The effects of force level and force direction on force discrimination and isometric tracking performance

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Virginia Polytechnic Institute and State University

An isometric, zero-order, two-dimensional pursuit tracking task utilizing a tracking path that incorporated both linear and circular segments was used to examine the effects of variation in force magnitude, force direction, and direction of target movement upon tracking performance. A contralateral force-matching procedure was also employed to assess the effects of variation in force level and force direction upon force discrimination capabilities.

Increases in force demand were hypothesized to result in degradation of both force discrimination capabilities and accuracy of tracking performance. Variation in force direction was hypothesized to result in tracking performance degradation and force discrimination deterioration that were inversely related to strength-related differences associated with each direction. It was hypothesized that linear tracking performance during inward (force-decreasing) movement of the target would be superior to linear tracking performance in the outward (force-increasing) direction. Finally, it was hypothesized that the direction-sensitive strength:demand ratio, SDR, (a measure of the relationship between strength and the magnitude of force demand) would correlate with both tracking performance and force discrimination measures and that force discrimination measures would correlate with tracking performance.

The findings robustly supported the hypothesized Force Level effect. The strength-related Force Direction effects were also supported, but somewhat less consistently than those for Force Level. As also hypothesized, linear inward tracking was superior to linear outward tracking. Previously unreported direction-of-movement effects were found for the circular tracking conditions. When measured along the path, clockwise (CW) tracking was superior to counterclockwise (CCW) tracking with the differences being greatest at the higher force level and in the longitudinal (forward and aft) directions. CCW tracking was superior to CW tracking when measured orthogonal to path. The hypothesized correlations among SDR, two of the three force discrimination measures, and tracking performance were found to be small but significant.