S2 dermatome and nerve root

Sacral spinal root

Where is the S2 dermatome?

The S2 dermatome covers a strip down the back of the thigh and calf, through the back of the knee. Its ISNCSCI key sensory point is the middle of the popliteal fossa, behind the knee. No disc compresses S2 on its own: it lies inside the sacrum, reached only by a large central disc higher up.

S2 at a glance
Type	Sacral spinal root
Key sensory point	The middle of the popliteal fossa, behind the knee
Key movement	None graded: the standard grades C5 to T1 and L2 to S1
Tendon reflex	Ankle jerk, in part
Usual disc	None on its own: inside the sacrum, reached only by a large central disc higher up

Try in: Dermatome and Myotome Map Explorer →

S2 myotome: the muscles it supplies

ISNCSCI grades no key movement for S2: it grades only C5 to T1 and L2 to S1.

S2 is a major root for 1 of the 7 muscles below and a minor root for the rest.

Muscles S2 supplies, and every root each takes
Muscle	From S2	All its roots
The small muscles of the foot	Major	S1 and S2
Gluteus maximus	Minor	L5 and S1; S2 in part
Hamstrings	Minor	L5 and S1; L4 and S2 in part
Gastrocnemius	Minor	S1; S2 in part
Soleus	Minor	S1; S2 in part
Pelvic floor muscles	Minor	S3 and S4; S2 in part
External anal sphincter	Minor	S4 and S5; S2 and S3 in part

Tendon reflexes through S2

Tendon reflexes S2 carries, how each is tested and its roots
Reflex	Role of S2	Tap	Response	Roots
Ankle jerk	In part	The Achilles tendon, with the foot held slightly up	The foot points down	S1 and S2, mainly S1

Why no disc compresses S2 on its own

The sacral roots run down inside the sacrum together, so a disc does not press on one alone. They carry sensation from the saddle area and supply the bladder, bowel and pelvic floor, which is why numbness between the legs with new bladder or bowel trouble is treated as an emergency: it can mean the whole bundle, the cauda equina, is compressed.

What an S2 root lesion does

Sensation. Numbness or tingling in the S2 band. Usually a smaller patch than the whole band, because S1 and S3 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where S2 is a major supply: the small muscles of the foot.
Reflexes. The ankle jerk may be reduced.

A teaching map. Root territories overlap and vary between people, and a root lesion is diagnosed by examination and imaging, not by a map. Numbness between the legs or around the anus, new bladder or bowel problems, or quickly spreading weakness need emergency care.

Neighboring dermatomes

S1: the outer edge and sole of the foot, the heel and the lower back of the calf
S3: a ring round the anus on the inner buttocks, at the fold under the buttock

Dermatome and Myotome Map Explorer, with S2 lit on a front and back body map
Every dermatome, with the disc levels and key movements side by side
Lumbar Spine and Pelvis Explorer, the lower back’s vertebrae and the sacrum in 3D
Lower Limb Muscles Explorer, the leg’s muscles in 3D
Reflex Arc Simulator, the loop a tendon reflex runs through the spinal cord
Muscles Worked by Exercise 3D Explorer, a whole body colored by root, in its root mode

Sources. Key sensory points and key movements: International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI), American Spinal Injury Association, 2019 revision. Muscles: Aids to the Examination of the Peripheral Nervous System, Medical Research Council and the Guarantors of Brain, 5th ed. (2010), cross-checked with Kendall et al. (2005). Reflex roots: Walker (1990), Clinical Methods, 3rd ed., Table 72.1, and StatPearls, “Physiology, Deep Tendon Reflexes”; the main root as the MSD Manual’s “How to Assess Reflexes” names it. Overlap: Sherrington (1893), Phil Trans R Soc Lond B 184:641-763.