Rhythmic affects on stroke-induced joint synergies across a range of speeds

Joint synergies are one among several diminished motor capabilities that are associated with stroke. These synergies are characterized by a stereotypical combination of involuntary joint coactivations. This research measured the synergistic rotations of the shoulder in response to voluntary rhythmic motion of the elbow across a range of speeds. The experimental protocol included a total of 22 subjects divided into two groups: (1) stroke survivors and (2) neurologically intact controls. Rhythmic motion in stroke survivors resulted in comparable synergies to discrete movement. It was found that hemiparetic subjects had greater synergy than neurologically intact individuals for all speeds. Synergy was quantified using a synergy ratio. This ratio uses elbow rotation as an input in the denominator and shoulder rotation as an output in the numerator. The amount of shoulder synergy varied with the subject's level of impairment as measured by a modified Fugl-Meyer assessment. As rhythmic speeds increased, the synergy ratios became higher for stroke subjects. This effect was especially pronounced for subjects with higher impairment. The relationships between synergies that arise from rhythmic and discrete movements are also discussed. The results of this study may have implications for therapeutic interventions, robotic rehabilitation approaches, and for the design of orthotic devices. More generally, these results shed light on the role of central pattern generators in hemiparetic motion.