Measuring low frequency fatigue with 2 Hertz stimulation. I. Stimulus-related potentiation effects

The purpose of this study was to identify sources of variability associated with the muscle twitch and determine whether muscle twitches resulting from 2 Hz electrical stimulation could be used as a reliable measure for the state of the muscle. Muscle twitches were measured from the Flexor Digitorum Superficialis (FDS) muscle of five subjects, while resting, at six time periods: initially, and 30, 60, 120, 180, 240 minutes thereafter. No significant differences in group mean twitch forces were measured over the six time periods. Changes in skin resistance and changes in skin surface temperature did not affect twitch force measurements. During 60 seconds of continuous 2 Hz stimulation twitch forces increased by 35% (potentiated) and then stabilized. Pre-conditioning the muscle with at least 60 seconds of continuous 2 Hz stimulation was found to be critical for obtaining reliable muscle twitch measurements