S1 dermatome and nerve root

Sacral spinal root

Where is the S1 dermatome?

The S1 dermatome covers the outer edge and sole of the foot, the heel and the lower back of the calf. Its ISNCSCI key sensory point is the outer side of the heel. Its key movement is ankle plantarflexion (gastrocnemius and soleus). The L5/S1 disc is the one that usually compresses S1.

S1 at a glance
Type	Sacral spinal root
Key sensory point	The outer side of the heel
Key movement	Ankle plantarflexion
Tendon reflex	Ankle jerk
Usual disc	L5/S1

Try in: Dermatome and Myotome Map Explorer →

S1 myotome: the muscles it supplies

S1 is a major root for 6 of the 11 muscles below and a minor root for the rest.

Muscles S1 supplies, and every root each takes
Muscle	From S1	All its roots
Gluteus maximus	Major	L5 and S1; S2 in part
Hamstrings	Major	L5 and S1; L4 and S2 in part
Fibularis longus	Major	L5 and S1
Gastrocnemius	Major, ISNCSCI key muscle	S1; S2 in part
Soleus	Major, ISNCSCI key muscle	S1; S2 in part
The small muscles of the foot	Major	S1 and S2
Gluteus medius	Minor	L5; L4 and S1 in part
Gluteus minimus	Minor	L5; L4 and S1 in part
Tensor fasciae latae	Minor	L5; L4 and S1 in part
Extensor hallucis longus	Minor	L5; L4 and S1 in part
Extensor digitorum longus	Minor	L5; L4 and S1 in part

Tendon reflexes through S1

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

Why an L5/S1 disc compresses S1

In the lumbar spine each root leaves below its own vertebra, high in the foramen and above the disc, so a typical posterolateral L5/S1 herniation misses the exiting L5 root and catches S1, which crosses the back of that disc on its way down to the next foramen. The level of the disc and the root therefore differ by one.

What an S1 root lesion does

Sensation. Numbness or tingling in the S1 band. Usually a smaller patch than the whole band, because L5 and S2 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Ankle plantarflexion, the standard’s key movement for S1 (gastrocnemius and soleus). Also weaker where S1 is a major supply: gluteus maximus, hamstrings, fibularis longus and the small muscles of the foot.
Reflexes. The ankle jerk is reduced or absent.

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.

A way to remember S1

S1: you stand on it, the sole and the heel.

Neighboring dermatomes

L5: the outer shin, the top of the foot and the big toe
S2: a strip down the back of the thigh and calf, through the back of the knee

Where published maps disagree

The inner sole is given to L4, L5 or S1, depending on the map.

Dermatome and Myotome Map Explorer, with S1 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.