The S2 myotome is commonly tested by knee flexion through the hamstrings, together with S1, and also supplies the long toe flexors and small muscles of the foot. S2 contributes to the pelvic floor and to parasympathetic control of the bladder. It is not one of the ISNCSCI key muscle levels.
| Nerve root | S2 |
|---|---|
| Area | Knee flexion (with S1); also toe flexion |
| How to test | The prone or seated patient bends the knee against resistance at the back of the ankle, and curls the toes against the examiner's fingers. Strength is graded 0-5 on the Medical Research Council scale and compared side to side. |
| Muscles | |
| Nerve Supply | Sciatic nerve (tibial division); Tibial nerve; Medial and lateral plantar nerves |
Bends the knee, curls the toes and helps control the pelvic floor and bladder.
Weak toe flexion or knee flexion with a normal ankle reflex may suggest an S2 problem, but isolated S2 root lesions are rare. Sacral root involvement with bladder, bowel or sexual symptoms should be treated as possible cauda equina syndrome. 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 of several sacral roots causing leg weakness, saddle numbness and bladder or bowel dysfunction.
Injury to the sciatic nerve, for example after hip surgery or posterior dislocation, weakens the hamstrings and all muscles below the knee.
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