Position and velocity cursor mappings contribute to distinct muscle forces in simulated isometric and movement reaching

Contributions of muscle forces in isometric and normal (movement) reaching have not been thoroughly compared. In this study, we ask if position-based and velocity-based cursor mappings during planar isometric reaching produce muscle forces that are similar to those exerted during unassisted movement. Healthy subjects pushed against a static manipulandum handle to direct a cursor toward a target using either a position or velocity mapping. Applied force and cursor path data were used to create dynamic musculoskeletal simulations (using the OpenSim platform) of targeted isometric and movement reaches. Isometric muscle forces in both position and velocity mappings were found to be distinct from the corresponding forces in movement simulations. These results motivate future research on the design of a physiology-based isometric mapping that incorporates arm dynamics and inertia. A mapping that produces comparable muscular activity in isometric and movement reaching may support the development of improved isometric and robotic rehabilitation strategies for patients with upper limb movement deficits.