L4 dermatome and nerve root

Lumbar spinal root

Where is the L4 dermatome?

The L4 dermatome covers the inner shin, the inner ankle and the inner edge of the foot. Its ISNCSCI key sensory point is the medial malleolus, the inner ankle bone. Its key movement is ankle dorsiflexion (tibialis anterior). The L3/L4 disc is the one that usually compresses L4, or the L4/L5 disc when a herniation is far-lateral.

L4 at a glance
Type	Lumbar spinal root
Key sensory point	The medial malleolus, the inner ankle bone
Key movement	Ankle dorsiflexion
Tendon reflex	Knee jerk
Usual disc	L3/L4, or a far-lateral L4/L5

Try in: Dermatome and Myotome Map Explorer →

L4 myotome: the muscles it supplies

L4 is a major root for 3 of the 14 muscles below and a minor root for the rest.

Muscles L4 supplies, and every root each takes
Muscle	From L4	All its roots
Quadriceps	Major	L3 and L4; L2 in part
Adductor magnus	Major	L3 and L4; L2 in part
Tibialis anterior	Major, ISNCSCI key muscle	L4; L5 in part
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
Adductor longus and brevis	Minor	L2 and L3; L4 in part
Gracilis	Minor	L2 and L3; L4 in part
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
Hamstrings	Minor	L5 and S1; L4 and S2 in part
Extensor hallucis longus	Minor	L5; L4 and S1 in part
Extensor digitorum longus	Minor	L5; L4 and S1 in part

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

Tendon reflexes through L4

Tendon reflexes L4 carries, how each is tested and its roots
Reflex	Role of L4	Tap	Response	Roots
Knee jerk	Main root	The patellar tendon below the kneecap, with the leg hanging	The knee straightens with a kick	L2, L3 and L4, mainly L4

Why an L3/L4 disc compresses L4

In the lumbar spine each root leaves below its own vertebra, high in the foramen and above the disc, so a typical posterolateral L3/L4 herniation misses the exiting L3 root and catches L4, 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.

A far-lateral herniation pushes out into or beyond the foramen, where the exiting L4 root lies, so a far-lateral L4/L5 disc catches L4 rather than the root a typical one would.

What an L4 root lesion does

Sensation. Numbness or tingling in the L4 band. Usually a smaller patch than the whole band, because L3 and L5 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Ankle dorsiflexion, the standard’s key movement for L4 (tibialis anterior). Also weaker where L4 is a major supply: quadriceps and adductor magnus.
Reflexes. The knee 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 L4

L4, down on all fours: the skin just below the kneecap that you kneel on, and the inner shin.

Neighboring dermatomes

L3: the lower front of the thigh and the inner knee
L5: the outer shin, the top of the foot and the big toe

Where published maps disagree

The top of the big toe is L5 in some maps and L4 in others.
The inner sole is given to L4, L5 or S1, depending on the map.

Dermatome and Myotome Map Explorer, with L4 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.