The activation of leg muscles was analyzed in respect to ankle joint movement and the changes in tension produced by the triceps surae muscle during slow gait in spastic adults and children with cerebral palsy. In normal subjects the increase in tension of the triceps surae in the stance phase of gait is mainly due to an increase in gastrocnemius and soleus EMG. In spastic patients the abnormally high tension development in triceps surae is due more to passive muscle stretch, for the reciprocally organized leg muscle EMG is reduced. It is concluded that the leg extensor muscles in spastic patients exhibit a pseudostretch-reflex behavior due to their mechanical properties, and that this is mainly responsible for muscle hypertonia. The coactivation of the leg muscles seen in children with cerebral palsy, which also is seen in the stepping of the newborn, suggests impaired maturation of the neuronal locomotor pattern.