Task performance is prioritized over energy reduction.

The objective of this study was to characterize the temporal relationship between hand stiffness and task performance during adaptation to a brief contact task that required precision at the time of contact. The experiment required subjects to control the vertical position of a paddle on a computer display by grasping a robot's instrumented handle, with the goal of intercepting a virtual ball within 1 mm from the paddle center. A force transient was applied to the hand immediately after the ball-paddle impact to estimate the intrinsic hand impedance. There were two main results: 1) more trials were required for a brief contact task to find a low-energy strategy when compared with tasks that received feedback through the entire movement trajectory and 2) when the whole course of adaptation is long for brief contact tasks, viscoelastic forces were increased to achieve the task goal before the energy reduction initiated. Also, as the accuracy requirement was increased by changing the gain between handle and paddle motion through visual amplification, peak stiffness increased and occurred later, indicating that higher energy strategies are used for longer when the task's accuracy requirements were increased. These results indicated that task performance may be prioritized over energy reduction for a brief contact task.