Cycle to cycle variability in a repetitive upper extremity task

The purpose of this study was to examine the variability in muscle activity at rest and work during a repetitive task. A total of 20 participants performed a bimanual push task using three frequencies (4, 8, 16 pushes/min), three loads (1 kg, 2 kg, 4 kg) and two grip conditions (no grip, 30% maximum). The coefficient of variation (CoV) of muscle activity was determined for the anterior deltoid, biceps brachii, extensor digitorum and flexor digitorum superficialis. Faster push frequencies and heavier loads had lower work-rest ratio CoV and higher mean muscle activity (p < 0.01). Sixteen pushes per minute produced the lowest CoV for the anterior deltoid (p < 0.01), while the 1- kg load produced the lowest CoV for the extensor digitorum and flexor digitorum superficialis (p < 0.01). Changes were driven by the rest phase rather than by the work phase, except for grip decreasing forearm muscle CoV. These findings underscore the importance of variability at rest and indicate that low variability of muscle activity is associated with ergonomic risk factors. PRACTITIONER SUMMARY: Decreased motor variability has been associated with pain and injury. A cyclical push task, evaluated in terms of work and rest phases, found that greater workloads increased variability primarily due to changes in the rest phase. Muscle variability, especially for the rest phase, may provide insight into injury risk.