Forearm muscles fatigue during repetitive ulnar deviation tasks


Jack Tigh Dennerleina, Vincent M. Ciriellob, Kirsty J. Benniea,c, Peter W. Johnsond 



While psychophysical methods have been developed to determine the acceptable workloads for repetitive ulnar deviation at different repetition rates, these methods would be better supported if physiological data were collected concurrently.  Our goal was to measure low frequency fatigue (LFF) in the extensor carpi ulnaris (ECU) muscle while workers completed a repetitive ulnar deviation task.  Using a repeated measures design, ten healthy females participated in three conditions, each lasting two days: a control condition where 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 eight hours through a psychophysical protocol. LFF of the ECU muscle and Borg scale ratings of perceived fatigue were recorded eight times throughout the control and work days 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 to the isometric force produced by 50 pulses per second provided the measure of LFF.  The ratios were lower on workdays compared to the control days, indicating the presence of LFF during repetitive work. The ratios were similar at the beginning of all the days across conditions.  During repetitive work, the ratios decreased during the day, especially when compared to the control condition, indicating the muscles fatigued as the days progressed.  The psychophysically determined workloads, while not creating noticeable discomfort to the subjects, did create low levels of subjective fatigue throughout the day, which appeared to recover by the next morning. 

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