Abnormal muscle activation characteristics associated with loss of dexterity after stroke

The aim of this study was to characterise the abnormalities of muscle activ ation which underlie low dexterity after stroke. A broad definition of dext erity was adopted, where loss of dexterity refers to an inability to coordi nate muscle activity in the performance of a motor task (i.e. dexterity was not confined to manual dexterity). EMG of biceps brachii and triceps brach ii were monitored from 16 people after stroke and 10 neurologically normal controls as they performed a tracking task requiring coordinated elbow flex ion and extension. Weakness could not interfere with performance since the task was designed to require minimal strength. Stroke subjects were assigne d to a low (n=10) or high (n=6) dexterity group based on their performance. Spatiotemporal aspects of biceps and triceps EMG were analysed. Low dexter ity performance after stroke was characterised by excessive biceps muscle a ctivation (P=0.002) and decreased coupling of muscle activation to target m otion (P=0.002). In this study, we could rule out weakness, slowness of mus cle activation, excessive co-contraction and spasticity as causes of these abnormalities. Therefore, the loss of dexterity after stroke can be seen as a specific negative impairment which can exist independently of other moto r impairments and reflects a loss of skill in generating spatial and tempor al muscle activation patterns which conform with environmental demands. (C) 2000 Elsevier Science B.V. All rights reserved.