Influence of the variability in motor unit discharge times and neural drive on force steadiness during submaximal contractions with a hand muscle.

Our purpose was to compare the influence of the spectral content of motor unit recordings on the calculation of electromechanical delay and on the prediction of force fluctuations from measures of the variability in discharge times and neural drive during steady isometric contractions with the first dorsal interosseus muscle. Participants ( = 42; 60 ± 13 yr) performed contractions at 5% and 20% MVC. After satisfying the inclusion criteria, high-density surface EMG recordings from a subset of 23 participants were decomposed into the discharge times of 530 motor units. The force and cumulative spike train (CST) signals were cross-correlated with a novel filtering approach to determine the electromechanical delay. Force and CST signals were bandpass filtered with three bandwidths (0.75-5 Hz, 0.75-2 Hz, and 2-5 Hz) to determine the influence of spectral content on the precision of the electromechanical delay measurement. Subsequently, the variability in the discharge times of motor units was quantified as the coefficient of variation for interspike interval (CVISI), and the variability in neural drive was represented as the standard deviation of the cumulative spike train (SDCST). The main findings were that all frequencies (0.75-5 Hz) were needed to determine the electromechanical delay and that the force fluctuations were best explained by measures of variability in both discharge times and neural drive (CVISI and SDCST) at 5% MVC force but only the variability in neural drive (SDCST) at 20% MVC force. These findings indicate that the source of the force fluctuations during the steady submaximal contractions with the hand muscle differed for the two target forces. The fluctuations in force during steady submaximal contractions can be caused by either or both the variability in discharge times of individual motor units and in the neural drive. After careful alignment of the force and discharge times within an optimal bandwidth (0.75-5 Hz), the fluctuations in force at the lower target force were strongly correlated with both measures of variability, whereas those at the higher target force were best explained by the variability in neural drive.