Discharge characteristics of motor units in FDI vary with load compliance but comprise a single neural module during submaximal isometric contractions

The purpose of our study was to determine the influence of load compliance on the discharge characteristics of the same motor units in the first dorsal interosseus (FDI) muscle during submaximal isometric contractions. Motor unit activity in FDI was recorded using high-density, surface electromyography while participants performed isometric contractions either by pushing against a rigid force transducer (force task) or supporting an equivalent inertial load (position task). The coefficient of variation for force decreased as the target force increased from 10% to 30% of maximal FDI force (index finger abduction) during the force task, whereas the standard deviation (SD) of acceleration increased with corresponding loads during the position task (both p < 0.001). The variability in discharge rate was greater during the position task and at the higher target (p < 0.001). There were two main findings: (1) factor analysis of the smoothed discharge rates yielded one motor unit mode for both compliance tasks and the two target forces, and (2) the SD of the discharge rates for the motor units included in the mode were more correlated with fluctuations in force or acceleration (0.52 - 0.84) than was the SD of the cumulative spike train (0.48 - 0.76). The emergence of a single motor unit mode for both tasks suggests that load compliance did not have a significant influence on the distribution of shared synaptic input to the involved motor neurons at either target force.