Surface electromyographic "EMG" activity in biceps brachii (agonist muscle) and triceps brachii (antagonist muscle) as well as the discharge behaviour of motor units "MUs" (needle recording) in biceps brachii muscle were recorded during slow (0.33 Hz) and fast (0.66 Hz) voluntary elbow flexion movements (auditory matching task) in fifty patients with spastic hemiplegia. In the spastic limbs, a long lasting, small amplitude tonic co-contraction of antagonist muscles was seen during slow flexions (SF) in 29 cases. This effect was strongest during the fast elbow flexion movements (FF). In 33 patients a triphasic pattern of muscle activation was observed on the unaffected side but not on the spastic side. The amplitude of the agonist surface EMG was significantly reduced and the amplitudes of the MU potentials recruited during maximal effort were generally smaller on the spastic side compared to the unaffected side. The agonist-antagonist activation pattern was analysed with respect to three clinically identifiable functional recovery stages of voluntary movements in the spastic limbs, namely synergistic, isolated and useful movements. The MU amplitudes and the amplitude of the surface EMG activity in the agonist muscle recorded during FF movements became significantly larger whereas the amplitude of the antagonist tonic activity became smaller with increasing functional recovery of the limb. It is concluded that impaired recruitment of type II motor units in the agonist muscles and the inability to selectively activate the agonist muscle contribute to the deficit in motor performance in spastic paresis.