Muscle synergy interprets the neural strategy adopted by the central nervous system (CNS) to simplify the coordination of muscles recruitments when performing useful movements. The computational mechanism of defining the optimal muscle combinations, however, still debatable. Muscle synergy deals with muscle activations pattern and time-dependent variables. The synergy space defines the suitable combinations of muscles, and time-dependent variables vary in lower-dimensional space to drive the behavior. In this study, we investigated the role of the CNS to optimize muscle patterns when performing skilled behavior. We introduced two synergy indices: the synergy stability index that indicates the similarity of the recruited synergies, and the synergy coordination index that indicates the size of the synergy space. The results on automatic posture response experiments on seven healthy participants show that both indices are positively correlated with the overall balance skill of the participants. Results suggest the optimal mechanisms adopted by the CNS to recruit muscles.