Does increasing the number of channels during neuromuscular electrical stimulation reduce fatigability and produce larger contractions with less discomfort?

Abstract Purpose: Neuromuscular electrical stimulation (NMES) recruits motor units (MUs) at unphysiologically high rates, leading to contraction fatigability. Rotating NMES pulses between multiple electrodes recruits different MUs from each site, reducing MU firing rates and fatigability. This study aimed 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 4 sessions. NMES was delivered over the quadriceps through 1 (NMES1), 2 (NMES2), 4 (NMES4) or 8 (NMES8) channels. Fatigability was assessed over 100 contractions (1s on/1s off) at an initial contraction amplitude that was 20% of a maximal voluntary contraction (MVC). Torque-frequency relationships were characterized over 6 frequencies from 20-120Hz. Results: NMES4 and NMES8 resulted in less decline in torque (42% and 41%) and generated more torque over the 100 contractions than NMES1 and NMES2 (53% and 50% decline in torque). Increasing frequency from 20-120Hz increased torque by 7, 13, 21 and 24% MVC, for NMES1, NMES2, NMES4 and NMES8, respectively. Perceived discomfort was highest during NMES8 . Conclusion: NMES4 and NMES8 reduced contraction fatigability and generated larger contractions across a range of frequencies than NMES1 and NMES2 . NMES8 produced the most discomfort, likely due to small electrodes and high current density. During NMES, more is not better and rotating pulses between 4-channels may be optimal to reduce contraction fatigability and produce larger contractions with minimal discomfort compared to conventional NMES configurations.