Directional effects of changes in muscle torques on initial path during simulated reaching movements

Adults are able to reach for an object for the first time with appropriate direction, speed, and accuracy. The rules by which the nervous system is ab le to set muscle activities to accomplish these outcomes are still debated and, indeed, the sensitivity of kinematics to variations in muscle torques is unknown for complex arm movements. As a result, this study used computer simulations to characterize the effects of change in muscle torque on init ial hand path. The same change was applied to movements towards 12 directio ns in the horizontal plane, and changes were systematically manipulated suc h that: (1) torque amplitude was changed at one joint, (2) timing of torque was changed at one joint, and (3) amplitude andlor timing was changed at t wo joints. Results showed that simultaneous changes in torque amplitude at shoulder and elbow joints affected initial speed uniformly across direction . These results add to conclusions from previous experimental and modeling work that the simplest rule to produce a desired change in speed for any di rection is to scale torque amplitude at both joints. In contrast, all simul ations showed nonuniform effects on initial path direction. For some region s of the workspace, initial path direction was little affected by either a +/-30% change in amplitude or a +/-100-ms change in timing, whereas for oth er regions the same changes produced large effects on initial path directio n. These findings suggest that the range of possible torque solutions to ac hieve a particular initial path direction varies within the workspace and, consequently, the requirements for an accurate initial path will vary withi n the workspace.