Innervation of the Muscles
in the Lower Extremity
Grading || Innervation and Function || Overview ||
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References and Resources ||
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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:
- 0/5 indicates no movement apparent (not even a "flicker" of muscle activity)
- 1/5 indicates muscle contraction that can be felt by the examiner, but is not sufficient to move the joint
- 2/5 indicates the muscle can move the joint, but only when gravity is eliminated
- 3/5 indicates that the muscle can move the joint against gravity, but not against resistance
- 4/5 (and its shades 4-/5 and 4+/5) indicates the patient can provide some resistance against the examiner, but is not full strength
- 5/5 indicates full strength
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) |
Adductor longus |
Hip adduction |
L4 |
Deep peroneal (L4, L5, S1) |
Tibialis anterior |
Ankle dorsiflexion |
L5 |
Superficial peroneal (L4, L5) |
Peroneus longus |
Foot eversion |
S1 |
Tibial (L5, S1) |
Flexor hallucis longus |
Big toe 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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