L3 dermatome and nerve root

Lumbar spinal root

Where is the L3 dermatome?

The L3 dermatome covers the lower front of the thigh and the inner knee. Its ISNCSCI key sensory point is the medial femoral condyle, just above the knee. Its key movement is knee extension (quadriceps). The L2/L3 disc is the one that usually compresses L3.

L3 at a glance
Type	Lumbar spinal root
Key sensory point	The medial femoral condyle, just above the knee
Key movement	Knee extension
Tendon reflex	Knee jerk, in part
Usual disc	L2/L3

Try in: Dermatome and Myotome Map Explorer →

L3 myotome: the muscles it supplies

L3 is a major root for 8 of the 10 muscles below and a minor root for the rest.

Muscles L3 supplies, and every root each takes
Muscle	From L3	All its roots
Quadratus lumborum	Major	T12 to L3
Iliopsoas	Major	L2 and L3; L1 and L4 in part
Sartorius	Major	L2 and L3
Quadriceps	Major, ISNCSCI key muscle	L3 and L4; L2 in part
Pectineus	Major	L2 and L3
Adductor longus and brevis	Major	L2 and L3; L4 in part
Gracilis	Major	L2 and L3; L4 in part
Adductor magnus	Major	L3 and L4; L2 in part
Erector spinae	Minor	T1 to L5 in part
Multifidus	Minor	T1 to L5 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 L3

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

Why an L2/L3 disc compresses L3

In the lumbar spine each root leaves below its own vertebra, high in the foramen and above the disc, so a typical posterolateral L2/L3 herniation misses the exiting L2 root and catches L3, 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 L3 root lies, so a far-lateral L3/L4 disc catches L3 rather than the root a typical one would.

What an L3 root lesion does

Sensation. Numbness or tingling in the L3 band. Usually a smaller patch than the whole band, because L2 and L4 overlap it: Sherrington found in 1893 that cutting a single root in monkeys left no skin entirely without feeling.
Strength. Knee extension, the standard’s key movement for L3 (quadriceps). Also weaker where L3 is a major supply: quadratus lumborum, iliopsoas, sartorius, pectineus, adductor longus and brevis, gracilis and adductor magnus.
Reflexes. The knee 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

L2: the front and outer thigh, running down and in
L4: the inner shin, the inner ankle and the inner edge of the foot

Dermatome and Myotome Map Explorer, with L3 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.