The aim of this study was to examine the pathophysiological mechanisms underlying co-contraction in patients with dystonia (n = 6) and writer's cramp (n = 5). Multi-unit needle and surface EMGs were recorded from extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles during motor tasks that elicited dystonia or writer's cramp. The EMGs from ECR and FCR were recorded simultaneously and analysed using cross-correlation analysis. Similar recordings were obtained from healthy age- and sex-matched control subjects (n = 8). Despite co-contraction of the muscles, cross-correlograms from the healthy subjects did not reveal evidence of motor unit synchronization. Cross-correlograms from the dystonic subjects revealed a central peak with a median duration of 37 ms, indicating broad-peak motor unit synchronization. Cross-correlograms from patients with writer's cramp were either flat or modulated by a 11-12-Hz tremor. Frequency-domain analysis of ECR and FCR EMGs demonstrated significant coherence in the patients with dystonia and writer's cramp. These results indicate that co-contraction in dystonia is neurophysiologically distinct from voluntary co-contraction and is produced by abnormal synchronization of presynaptic inputs to antagonist motor neuron pools. ECR and FCR co-contraction in writer's cramp may be a compensatory process under voluntary control.