A synergy-based motor control framework for the fast feedback control of musculoskeletal systems.

This paper presents a computational framework for the fast feedback control of musculoskeletal systems using muscle synergies. The proposed motor control framework has a hierarchical structure. A feedback controller at the higher level of hierarchy handles the trajectory planning and error compensation in the task space. This task space controller only deals with the task-related kinematic variables, thus is computationally efficient. The output of the task space controller is a force vector in the task space, which is fed to the low-level controller to be translated into muscle activity commands. Muscle synergies are employed to make this force-to-activation (F2A) mapping computationally efficient. The explicit relationship between the muscle synergies and task space forces allows for the fast estimation of muscle activations that result in the reference force. The synergy-enabled F2A mapping replaces a computationally-heavy non-linear optimization process by a vector decomposition problem that is solvable in real-time. The estimation performance of the F2A mapping is evaluated by comparing the F2A-estimated muscle activities against measured EMG data. The results show that the F2A algorithm can estimate the muscle activations using only the task-related kinematics/dynamics information with ~70% accuracy. An example predictive simulation is also presented; the results show that this feedback motor control framework can control arbitrary movements of a 3D musculoskeletal arm model quickly and near-optimally. It is two orders-of-magnitude faster than the optimal controller, with only 12% increase in muscle activities compared to the optimal.