Ankle clonus is elicited by stretching the Archille's
tendon by dorsiflexing the foot. Sustained tension on
the ligament will cause a rhythmic contraction of the
gastrocnemius and soleus muscles causing the foot to
alternate between dorsiflexion and plantarflexion.
Sustained clonus is a manifestation of upper motor
neuron disease in which the normal cortical suppression
of the reflex arc is diminished or absent. It is the
defining characteristic of hyperreflexia.
Clonus can also be elicited by hyperextending the wrist
(finger flexor clonus), and by thrusting the patella
downward (quadriceps clonus).