Does Increasing the Number of Channels During Neuromuscular Electrical Stimulation Reduce Fatigability and Produce Larger Contractions with Less Discomfort?

Purpose: Neuromuscular electrical stimulation (NMES) is often delivered at frequencies that recruit motor units (MUs) at unphysiologically high rates, leading to contraction fatigability. Rotating NMES pulses between multiple electrodes recruits subpopulations of MUs from each site, reducing MU firing rates and fatigability. This study was designed to determine whether rotating pulses between an increasing number of stimulation channels (cathodes) reduces contraction fatigability and increases the ability to generate torque during NMES. A secondary outcome was perceived discomfort.

Methods: Fifteen neurologically intact volunteers completed four sessions. NMES was delivered over the quadriceps through 1 (NMES), 2 (NMES), 4 (NMES) or 8 (NMES) channels. Fatigability was assessed over 100 contractions (1-s on/1-s off) at an initial contraction amplitude that was 20% of a maximal voluntary contraction. Torque-frequency relationships were characterized over six frequencies from 20 to 120 Hz.

Results: NMES and NMES resulted in less decline in peak torque (42 and 41%) over the 100 contractions than NMES and NMES (53 and 50% decline). Increasing frequency from 20 to 120 Hz increased torque by 7, 13, 21 and 24% MVC, for NMES, NMES, NMES and NMES, respectively. Perceived discomfort was highest during NMES.

Conclusion: NMES and NMES reduced contraction fatigability and generated larger contractions across a range of frequencies than NMES and NMES. NMES produced the most discomfort, likely due to small electrodes and high current density. During NMES, more is not better and rotating pulses between four channels may be optimal to reduce contraction fatigability and produce larger contractions with minimal discomfort compared to conventional NMES configurations.