Muscle Synergies Underlying Isometric and Kinematic Reaching in the Human Upper Extremity

Muscle synergies have been used to study the organization of neuromotor control in humans as a means of developing a better understanding of rehabilitation strategies for people with neurological impairments. This study is an extension of our previous research that aimed to characterize the generalizability of muscle synergies under isometric and point–to–point reaching conditions. Here, we wanted to characterize muscle synergies underlying isometric and kinematic reaching conditions to understand the differences in how muscle synergy characteristics are expressed under different biomechanical conditions. To do so, we recruited 10 right–handed participants to perform isometric or kinematic reaching tasks in eight directions using the KULSIS robotic system. Non–negative matrix factorization was used to analyze the EMG of 12 muscles collected from the arm and back muscles of the participants. We found that across the different biomechanical conditions, five synergies were enough to reconstruct the EMG data. We also saw that the muscle synergies exhibited shared and task–specific synergy composition but differed in the activation profile when the muscle synergy was constant across conditions. In future studies, we hope to characterize the same muscle synergy characteristics in stroke survivors to develop better rehabilitation methods.