Estimating time to contact and impact velocity when catching an accelerating object with the hand.

To catch a moving object with the hand requires precise coordination between visual information about the target's motion and the muscle activity necessary to prepare for the impact. A key question remains open as to if and how a human observer uses velocity and acceleration information when controlling muscles in anticipation of impact. Participants were asked to catch the moving end of a swinging counterweighted pendulum, and resulting muscle activities in the arm were measured. The authors also simulated muscle activities that would be produced according to different tuning strategies. By comparing data with simulations, the authors provide evidence that human observers use online information about velocity but not acceleration when preparing for impact.