Manual obstacle avoidance takes into account visual uncertainty, motor noise, and biomechanical costs.

Moving around obstacles requires balancing the need to avoid collisions with the need to minimize biomechanical costs. We investigated this tradeoff by studying the effects of visual uncertainty, motor noise, and practice on clearance over obstacles in a manual positioning task. Participants moved a manipulandum back and forth over a stationary obstacle. We varied visual uncertainty by placing the obstacle at different heights relative to participants' eyes, and we varied motor noise by having participants hold the object to be moved at different positions relative to the range of motion of the arm joints. Clearance was larger in conditions of higher visual uncertainty than in conditions of lower visual uncertainty, larger in the higher motor noise conditions than in the lower motor noise conditions, and larger early in practice than late in practice. The results indicate that spatial accuracy and biomechanical costs are both taken into account during reaching over obstacles, but to differing degrees across practice.