Low-threshold motor unit membrane properties vary with contraction intensity during sustained activation with surface EMG visual feedback

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Abstract
Single-motor unit (MU) activities were detected from the abductor pollicis and abductor digiti minimi muscles providing the subjects with visual feedback of multichannel surface electromyogram (EMG) signals. The subjects could modulate the force to observe on the surface recordings a single dominant MU and modulate its firing rate for contractions of 300 s with a noninvasive EMG feedback. The firing rate was maintained at ∼8 pulses per second [low-frequency (LF) contraction] and at ∼12 pulses per second [high-frequency (HF) contraction]. Single-MU conduction velocity (CV) decreased slightly but significantly over time, and it was possible to identify a significantly larger rate of decrease of CV during the HF with respect to the LF contractions. CV initial value significantly increased with the average firing rate, and CV values were significantly correlated to the instantaneous firing rate (R ranging from 0.21 to 0.39). Both additional MU recruitment and substitution were observed during the contractions. The study provides evidence that 1) it is possible to follow the same MU in a hand muscle at two different intensities (HF and LF) for 300-s durations by using visual feedback of surface EMG, 2) low-threshold single-MU CV changes over time since the beginning of the contraction, and 3) it is possible to distinguish between CV changes of the same MU at slightly different firing rates. The technique provides a practical method for the noninvasive assessment of both control and membrane properties of single MUs.