Effects of trunk support and target distance on postural adjustments prior to a rapid reaching task by seated subjects

Abstract

The purpose of this study was to determine the presence of postural adjustments before a reaching task by seated subjects. Nine seated subjects performed a rapid reaching task to a target placed at shoulder height, 45 degrees to the right of midline. Reach time, timing and magnitude of forces produced by the movement, and onset of deltoid muscle activity were analyzed under supported versus unsupported, and near versus far reach conditions. Reach time was proportional to reach distance. For all reach conditions, initial movement of the center of body mass occurred in a direction toward the target, ie, opposite in direction to reactive forces produced by the accelerating arm. Regardless of support, reactive forces were recorded approximately 30 to 50 msec before arm movement, and represented a greater percentage of body weight, for the far reach conditions. Peak acceleration to the right occurred at proportionately the same time into reach regardless of support or reach distance. Peak acceleration occurred earlier in the supported conditions. This paper provides kinetic evidence of postural adjustments before a rapid reaching task performed by seated subjects.