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Innervation of the Muscles
in the Lower Extremity

Grading || Innervation and Function || Overview || Related Articles
References and Resources || Leave a Comment

Grading Strength

A thorough lower extremity motor examination can be done quickly and provides a significant amount of information to the clinician.

The universal way to grade strength is on a scale from 0 to 5:

Side note: you will frequently see people write 5-/5 in the chart... This is a huge pet peeve! The patient is either full strength (ie: 5/5) or weak; if you think the patient has minor weakness document it as 4+/5, not 5-/5!

Innervation and Function

I break the motor examination into three components: the muscle being tested, the primary spinal level served by that muscle (ie: myotome), and the peripheral nerve that innervates the muscle. Remember that muscles are usually innervated by more than one spinal level (ie: myotomes overlap); this is why injury to a single root rarely causes full paralysis.

The table below gives the major spinal level (myotome) that innervates the specified muscle(s); note that each peripheral nerve receives axons from several spinal levels (given in parenthesis next to nerve), but usually one of these levels provides the dominant supply of innervation to a given muscle!

Spinal Level Peripheral Nerve Muscle Function
L2
Femoral (L2, L3, L4)
Iliopsoas
Hip flexion
L3
Obturator (L2, L3, L4)
Femoral (L2, L3, L4)
Adductor longus
Vastus medialis/lateralis (quadriceps)
Hip adduction
Knee extension
L4
Deep peroneal (L4, L5, S1)
Sciatic (L4, L5, S1)
Tibialis anterior
Biceps femoris (hamstring)
Ankle dorsiflexion
Knee flexion
L5
Superficial peroneal (L4, L5)
Deep peroneal (L4, L5, S1)
Sciatic (L4, L5, S1)
Peroneus longus
Extensor hallucis longus
Biceps femoris (hamstring)
Foot eversion
Big toe extension
Knee flexion
S1
Tibial (L5, S1)
Tibial (L5, S1)
Inferior gluteal nerve (L5, S1, S2)
Sciatic (L4, L5, S1)
Flexor hallucis longus
Gastrocnemius
Gluteus maximus
Biceps femoris (hamstring)
Big toe flexion
Plantarflexion
Hip extension
Knee flexion
S2-S4
Pudendal (S2, S3, S4)
Anal sphincter
Fecal continence

In general, hip flexion (iliopsoas) is the best way to test L2 nerve root function. Knee extension (quadriceps, which are composed of the vastus muscles) is the best way to test L3 nerve root function. Ankle dorsiflexion (predominately anterior tibialis) is the best way to test L4 nerve root function. Large toe extension (extensor hallucis longus) is the best way to test L5 function and plantarflexion (gastrocnemius) is the best way to test S1 function.

Overview

The muscles of the lower extremity can be grouped by the spinal level that innervates them. Each muscle has a "dominant" spinal level, but also receives input from adjacent levels (the myotomes overlap slightly). Understanding the innervation of the muscles in the lower extremity can help the clinician elucidate the cause of weakness.

References and Resources

(1) Phillips LH, Park TS. Electrophysiologic mapping of the segmental anatomy of the muscles of the lower extremity. Muscle and Nerve. Volume 14, Issue 12. Dec, 1991.

(2) Liguori R, Krarup C, Trojaborg W. Determination of the segmental sensory and motor innervation of the lumbosacral spinal nerves: an electrophysiological study. Brain (1992).

(3) Baehr M, Frotscher M. Duus' Topical Diagnosis in Neurology: Anatomy, Physiology, Signs, Symptoms. Fourth Edition. Stuttgart: Thieme, 2005.

(4) Bickley LS, Szilagyi PG. Bates' Guide to Physical Examination and History Taking. Ninth Edition. New York: Lippincott Williams and Wilkins, 2007.

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