Identification of neuromuscular synergies in natural upper-arm movements

In this paper we study a natural upper-arm movement composed of sequential phases of reaching, grasping, and retrieval in the horizontal plane; in the present context, natural means that the spatiotemporal constraints active in the execution of the movement are low. The proposed method of quantitative analysis, which is applied to data obtained from healthy subjects, combines quantification of arm kinematics and kinesiologic electromyography assessment of neuromuscular control for each subject; to this aim, we propose using factor analysis. Despite the existence of a time-profile invariance of velocity traces, the identified factors may differ in the muscles that load on them; the interpretation is that different neuromuscular synergies are involved in the execution of the movement, which leads to cluster the subjects into two distinct groups. Compared with group-2 subjects, group-1 subjects present a more lateral hand path direction and a less smooth hand trajectory in their approach to the grasped object. To explain the identified factors and the kinematic findings, it is hypothesized that the feedforward motor commands involved in trajectory planning are different.