Stroke-caused synergies may result from 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 muscle synergies, and accordingly the controllability of individual muscles is reduced. It is not clear whether the muscles involuntarily activated by abnormal synergy vary with the muscles voluntarily activated when motor commands descend through the RST. Studies showed that abnormal synergies may originate from merging and re-weighting of synergies individuals without neurological deficit. 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 (FMA) that could characterize the neuroanatomical architecture in individuals with a wide range of impairment. 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%CI: [5.0%, 23.1%]) of the participants with severe impairment (total FM 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 undergo a different course of neural reorganization which enhanced abnormal synergies in comparison with individuals with mild impairment (p<0.05). These results 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.
Keywords: Reticulospinal Tract; Stroke; Synergy Module.