Alterations in upper limb muscle synergies underlying coupling of end-point force post stroke during isokinetic force generation

Abstract

To perform a given motor task, muscles in the human body coordinate with one another. Intermuscular coordination can be quantitatively described as weights of muscles that form muscle synergies (i.e., modules of the coordination) and activation of the synergies. Stroke, which induces motor impairments in the affected limbs, results in alteration of muscle synergies, but different mechanisms of alteration have been reported depending on the motor tasks performed by a subject. A comparison of the different patterns of muscle synergies post stroke across a variety of motor tasks not only improves our understanding of how stroke affects intermuscular coordination but also enables systematic development of rehabilitation training for the stroke-affected intermuscular coordination. We evaluated muscle synergies underlying the shoulder and elbow joints of stroke-affected upper limbs during isokinetic force generation tasks, whose results have been unknown. Chronic stroke survivors and neurologically intact participants (as control subjects) intended to move their hands straight along six orthogonal directions (e.g. anterior, posterior, medial, lateral, superior, and inferior) at a constant speed and to generate end-point forces at maximum effort toward the movement directions. In addition to the muscle synergies, we evaluated the coupling of force components in different directions to quantify how accurate the control of end-point force direction was in both groups. Compared to the control group, the stroke survivors showed coupling among the force components, which indicated that the end-point forces were less accurately controlled in the stroke-affected limbs. Specifically, the end-point forces along the anterior, medial, and inferior directions were coupled with one another while the posterior, lateral, and superior forces were coupled. In terms of muscle synergy, in the stroke group, we observed that activation of shoulder-related synergies was altered differently depending on whether the elbow joint was flexed or extended. The altered activation of muscle synergies tended to explain the coupling of the end-point force. These results suggest that the coupling of the end-point force during isokinetic force generation can be utilized as an indicator of abnormal coordination between muscles underlying the shoulder and elbow joints. In addition, training independent activation of muscle synergies, which involve excitation of muscles around the elbow or shoulder joint, can improve motor control performance in the stroke-affected limbs such as reduction of the end-point force coupling.