Timing variability and not force variability predicts the endpoint accuracy of fast and slow isometric contractions

The purpose of the study was to determine the contributions of endpoint variance and trajectory variability to the endpoint accuracy of goal-directed isometric contractions when the target force and contraction speed were varied. Thirteen young adults (25 +/- 6 years) performed blocks of 15 trials at each of 2 contraction speeds and 4 target forces. Subjects were instructed to match the peak of a parabolic force trajectory to a target force by controlling the abduction force exerted by the index finger. The time to peak force was either 150 ms (fast) or 1 s (slow). The target forces were 20, 40, 60, and 80% of the maximal force that could be achieved in 150 ms during an MVC. The same absolute forces were required for both contraction speeds. Endpoint accuracy and variability in force and time along with intramuscular EMG activity of the agonist (first dorsal interosseus) and antagonist (second palmar interosseus) muscles were quantified for each block of trials. The principal dependent variables were endpoint error (shortest distance between the coordinates of the target and the peak force), endpoint variance (sum of the variance in peak force and time to peak force), trial-to-trial variability (SD of peak force and time to peak force), SD of the force trajectory (SD of the detrended force from force onset to peak force), normalized peak EMG amplitude, and the SD of normalized peak EMG amplitude. Stepwise multiple linear regression models were used to determine the EMG activity parameters that could explain the differences observed in endpoint error and endpoint variance. Endpoint error increased with target force for the fast contractions, but not for the slow contractions. In contrast, endpoint variance was greatest at the lowest force and was not associated with endpoint error at either contraction speed. Furthermore, force trajectory SD was not associated with endpoint error or endpoint variance for either contraction speed. Only the trial-to-trial variability of the timing predicted endpoint accuracy for fast and slow contractions. These findings indicate that endpoint error in tasks that require force and timing accuracy is minimized by controlling timing variability but not force variability, and that endpoint error is not related to the amplitude of the activation signal.