Two anatomically distinct motor unit modes in tibialis anterior during submaximal isometric contractions

The purpose of our study was to compare motor unit modes derived from high-density surface electromyogram recordings from proximal and distal regions of the tibialis anterior muscle during submaximal isometric contractions. Motor unit activity was recorded with two grid electrodes attached to the skin over the muscle while participants performed isometric contractions to five targets that ranged from 5 to 60% of maximal torque. Factor analysis of the smoothed discharge rates yielded two distinct motor unit modes-defined as subsets of motor units in which the modulation of discharge rate was correlated presumably due to shared synaptic input. Motor units were classified into two modes based on the proximity of each correlation value to two independent centroids. The analysis indicated that most proximal motor units were associated with mode 1, whereas distal motor units were more associated with mode 2. Low cross-correlation values between the two modes for each participant (range: 0.11-0.28) indicated that the modes were independent. Moreover, median Z scores of the pooled coherence for the discharge rates derived from pairs of motor units were less between the two modes than within either mode. The percentage of motor units associated with mode 1 increased with target torque in most participants but decreased in the others, indicating regional differences in the modulation of discharge rate across individuals. These findings suggest that motor units in the proximal and distal regions of tibialis anterior can exhibit differential modulation of discharge rate during submaximal isometric contractions.NEW & NOTEWORTHY The findings of this study suggest that the force produced by the tibialis anterior muscle during submaximal isometric contractions is controlled by two anatomically distinct motor unit modes. Moreover, factor and coherence analyses indicated that motor units in proximal and distal regions of the muscle received independent shared synaptic inputs. These results challenge the assumption of a uniform neural drive to the tibialis anterior muscle during submaximal isometric contractions.