  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.
