Activation deficit correlates with weakness in chronic stroke: Evidence from evoked and voluntary EMG recordings

Objective: To use evoked (M-wave) and voluntary (during maximal voluntary contraction (MVC)) EMG recordings to estimate the voluntary activation level in chronic stroke. Methods: Nine chronic hemiparetic stroke subjects participated in the experiment. M-wave (EMG_M-wave) and MVC (EMG_MVC) EMG values of the biceps brachii muscles were recorded. Results: Peak torque was significantly smaller on the impaired than non-impaired side. EMG_M-wave was also significantly smaller on the impaired than non-impaired side. However, the normalized EMG_M-wave/Torque_MVC ratio was not significantly different between two sides. In contrast, both absolute EMG_MVC and normalized EMG_MVC/Torque_MVC were smaller on the impaired than non-impaired side. The voluntary activation level, EMG_MVC/M-wave, was also smaller on the impaired than non-impaired side. The voluntary activation level on the impaired side was highly correlated with weakness (R=0.72), but very low (R=0.32) on the non-impaired side. Conclusion: Collectively, our findings suggest that both peripheral and central factors contribute to post-stroke weakness, but activation deficit correlates most closely with weakness as estimated from maximum voluntary torque generation. Significance: These findings serve to highlight the potential benefit from high-intensity exercises to enhance central activation for facilitation of motor recovery.