Clonus in man: a rhythmic oscillation maintained by a reflex mechanism.

The mechanisms underlying clonus were studied in 7 patients with spasticity of the lower limbs arising from upper motor neuron lesions. Clonus sensitivity to resetting was studied by evoking a soleus H reflex at different time intervals between two successive clonic beats. The relationship between the interval separating the H reflex from the preceding beat and the inter-beat interval in which the H reflex was delivered was used to calculate the resetting index. A high resetting index, close to 1.0, was found. Calf compression, applied to the upper third of the leg to block group I afferent fibres from gastrocnemius-soleus muscle, completely abolished ankle clonus within 10 min of the onset of compression. Clonus disappearance was preceded by a progressive reduction in duration and frequency. Finally, evidence is presented that clonus duration is contingent upon joint angle and therefore presumably on the amount of stretch placed on the triceps surae muscle group. Our findings demonstrate that clonus is largely dependent on reflex and mechanical factors and do not confirm the recent hypothesis that clonus relies on a central spinal generator.