Generalizability of Neuromuscular Coordination in the Human Upper Extremity after Stroke and its Implications in Neurorehabilitation

Background Previous studies have shown that stroke often impairs neuromuscular coordination (i.e., muscle synergies) across various biomechanical conditions. In our previous study, we investigated the generalizability of muscle synergies between isometric and free dynamic reaching in healthy individuals. However, the extent to which muscle synergy characteristics after stroke are generalized across these conditions remains unclear. Methods Electromyographic (EMG) signals from eight upper extremity muscles were recorded from 14 chronic stroke survivors with mild-to-severe motor impairment and eight age-range matched controls while performing isometric force generation and point-to-point dynamic reaching tasks. Non-negative matrix factorization was applied to identify muscle synergy characteristics underlying each task. Results In both groups, muscle activation patterns were effectively reconstructed using a small set of muscle synergies. The neurologically intact participants recruited four and five muscle synergies during the static and dynamic tasks, respectively. However, stroke survivors typically recruited four muscle synergies to perform both tasks. In addition, the composition of muscle synergies within each participant in both groups was largely conserved across the two tasks, though alterations in intermuscular coordination patterns were observed in post-stroke individuals, particularly in moderate and severe impairment cases. The majority of the altered, stroke-induced synergy patterns were explained by merging synergies underlying dynamic reaching of healthy individuals. The characteristics of muscle synergy activation profiles differed between the isometric and dynamic motor tasks in both groups. Stroke-induced alterations in correlation of pairs of synergy activation profiles were observed in dynamic reaching, but not in isometric conditions. Conclusion This study provides several implications to stroke neurorehabilitation. First, accessible isometric conditions, especially for severely impaired stroke survivors, can be adopted as biomechanical conditions of therapeutic exercises expecting potential transferability of motor learning effects to dynamic conditions. Second, fractionation of merged synergies after stroke can be a potential rehabilitation target to enhance motor control. Finally, dynamic tasks can be effective in assessing and intervening in potential motor abnormalities that may not be prominent during isometric conditions. These results highlight the importance of developing novel stroke rehabilitation strategies that aim at improving intermuscular coordination characteristics to enhance motor function across varying biomechanical conditions after stroke.