Using electrical stimulation to measure physiological changes in the human extensor carpi ulnaris muscle after prolonged low-level repetitive ulnar deviation.

The objective of this study was to determine whether muscle fatigue would result from repetitive voluntary contractions performed consecutively over four, 8-h workdays. Using a repeated measures design, ten healthy females participated in three conditions: a control and two repetitive work conditions involving 8 h of repeated ulnar deviation of the wrist, at self-selected workloads at 20 and 25 repetitions per minute (RPM). The 2, 20 and 50 Hz force response of the Extensor Carpi Ulnaris muscle was measured before, during work, and in three hours of recovery. Twitch contraction times (CT), one-half relaxation times (½ RT) and 20:50 Hz ratios (low frequency fatigue ratios) were also compared. The average workloads for the 20 and 25 RPM conditions were 20.3% (±11.6%) and 16.3% (±10.8%) MVC respectively. In the exposure conditions there was a decrease in the 20:50 Hz ratios indicating low frequency fatigue (LFF), a significant increase in the muscle's force response across all stimulation frequencies (potentiation), and a corresponding decrease (quickening) in the twitch CTs and ½ RTs. During recovery, the 20:50 Hz ratios, muscle forces and twitch CTs and ½ RTs returned to pre-exposure/baseline levels. There were no carryover effects or significant differences between the two consecutive workdays. For the low-level dynamic workloads tested in this study, LFF coexisted with muscle potentiation and the results indicated that both LFF ratios and the individual force responses at each frequency needs to be evaluated in order to understand the underlying state of the muscle.