Achilles reflex
The ankle jerk reflex, also known as the Achilles reflex, occurs when the Achilles tendon is tapped while the foot is dorsi-flexed. A positive result would be the jerking of the foot towards its plantar surface. Being a deep tendon reflex, it is monosynaptic. It is also a stretch reflex. These are monosynaptic spinal segmental reflexes. When they are intact, integrity of the following is confirmed: cutaneous innervation, motor supply, and cortical input to the corresponding spinal segment.
This reflex is mediated by the S1 spinal segment of the spinal cord
Ankle of the patient is relaxed. 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. If you are not able to elicit a response, a Jendrassik maneuver can be tried by having the patient cup their fingers on each hand and try to pull the hands apart. A positive response is marked by a brisk plantarflexion of the foot. The response is also graded into Grade 1-4 according to the reflex grading system
The Achilles reflex checks if the S1 and S2nerve roots are intact and could be indicative of sciatic nerve pathology. It is classically delayed in hypothyroidism. This reflex is usually absent in disk herniations at the L5—S1 level. A reduction in the ankle jerk reflex may also be indicative of peripheral neuropathy.
...
Wikipedia