It is a type of stretch reflex that tests the function of the gastrocnemius muscle and the nerve that supplies it.
When they are intact, integrity of the following is confirmed: cutaneous innervation, motor supply, and cortical input to the corresponding spinal segment.
It is helpful to support the ball of the foot at least somewhat to put some tension in the Achilles tendon, but don’t completely dorsiflex the ankle.
A small strike is given on the Achilles tendon using a rubber hammer to elicit the response.
Any spinal cord lesions, be it traumatic, neoplastic, pyogenic, vascular above the level of S1 can cause clonus.