The functional role of automatic body response in shaping voluntary actions based on muscle synergy theory

The functional role of automatic body response in forming voluntary actions remain controversial. We here support the hypothesis that the automatic body responses could be used as a reference to adapt voluntary actions to the environment. We validate this hypothesis by analyzing human body movements from the perspective of muscle synergy. In this study, a horizontal shoulder adduction of the dominant arm of four healthy subjects was examined in various tasks. The tasks include reflex and voluntary movements in regular and modified environments. Preliminary results were encouraging; the number and the consistency between the utilized synergies in automatic and voluntary tasks were fairly correlated. In contrast, there was a lack of the correlation when the environment was abruptly modified (an additional resistance applied to the voluntary movement). This lack of correlation, however, was gradually adjusted through training. Our results suggest that automatic synergy may encode some features which could be used by the central nervous system to shape the voluntary synergy.