Muscular Fatigue and Endurance During Intermittent Static Efforts: Effects of Contraction Level, Duty Cycle, and Cycle Time

Abstract

To determine the effects of intermittent task parameters on muscle fatigue and endurance time during static shoulder abductions, with a long-term goal of establishing relationships between intermittent task parameters and short-term performance. Effects of intermittent work on endurance and fatigue development have been reported, and certain combinations of contraction levels and duty cycles have been proposed as design guidelines. These findings, however, were not derived from systematic manipulations of the task parameters. Prolonged (1-hr maximum) intermittent shoulder abductions were performed at different combinations of contraction level (12% or 28% of muscle strength), duty cycle (.25 or .75), and cycle time (34 or 166 s). Fatigue was measured based on reductions in muscle strength and indirectly by changes in ratings of discomfort, electromyographic (EMG) amplitude, and EMG spectral distribution. Contraction level and duty cycle significantly affected endurance time and muscle fatigue, and interactive effects between these parameters were observed for some of the measures. Significant effects of cycle time were found only for EMG spectral measures. Endurance time and local fatigue were dependent on the comprehensive effects of the different task parameters. Design changes to reduce the occurrence of localized fatigue during intermittent work need to take into account all the task parameters simultaneously.