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 approximately 8 pulses per second [low-frequency (LF) contraction] and at approximately 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.

Gazzoni, Marco, Federico Camelia, and Dario Farina. Conduction velocity of quiescent muscle fibers decreases during sustained contraction.

Farina, Dario, Marco Gazzoni, and Federico Camelia. Conduction velocity of low-threshold motor units during ischemic contractions performed with surface EMG feedback. J Appl Physiol 98: 1487-1494, 2005. First published November 12, 2004 doi:10.1152/ japplphysiol.01032.2004.-The aim of this study was to analyze the effect of ischemia on low-threshold motor unit conduction velocity. Nine subjects were trained to isolate the activity of a single motor unit (target motor unit) in the abductor pollicis brevis muscle with feedback on surface EMG signals recorded with a 16-electrode linear array. After training, the subjects activated the target motor unit at ϳ8 pulses per second (pps) for five 3-min-long contractions. During the third and fourth contractions, a cuff inflated at 180 mmHg around the forearm induced ischemia of the hand. The exerted force (mean Ϯ SE, 4.6 Ϯ 2.1% of the maximal voluntary contraction force), discharge rate (8.6 Ϯ 0.4 pps), interpulse interval variability (34.8 Ϯ 2.5%), and peak-to-peak amplitude of the target motor unit action potentials (176.6 Ϯ 18.2 V) were not different among the five contractions. Conduction velocity, mean power spectral frequency, and action potential duration were the same in the beginning of the five contractions (2.8 Ϯ 0.2 m/s, 195.2 Ϯ 10.5 Hz, and 5.4 Ϯ 0.3 ms, respectively) and changed over the 3 min of sustained activation only during the fourth contraction. Conduction velocity and mean power spectral frequency decreased (10.05 Ϯ 1.8% and 8.50 Ϯ 2.18% during the 3 min, respectively) and action potential duration increased (8.2 Ϯ 4.6% in the 3 min) during the fourth contraction. In conclusion, 1) subjects were able to isolate the activity of a single motor unit with surface EMG visual feedback in ischemic conditions maintained for 16 min, and 2) the activation-induced decrease in single motor unit conduction velocity was significantly larger with ischemia than with normal circulation, probably due to the alteration of mechanisms of ion exchange across the fiber membrane.