An Interactive Simulator for Imposing Virtual Musculoskeletal Dynamics

Objective: disease processes are often marked by both neural and muscular changes that alter movement control and execution, but these adaptations are difficult to tease apart because they occur simultaneously. This is addressed by swapping an individual's limb dynamics with a neurally controlled facsimile using an interactive musculoskeletal simulator (IMS) that allows controlled modifications of musculoskeletal dynamics. This paper details the design and operation of the IMS, quantifies and describes human adaptation to the IMS, and determines whether the IMS allows users to move naturally, a prerequisite for manipulation experiments. Methods: healthy volunteers (n = 4) practiced a swift goal-directed task (back-and-forth elbow flexion/extension) for 90 trials with the IMS off (normal dynamics) and 240 trials with the IMS on, i.e., the actions of a user's personalized electromyography-driven musculoskeletal model are robotically imposed back onto the user. Results: after practicing with the IMS on, subjects could complete the task with end-point errors of 1.56 degrees, close to the speed-matched IMS-off error of 0.57 degrees. Muscle activity, joint torque, and arm kinematics for IMS-on and -off conditions were well matched for three subjects (root-mean-squared error [RMSE] = 0.16 N.m), but the error was higher for one subject with a small stature (RMSE = 0.25 N.m). Conclusion: a well-matched musculoskeletal model allowed IMS users to perform a goal-directed task nearly as well as when the IMS was not active. Significance: this advancement permits real-time manipulations of musculoskeletal dynamics, which could increase our understanding of muscular and neural co-adaptations to injury, disease, disuse, and aging.