Enhanced motor unit rate coding with improvements in a force-matching task

These data describe improved modulation of discharge rates (rate coding) of first dorsal interosseous motor units throughout the acquisition of a complex force-matching skill involving isometric index finger abduction. In each of 15 consecutive trials, subjects attempted to match their force to a trajectory consisting of the sum of two sine waves (0.15 and 0.5 Hz) and random oscillations (overall mead force level similar to20% MVC). Reductions in root-mean-square (RMS) error of each subject's force relative to the trajectory indicated substantial improvements in force-matching ability (F = 33.8, p < 0.001). With the acquisition of this new skill, there was increased amplitude modulation of muscular force near both dominant frequencies of the force-matching trajectory (F = 10.6, p = 0.008). The standard deviation and coefficient of variation of motor unit inter-spike intervals both decreased with improved performance indicating a general reduction in the amplitude of firing rate modulations (SD: F = 18.69, p = 0.001; CV: F = 43.6, p < 0.001). After skill acquisition, there was decreased firing rate modulation outside of the two dominant frequencies and increased amplitude of firing rate modulation at the higher of the two dominant frequencies (0.5 Hz, F = 8.23, p = 0.015). These findings indicate that improved precision of rate coding was a contributor to the acquisition of the new force-matching task. That the change in rate coding was frequency dependent suggests that factors other than frequency coding may contribute to the improved force matching at 0.15 Hz. (C) 2004 Elsevier Ltd. All rights reserved.