The directionality of interjoint neural coupling in the impaired arm post stroke

Yet, little is known about the direction of abnormal coupling in conjunction with its degree between joints—muscles—of interest. In this study, we address the directionality of pathological neural couplings between joints, and variability in the degree of them, in the upper extremity post stroke. To evaluate the direction of interjoint coupling, joint motion and muscle activity in an involuntary mode while another joint—muscle—was voluntarily activated were assessed in comparison to those made in a volitional mode, focusing on the shoulder, elbow and wrist joints. Overall, involuntary activation of the wrist flexor in the stroke group is observed when muscles of a more proximal joint voluntarily activate (p < 0.1 and p <0.05), while activation of the wrist flexor does not lead to involuntary activation of the shoulder muscles and elbow flexor, in comparison to the healthy group. In particular, in the stroke group who is assumed to have a severe loss of the corticospinal system, the wrist extensor is subordinately activated by those about the other joints voluntarily activated (p < 0.05) while it does not cause activations of those about the other joints. These findings indicate that stroke-caused synergies could be directional.