The distribution of muscle weakness in upper motoneuron lesions affecting the lower limb. To determine the distribution of weakness in the lower limb after upper motoneuron lesions the strength of 8 muscle groups was measured. Four groups of patients were studied: 22 control subjects, 16 patients with unilateral leg paresis, 4 patients with severe unilateral paralysis and 5 patients with paraparesis. In the testing posture (seated), patients with cerebral upper motoneuron lesions showed no selective loss of power in flexors or extensors on the contralateral side. Gravitational torques were included in the measurements. However, proximal muscles (acting at hip and knee) were significantly less severely affected than more distal muscles (acting at ankle and hallux). At any particular joint, physiological flexors and extensors were affected equally in both the hemiparetic and paraparetic subjects. As in the upper limb (Colebatch and Gandevia, 1989), the strength of muscles on the clinically unaffected side was reduced compared with control subjects, although no muscle groups were especially affected. 