Fluctuations In Force Trajectory Predict End-point Accuracy During Rapid Isometric Contractions

Abstract

The minimum-variance theory predicts that the central nervous system preferentially selects trajectories that comprise low levels of force fluctuations to minimize signal-dependent noise and maximize end-point accuracy. Purpose To determine the association between fluctuations in the force trajectory and end-point accuracy of rapid, goal-directed isometric contractions. Methods Eleven men and 11 women (23±5 yrs), performed 100 discrete isometric contractions with the first dorsal interosseus muscle. Each subject was instructed to match a force trajectory to a target by controlling the abduction force exerted by the index finger. Visual feedback of the force-time curve, the force-time trajectory, and target location was provided to each subject 1 s after each trial. The time to peak force was 150 ms and the peak force required was 25% of the maximal force that each subject could exert at 150 ms. End-point accuracy was quantified in two ways: (1) overall error — the average distance from the target based on the force and time coordinates; (2) ellipsoid — the product of the SD of the force and SD of time, as used previously by the proponents of the minimum-variance theory. Trajectory variability was quantified as the SD of the detrended force (SDF) from the initial rise in force to the peak force. Results Both the overall error and ellipsoid measures declined with trial number and indicated that ten trials were needed to reach a plateau in accuracy. The two measures of end-point accuracy, however, were not significantly associated (r2 = 0.10, P = 0.15). The SDF was strongly associated with the overall error (r2 = 0.80, P <0.001), but only weakly associated with the ellipsoid (r2 = 0.18, P = 0.049). The overall error was predicted (r2 = 0.80, P = 0.001) equally from the error in force (r = 0.56) and time (r = 0.53). Further, end-point accuracy was also predicted by the impulse to peak force, which was highly associated with overall error (r2 = 0.81, P <0.001). Conclusion As predicted from the minimum-variance theory, endpoint accuracy during open-loop isometric contractions appears to be strongly related to the fluctuations in the force trajectory. The measure of end-point accuracy that is most sensitive to the fluctuations in the force trajectory, however, appears to be the distance from the target (overall error) and not the total variability of the responses (ellipsoid). Supported by NIA R03 AG024662-01 to EAC.