Additional load decreases movement time in the wrist but not in arm movements at ID 6.

An experiment using reciprocal arm and wrist aiming movements with an amplitude of 16(o) and target width of .5° (ID = 6) was conducted to determine the impact of adding external loads. We predicted that wrist and arm performance may be differentially impacted by the added mass. Participants were asked to flex/extend their limb/lever in a horizontal plane at the wrist (arm stabilized) or elbow joint (wrist stabilized) in an attempt to move back and forth between the two targets as quickly and accurately as possible. External loads of 0, .568, or 1.136 kg were fixed at the distal end of the limb/lever. The targets and the current position of the limb were projected on the screen in front of the participant. The results indicated significant Group × Load interactions for movement time and percent time to peak velocity. Movement time decreased as load increased for the wrist but remained stable across loads for arm movements. Percent movement time utilized to accelerate the limb increased as load increased for wrist movements but only increased from 0 to .568 kg load for the arm movements. For both groups increased load had no significant effect on endpoint variability. The present findings suggest that the additional load allowed the control advantages of the wrist muscles to be exploited.