Effects of practice on magnitude and structure of force variability during periodic unimanual isometric force production.

The present study examined practice effects on the magnitude and structure of force variability using multiscale entropy analysis. Ten male participants (M age = 22.5 yr., SD = 1.7) produced unimanual isometric forces cycling between low levels of maximum voluntary contraction. The participants practiced the motor task over ten blocks: one block consisted of one self-paced recall trial immediately after nine audibly-synchronized practice trials with feedback of force output. Although the coefficient of variation of the force-time series in the recall trial had a greater magnitude than that in the practice, the recall trial had lower sample entropy values of force output (higher time structure of force variability). Multiscale entropy analysis did not find a practice effect on the time structure of force variability on a short time scale (10 msec.) but did on a long time scale (20-60 msec.). Different tasks have different time scales of force output, and the task demands of the present experiment emphasized the long time scales.