Task Complexity Amplifies the Stroke-Induced Temporal and Spatial Asymmetry in Muscle Synergy Plasticity

Background : motor synergy patterns are recognized as physiological markers of motor cortical damage, providing insight into how motor cortex coordinates spinal motor modules to generate movement. However, how these patterns adapt to tasks of varying complexity following post-stroke cortical damage is not yet fully understood. Objective : we aimed to understand how motor synergy patterns are distorted across tasks of increasing complexity after stroke induced cortical damage, also to provide a reference for task selection when using muscle synergy patterns as biomarkers for stroke evaluation or intervention. Methods : This is a pilot, cross sectional study. We investigated the muscle synergies during 5 tasks with varying complexity in 20 healthy individuals (13 females and 7 males, aged 64.33 ±6.94 years) and in 12 chronic stroke participants (4 females and 8 males, aged 64.4 ±6.54 years) by recording the surface electromyographic activities of 16 upper limb muscles (eight muscles unilaterally). Non-negative matrix factorization was performed to extract the muscle synergies. We categorized the stroke-induced synergy plasticity based on healthy synergy centroids, compared the synergy plasticity between affected and unaffected limb, and investigated the correlation between synergy plasticity and patient’s motor function, Results: In healthy individuals, the number of muscle synergies exhibits a U-shaped pattern as task complexity increases, whereas in stroke patients, both the affected and unaffected limbs show a decreasing trend in muscle synergy number with increasing task complexity. The proportion of preservation synergies was significantly higher in the unaffected arm compared to the affected arm in moderate and high complexity tasks. In contrast, the number of mutation synergies as well as mutation synergy activation were lower in the unaffected arm than in the affected arm in moderate and high complexity task. Notably, this asymmetry in preservation is significantly correlated with motor function of stroke patients. Conclusion : This study is the first to investigate how task complexity influences muscle synergy plasticity and their asymmetry in stroke participants. stroke patients demonstrate spatial and temporal asymmetry in muscle synergies between the unaffected and affected sides. This asymmetry is magnified by task complexity and shows a strong correlation with motor performance. Therefore, we recommend that the use of muscle synergy patterns as biomarkers for stroke assessment or rehabilitation should also account for the factor of task complexity.