Gastrocnemius-soleus dysfunction is a frequent result of cauda equina lesions and peripheral neurophathies and of stroke and brain injury. Temporary tibial nerve paralysis constitutes a comparable laboratory condition which allows the controlled examination of aspects of these disabilities. The biomechanical effects of temporary tibial nerve paralysis in six normal young adult volunteers were examined to quantitatively define the gait abnormalities resulting from gastrocnemius-soleus paralysis and to provide a basis for the assessment of the effectiveness of different orthotic designs in restoring a normal gait pattern. The motion of the right lower extremity, ground reactive forces acting on the right lower extremity, timing of gait events and step length were recorded, first during normal ambulation and then during ambulation after a right tibial nerve block. Step length was reduced bilaterally after the nerve block (p less than 0.005); the reduction was greater for the left (unblocked) step than for the right (blocked) step (p less than 0.005). Right heeloff was delayed until the time of left heelstrike in all subjects after the right tibial block. Right heeloff occurred later (p less than 0.005) while left heelstrike occurred earlier than normal (p less than 0.005). The shorter left step length and earlier left heelstrike resulted from a reduction in the forward progression of the right hip (p less than 0.001). When subjects were deprived of the plantarflexion activity necessary to counteract dorsiflexion moments at the ankle, they delayed the forward progression of the center of pressure (p less than 0.001) to avoid the unstable collapse of the foot into dorsiflexion.(ABSTRACT TRUNCATED AT 250 WORDS)