Non-Linear Discharge of Human Motor Units During Linear Time-Varying Contractions Across Motor Pools

Increasing muscular force occurs when motor units, comprised of the spinal motor neuron and innervating muscle fibers, increase firing rates, or when the number of motor units recruited within that muscle increases[1] [2]. It is known that the recruitment limb of increasing linear force generation can elicit nonlinear firing rate patterns, and that the firing rate at the point of recruitment of motor units is higher than the firing rate at de-recruitment.[3] To the best of our knowledge, the de-recruitment limb firing rates during decreasing torque generation have yet to be characterized. We sought to test whether over the course of a linearly increasing and decreasing torque generation, we would observe similar and linear behavior of firing rates. High density surface EMG (HD-EMG) recordings of a human subject were decomposed into motor units during contractions using blind source separation. 13 subjects were analyzed over 10s triangular torque contractions. Slopes of firing rates vs time over the course of isometric triangular torque generation were generated using a bilinear fit for both the recruitment and de-recruitment limbs. Residuals of raw firing rates for the lines of best fit before and after the breakpoint were calculated. Linear increases and decreases in torque are accompanied by nonlinear increases and decreases in firing rates. These results suggest important characteristic intrinsic mechanisms to the sustained and released firing activity of motor neurons.