Evaluation of Postural Muscle Synergies during a Complex Motor Task in a Virtual Reality Environment

In this study, we investigate how the central nervous system (CNS) organizes postural control synergies when individuals perform a complex catch-and-throw task in a virtual reality (VR) environment. A Robotic Upright Stand Trainer (RobUST) platform, including surface electromyography and kinematics, was used to investigate how the CNS fine-tunes postural synergies with perturbative and assist-as-needed force fields. A control group without assistive forces was recruited to elucidate the effect of force fields on motor performance and postural synergy organization after the perturbation and during the VR reaching task. We found that the application of assistive forces significantly improved reaching and balance control. The group receiving assistive forces displayed four postural control synergies characterized by higher complexity (i.e., greater number of muscles involved). However, control subjects displayed eight synergies that recruited less number of muscles. In conclusion, assistive forces reduce the number of postural synergies while increasing the complexity of muscle module composition.