The Achilles reflex, also called the ankle reflex or ankle jerk, is a stretch reflex in which a tap on the Achilles tendon makes the calf contract and the foot point down. It tests the S1 root and the tibial nerve. It is the standard reflex for S1.
| Nerve root | S1 (S1-S2) |
|---|---|
| Area | Achilles (calcaneal) tendon at the back of the ankle |
| How to test | With the foot held in slight dorsiflexion and the leg relaxed (or kneeling on a chair), the examiner strikes the Achilles tendon and watches for plantarflexion. |
| Muscles | |
| Nerve Supply | Tibial nerve |
Confirms the integrity of the S1 reflex arc and contributes to calf muscle tone during standing.
A one-sided reduced ankle reflex suggests S1 radiculopathy, usually from an L5-S1 disc. Ankle reflexes are the first to disappear in length-dependent peripheral neuropathy and can be reduced in many healthy older adults. A slow relaxation phase is a classic sign of hypothyroidism. Tendon reflexes are graded 0 (absent) to 4+ (clonus), with 2+ normal; symmetry matters more than the absolute grade.
Compression of the S1 root at L5-S1 reduces or abolishes the ankle reflex, with calf weakness and outer foot numbness.
Generalised nerve damage, for example from diabetes, reduces or abolishes tendon reflexes, starting with the ankle reflexes.
Low thyroid hormone slows muscle relaxation, giving a delayed return phase of the ankle reflex.
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