Visuo-motor planning is influenced by previous experience with the visual world, the reward structure of the environment, and the state of one's motor system. A physical constraint on one's joints, be it through injury or an external restraint, can be thought of as changing both the cost structure of movement and the state of one's motor apparatus. Here we investigated how joint constraints at the wrist, fingers and elbow affect movement times and kinematics in a visually guided grasping and reaching task.

Experiment 1: Participants (n=6) were asked to place a series of small discs into slots as fast as possible. Blocks of unconstrained trials were alternated with blocks during which the participant wore a brace around the fingers, wrist or elbow. Each participant performed blocks with each constraint. Movement times were recorded and compared between the constrained and unconstrained trials and between the unconstrained trials prior to and after each constraint.

Experiment 2: Participants (n=9) performed the task above while their movement times and 3D position and orientation of their hand, wrist, elbow, and shoulder were recorded with a Polhemus Liberty. As in Experiment 1, unconstrained and constrained blocks were alternated, but each participant was exposed to only one constraint (wrist (n=5) or elbow (n=4)).

Results: In both experiments, while all constraints slowed movements significantly, repeatedly moving with the wrist constraint (but not with the elbow or finger constraint) led to significantly faster movements after the constraint was removed. Since this improvement in movement time was an aftereffect specific to the wrist constraint, it was not simply the result of repeated practice and must have been due to a new compensatory strategy learned as a result of experience with the constraint. Compensations that were identified from the joint position and orientation data will be discussed.