The S1 myotome is tested by ankle plantarflexion, the ISNCSCI key movement for S1, through the gastrocnemius and soleus. S1 also supplies foot eversion and hip extension. The Achilles reflex tests the same root.
| Nerve root | S1 |
|---|---|
| Area | Ankle plantarflexion (ISNCSCI key muscles: ankle plantarflexors) |
| How to test | The patient pushes the foot down against the examiner's hand on the sole, and then performs repeated single-leg heel raises to detect subtle weakness. Strength is graded 0-5 on the Medical Research Council scale and compared side to side. |
| Muscles |
|
| Nerve Supply | Tibial nerve; Superficial fibular nerve; Inferior gluteal nerve |
Points the foot down for push-off in walking and running, and helps turn the foot out and extend the hip.
Because the calf muscles are so strong, mild S1 weakness is easily missed on manual testing; repeated single-leg heel raises (often about 10 or more in healthy adults) are more sensitive. S1 radiculopathy, usually from an L5-S1 disc, also causes outer foot numbness and a reduced ankle reflex. Myotome charts vary slightly between sources, and most muscles receive more than one root, so a single root lesion usually weakens rather than paralyses.
Compression at L5-S1 causing weak plantarflexion, outer foot numbness and a reduced Achilles reflex.
Damage to the tibial nerve, for example in the popliteal fossa, weakens plantarflexion and causes sole numbness.
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