Electromyographic activity of the human extensor carpi ulnaris muscle changes with exposure to repetitive ulnar deviation

With little known about the changes in muscle physiology due to repetitive work during an 8 h workday, our objective was to quantify the changes in muscle activity due to this type of work. Using a repeated measurements design, 13 healthy women participated in three conditions, each lasting 2 days: a control condition where they remained inactive, and two repetitive work conditions involving repeated ulnar deviation of the wrist at 20 and 25 repetitions a minute at workloads which they themselves had deemed acceptable for 8 h through a psychophysical protocol. Electromyographic (EMG) activity of the extensor carpi ulnaris muscle was recorded during voluntary isometric contractions (20% and 60% maximal voluntary contraction) eight times throughout the work and control days at 0, 2, 4.25, 6.75, 8, 9, 10, and 11 h. The amplitude of the EMG signal was lower on workdays compared to the control days. Although power was significantly reduced in all spectrum bands of the EMG power spectra, the reductions were not uniform across the entire frequency range, giving rise to different shapes of the EMG power spectra. Initial median frequency of the EMG signal showed no change between the control and workdays (P=0.51); however, the decline of the median frequency with respect to time over the course of each isometric contraction was steeper during workdays compared to control days (P=0.003). These changes suggest that the muscles are in an early stage of fatigue when working for an 8 h day.