Effect Of Neuromuscular Electrical Stimulation On Contralateral Motor Unit Activity In Biceps Brachii

Abstract

Neuromuscular electrical stimulation (NMES) elicits action potentials in intramuscular branches of peripheral nerves. Because the time constant for the strength-duration relation is longer for sensory axons than motor axons, narrow pulse widths (0.2-0.4 ms) preferentially activate motor axons, whereas wider pulse widths (0.5-1 ms) recruit relatively more sensory axons. PURPOSE: The purpose of the study was to determine the influence of NMES pulse width and frequency on the modulation of motor unit (MU) activity in a contralateral limb. The hypothesis was that wide-pulse NMES would cause greater modulation of MU activity due to augmented activation of sensory axons. METHODS: The discharge of action potentials by MUs in the left biceps brachii of young adults (18-35 yrs) was recorded with subcutaneous fine-wire electrodes. The protocol involved subjects performing submaximal isometric contractions to maintain the discharge of an isolated MU. In the middle 5 s of each contraction, NMES was applied to the right biceps brachii with either of two pulse durations (0.2 or 1 ms, each at 40 Hz) and three current intensities (80% motor threshold, 10%, or 20% of maximal voluntary contraction (MVC) force). In an additional group of 7 MUs (recruited between 4-22% MVC force), NMES was applied at both pulse widths and two stimulus frequencies (40 Hz and 100 Hz). The primary outcome was the influence of NMES on the discharge characteristics (mean and coefficient of variation (CV) of interspike interval (ISI)) of the isolated MUs before, during, and after stimulation. RESULTS: Forty-four MUs were discriminated, 16 of which had high recruitment thresholds (≥30% MVC force). Neither narrow- nor wide-pulse NMES elicited a significant change in mean ISI or CV for ISI. For example, mean ISI during an NMES-evoked contraction at 10% MVC force was 11.4 ms, 12.3 ms, and 11.8 ms before, during, and after stimulation (respectively) for narrow pulses and 11.4 ms, 12.3 ms, and 12.0 ms for wide pulses. Similarly, CV for ISI was 20.3%, 20.2%, and 21.2% for narrow pulses and 19.4%, 18.7%, and 19.9% for wide pulses. Moreover, stimulus frequency did not influence the discharge characteristics of the isolated MUs. CONCLUSION: NMES applied to the right biceps brachii did not influence the discharge characteristics of motor units in the left biceps brachii during brief voluntary contractions.