The effect of the stimulation pattern on the fatigue of single motor units.

Abstract

The aims of this research were: (1) to examine the effect of subtle changes in the stimulation pattern on the fatigability (force decline) of single motor units during repetitive activation; and (2) to determine the relationship between the magnitude of submaximal force developed and the subsequent fatigue within these units. The experiments were performed on motor units of the tibialis posterior muscle in deeply anesthetized cats. Preliminary experiments revealed that fast-twitch motor units remained fatigued for several hours following short periods of repetitive stimulation, such that sequential testing of a single motor unit with different stimulation protocols was not feasible. Hence, a new fatigue test was developed to compare the effects of two stimulation patterns during fatigue. Two different stimulation patterns were pseudo-randomly delivered at 1 train/second for 360 seconds to single motor axons. One pattern (regular) was composed of a 500-millisecond duration train with constant interpulse intervals customized to the twitch contraction time of each unit (stimulus interval range, 27-51 ms). The other pattern (optimized) was designed to increase the amount of force produced in the control (pre-fatigue) state. It consisted of three initial stimuli with short (10 ms) interpulse intervals, followed by a constant-interpulse interval train that was adjusted to have the same number of pulses and duration as the regular train (stimulus interval range, 29-62 ms). The force elicited during the fatigue test was decomposed to reveal a force profile attributable to each stimulation pattern. The results showed that the optimized pattern produced significantly more force during both control and fatiguing contractions. This suggested that force optimization is a potential mechanism by which the CNS might counteract fatigue. The association between the magnitude or submaximal force produced by an individual motor unit and its subsequent fatigue was tested within the same population of units. The results showed that the increased force due to optimized stimulation was associated with decreased fatigue, and that the cumulative force produced during fatiguing contractions could be predicted from the units' pre-fatigue force. It is concluded that an inverse relationship between force and fatigability does not exist at the level of the individual motor unit under these conditions. Rather, the results showed that the amount of force a unit can develop over time during fatiguing contractions is dependent, in part, on the pattern of stimulation.