Abnormal EMG-force slope estimates in the first dorsal interosseous of hemiparetic stroke survivors

Hemispheric brain injury resulting from a stroke is often accompanied by weakness in contralateral limbs. Appropriate motoneuronal recruitment and rate modulation is necessary to optimize muscle force production utilizing residual neuromuscular elements. We sought to determine whether weakness in a hand muscle in stroke survivors is partially attributable to alterations in the control of the motor units in the affected muscles. Specifically, our goal was to characterize whether surface EMG amplitude, a gauge of neural input, was systematically larger as a function of force, in paretic muscles when compared to the contralateral muscles in the same subject, and to neurologically intact subjects. We tested the first dorsal interosseous (FDI) in five hemiparetic and six neurologically intact subjects. In four of the stroke subjects the EMG-force slope was significantly greater on the affected side as compared to the contralateral side as well as compared to neurologically intact subjects. We discuss possible experimental as well as physiological factors that may contribute to an increased slope, concluding that a combination of abnormal firing rate patterns and changes in MU control are the most likely reasons for the observed changes.