Influence of workspace constraints on directional preferences of 3D arm movements.

We previously demonstrated a tendency to perform arm movements by using a trailing joint control pattern during which either the shoulder or elbow is rotated actively and the other (trailing) joint is rotated predominantly passively, by interaction torque during horizontal movements and by interaction and gravitational torque during 3D arm movements. This tendency was established with a free-stroke drawing task that required production of series of strokes in randomly selected directions from the center to the perimeter of a horizontal circle. The studies demonstrated that within a planar workspace, the usage of the trailing pattern depends on movement direction and the most frequently selected directions are those providing the opportunity to use the trailing pattern. Here, we studied whether the opportunity to use the preferred trailing pattern also depends on the orientation of the planar workspace. The free-stroke drawing task was performed with unconstrained arm movements within circles of a left-diagonal (LD) and right-diagonal (RD) orientation. Two pronounced preferred directions were revealed in the LD condition, and they were the directions in which the trailing pattern was used. Directional preferences were less pronounced, and the trailing pattern was not observed in any directions in the RD condition. Also, subjects identified the RD condition as inconvenient. The results reinforce the previous finding of the propensity to use the trailing pattern during arm movements. They also suggest that orientations of the workspaces in manual activities should be designed to support the trailing pattern as a favored type of joint control.