L1 dermatome and nerve root

Lumbar spinal root

Where is the L1 dermatome?

The L1 dermatome covers the groin, just below the inguinal ligament, and the upper buttock. Its ISNCSCI key sensory point is halfway between the T12 and L2 points. The T12/L1 disc is the one that usually compresses L1, or the L1/L2 disc when a herniation is far-lateral.

L1 at a glance
Type	Lumbar spinal root
Key sensory point	Halfway between the T12 and L2 points
Key movement	None graded: the standard grades C5 to T1 and L2 to S1
Tendon reflex	None tests it on its own
Usual disc	T12/L1, or a far-lateral L1/L2

Try in: Dermatome and Myotome Map Explorer →

L1 myotome: the muscles it supplies

ISNCSCI grades no key movement for L1: it grades only C5 to T1 and L2 to S1.

L1 is a major root for 3 of the 6 muscles below and a minor root for the rest.

Muscles L1 supplies, and every root each takes
Muscle	From L1	All its roots
Internal oblique	Major	T7 to L1
Transversus abdominis	Major	T7 to L1
Quadratus lumborum	Major	T12 to L3
Erector spinae	Minor	T1 to L5 in part
Multifidus	Minor	T1 to L5 in part
Iliopsoas	Minor	L2 and L3; L1 and L4 in part

Erector spinae and multifidus: supplied segment by segment by the dorsal rami, so each root reaches only its own short stretch.

L1 and the tendon reflexes

No tendon reflex tests L1 on its own.

Why a far-lateral L1/L2 disc compresses L1

In the lumbar spine each root leaves below its own vertebra, high in the foramen and above the disc. A far-lateral herniation pushes out into or beyond the foramen, where the exiting L1 root lies, so a far-lateral L1/L2 disc catches L1 rather than the root a typical one would.

What an L1 root lesion does

Sensation. Numbness or tingling in the L1 band. Usually a smaller patch than the whole band, because T12 and L2 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Also weaker where L1 is a major supply: internal oblique, transversus abdominis and quadratus lumborum.
Reflexes. No tendon reflex depends on L1.

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

T12: the lower abdomen above the groin crease, and the flank above the hip
L2: the front and outer thigh, running down and in

Dermatome and Myotome Map Explorer, with L1 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
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.