The aim of this study was to characterise the abnormalities of muscle activation which underlie low dexterity after stroke. A broad definition of dexterity was adopted, where loss of dexterity refers to an inability to coordinate muscle activity in the performance of a motor task (i.e. dexterity was not confined to manual dexterity). EMG of biceps brachii and triceps brachii were monitored from 16 people after stroke and 10 neurologically normal controls as they performed a tracking task requiring coordinated elbow flexion and extension. Weakness could not interfere with performance since the task was designed to require minimal strength. Stroke subjects were assigned 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 dexterity performance after stroke was characterised by excessive biceps muscle activation (P=0.002) and decreased coupling of muscle activation to target motion (P=0.002). In this study, we could rule out weakness, slowness of muscle 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 motor impairments and reflects a loss of skill in generating spatial and temporal muscle activation patterns which conform with environmental demands.