Contralesional Crossed Facilitation Impairs Bimanual Force Coordination in Chronic Stroke Survivors with Moderate-to-Mild Motor Impairment.

BACKGROUND

After stroke, impaired bimanual coordination reduces quality of life, where precise coordination of force between arms is essential for daily activities. Effective coordination relies on balanced interhemispheric communication, which induces crossed facilitation between primary motor cortices (M1). Intracortical inhibition influences both crossed facilitation and bimanual coordination in neurologically intact individuals. This study examines whether GABAB-mediated inhibition in ipsilesional M1 influences crossed facilitation from contralesional M1 and its relationship with bimanual coordination post-stroke.

METHODS

Thirteen chronic stroke participants performed dynamic and isometric bimanual force grip task. In the dynamic task, the paretic hand maintained 30% of maximal voluntary contraction while the non-paretic hand varied force levels (low-mid-high). Cross-covariance coefficient between hands measured interference from non-paretic hand to paretic hand. In the isometric task, transcranial magnetic stimulation assessed crossed facilitation via motor evoked potential (MEP) and intracortical inhibition via cortical silent period (CSP) in ipsilesional M1 under varying bimanual force conditions (paretic: rest, 5%, 30%; non-paretic: rest, 10%, 30%, 70%).

RESULTS

Results showed variable bimanual interference post-stroke, with greater interference in less impaired individuals and under high non-paretic force. Crossed facilitation increased with higher force asymmetry and lower paretic effort, particularly in less impaired participants, but became more variable as paretic effort increased (during PH 30%: NPH 70%). Under the high asymmetry condition, GABAB-mediated disinhibition was most pronounced and greater crossed facilitation was associated with increased bimanual interference.

CONCLUSION

These findings suggest that reduced inhibitory tone may contribute to the regulation of crossed facilitation, and bimanual coordination deficits may be driven by excessive crossed facilitation. Future work will examine other ipsilesional factors regulating crossed facilitation, as targeted asymmetric training and neuromodulation may help improve bimanual coordination in individuals with moderate-to-mild motor impairment.