Constraints on human arm movement trajectories.

Abstract

The underlying processes in movement organization and control were studied by varying the conditions under which arm movements were made. The three-dimensional movement trajectories of the following conditions were contrasted: pointing to a target with the index finger versus grasping a disk the same size as the target, grasping a fragile object versus a soft resilient object, and grasping a disk either to throw into a large box or place into a tight fitting well. Results showed that the arm trajectories, as represented by the resultant velocity profile of the wrist, varied considerably in their shape with the main factor being when peak velocity was reached as a function of the total duration of the movement. It appeared that when task demands required greater precision, the main deceleration phase of the trajectory was increased in duration. These results do not support a movement production mechanism that has access to an abstract representation of a base velocity profile and that creates trajectories by a simple scaling procedure in the temporal domain. Rather, the results support a view of movement production as relatively specific to the past experience of the performer and the constraints of the task.