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

• A novel method using evoked (M-wave) and voluntary (MVC) EMG recordings was used to estimate the voluntary activation level in chronic stroke.
• Decreased M-wave values were proportional between impaired and non-impaired biceps, but MVC EMG values were further decreased on the impaired side and highly correlated with weakness.
• 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.

ObjectiveTo use evoked (M-wave) and voluntary (during maximal voluntary contraction (MVC)) EMG recordings to estimate the voluntary activation level in chronic stroke.MethodsNine chronic hemiparetic stroke subjects participated in the experiment. M-wave (EMGM-wave) and MVC (EMGMVC) EMG values of the biceps brachii muscles were recorded.ResultsPeak torque was significantly smaller on the impaired than non-impaired side. EMGM-wave was also significantly smaller on the impaired than non-impaired side. However, the normalized EMGM-wave/TorqueMVC ratio was not significantly different between two sides. In contrast, both absolute EMGMVC and normalized EMGMVC/TorqueMVC were smaller on the impaired than non-impaired side. The voluntary activation level, EMGMVC/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.ConclusionCollectively, 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.SignificanceThese findings serve to highlight the potential benefit from high-intensity exercises to enhance central activation for facilitation of motor recovery.