thus include periods of complete relaxation between exertions to prevent the early onset of muscle fatigue.