Fatigue in the forearm resulting from low-level repetitive ulnar deviation

This study measured low-frequency fatigue (LFF) in the extensor carpi ulnaris (ECU) muscle while workers completed a repetitive ulnar deviation task. Using a repeated measures design, 10 healthy women participated in three conditions, each lasting 2 consecutive days: a control condition in which subjects remained inactive, and two repetitive work conditions involving repeated ulnar deviation of the wrist at 20 and 25 repetitions per minute at individual workloads deemed acceptable for 8 hours through a psychophysical protocol. LFF of the ECU muscle and self-reported levels of fatigue were recorded eight times throughout the control and workdays before (time 0), during (2, 4.25, 6.75, 8 hours), and after (9, 10, and 11 hours) exposure. The ratio of the isometric force produced by electrical stimulus at 20 pulses per second (pps) to the isometric force produced by 50 pps provided the measure of LFF. The ratios were lower on workdays compared with the control days, indicating the presence of LFF during repetitive work. During repetitive work the ratios decreased during the day, indicating the muscles fatigued as the day progressed. The psychophysically determined workloads, although not creating noticeable discomfort to the subjects, were high enough to create low levels of muscle fatigue.