Evidence of the Existence of Multiple Modules for the Stroke-caused Flexion Synergy from Fugl-Meyer Assessment Scores

Stroke-caused synergies may result from the preferential use of the reticulospinal tract (RST) due to damage to the corticospinal tract. The RST branches multiple motoneuron pools across the arm together resulting in gross motor control or abnormal synergies, and accordingly, the controllability of individual muscles decreases. However, it is not clear whether muscles involuntarily activated by abnormal synergy vary depending on the muscles voluntarily activated when motor commands descend through the RST. Studies showed that abnormal synergies may originate from the merging and reweighting of synergies in individuals without neurological deficits. This leads to a hypothesis that those abnormal synergies are still selectively excited depending on the context. In this study, we test this hypothesis, leveraging the Fugl-Meyer assessment that could characterize the neuroanatomical architecture in individuals with a wide range of impairments. We examine the ability to perform an out-of-synergy movement with the flexion synergy caused by either shoulder or elbow loading. The results reveal that about 14% [8/57, 95% confidence interval (5.0%, 23.1%)] of the participants with severe impairment (total Fugl-Meyer score <29) in the chronic phase (6 months after stroke) are able to keep the elbow extended during shoulder loading and keep the shoulder at neutral during elbow loading. Those participants underwent a different course of neural reorganization, which enhanced abnormal synergies in comparison with individuals with mild impairment ( < 0.05). These results provide evidence that separate routes and synergy modules to motoneuron pools across the arm might exist even if the motor command is mediated possibly via the RST. We demonstrate that abnormal synergies are still selectively excited depending on the context.