SUMMARY
In neural control of movement, musculoskeletal redundancy poses a problem of choosing the solution among multiple muscle coordination that give the same movement. It has been suggested that the nervous system may use a reduced set of solutions: muscle synergies and its functional motor outputs. The underlying principle of how the nervous system selects specific muscle activation pattern remaining unknown, people have made predictions on muscle coordination using minimum energy as the optimizing criteria. However, there are evidences that stability, as well as energetic efficiency, also needs to be considered. We hypothesized that the nervous system may choose the activation pattern of a muscle synergy to produce an output that is functionally stable. To test our hypothesis, we identified unique muscle patterns with minimum energy criteria that produced experimental force vectors used for balance control in cats. Using them as inputs to a detailed neuromechanical model of the cat hindlimb, we tested whether each muscle activation patterns satisfied functional stability criteria established in this study when system was perturbed. Results show that minimum-energy muscle synergies can be functionally stable and unstable where linearized system characteristics could also predict functional stability. We conclude that using functional stability is useful in assessing the physiological behavior of the complex and nonlinear biomechanical systems. We suggest that functional stability must be considered when choosing a muscle activation pattern for a given motor task.