Myotomes

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Original Editor - Lucinda hampton Top Contributors - Lucinda hampton, Kristen Holeman, Kim Jackson, Nikhil Benhur Abburi, Candace Goh, Rachael Lowe, Joao Costa, Rucha Gadgil, Oyemi Sillo, Naomi O'Reilly, Shaimaa Eldib and Tony Lowe

Introduction

A myotome (Greek: myo = muscle; tome = a cut or slice) refers to the collection of muscle fibres innervated by motor axons within a single spinal nerve root. Embryologically, myotomes originate from somites derived from the paraxial mesoderm and contribute to the development of skeletal muscle. This segmental organisation provides the developmental basis for the relationship between spinal nerve roots and the muscles they innervate. Myotomes represent the motor component of the neurological examination and are commonly assessed through specific muscle actions and strength testing. In contrast, a dermatome represents an area of skin supplied predominantly by sensory fibres from a single spinal nerve root and is evaluated through sensory testing. Together, assessment of myotomes, dermatomes, and reflexes can provide information about the neurological integrity of a spinal nerve root and assist with localisation of neurological dysfunction.

Figure 1: Differentiation of somites into dermatome, sclerotome and myotome showing embryological development of myotomes

Anatomy

Figure 2: Spinal cord cross-section showing nerve root emergence and myotome innervation patterns

There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Although there are only seven cervical vertebrae, there are eight cervical spinal nerves; the C8 spinal nerve exits between the C7 and T1 vertebrae.

Motor innervation of skeletal muscle originates from motor neurons within the spinal cord, whose axons exit through the ventral roots and join the dorsal roots to form mixed spinal nerves. These spinal nerves subsequently divide into rami, with the ventral rami contributing to nerve plexuses that ultimately give rise to the peripheral nerves supplying the muscles of the limbs.

Within the nerve plexuses, fibres from adjacent spinal nerve roots intermingle and are redistributed into peripheral nerves. As a result, a peripheral nerve typically contains fibres originating from multiple spinal nerve roots, and an individual muscle may receive motor contributions from more than one spinal level. The relative contribution of each spinal nerve root to a muscle may differ, with one root often providing the predominant motor contribution.

Myotome Distribution

Myotomal distributions describe the patterns of skeletal muscle innervation associated with individual spinal nerve roots. Although traditionally represented as specific roots corresponding to individual muscles or movements, human myotomes demonstrate considerable overlap, and the distribution of motor innervation may vary between individuals.

Human myotomal distributions have been investigated using anatomical studies, clinical observations of nerve root lesions, electromyography, and direct electrical stimulation of spinal nerve roots. Direct intraoperative nerve root stimulation has demonstrated that individual spinal nerve roots may activate a broader range of muscles than traditionally represented in standard myotome maps. Similar findings have been demonstrated in the lumbosacral region, where broad motor distributions and individual and side-to-side variation in root contributions have been identified.

It is important to distinguish a myotomal pattern from a peripheral nerve distribution. A spinal nerve root contributes motor fibres to multiple peripheral nerves; therefore, muscles sharing the same predominant root innervation may be supplied by different peripheral nerves. Conversely, an individual peripheral nerve typically contains fibres originating from multiple spinal nerve roots. Assessment of weakness across muscles supplied by different peripheral nerves but sharing a common spinal nerve root can therefore assist in localising neurological dysfunction to the nerve root rather than to an individual peripheral nerve.

For example, the C5 spinal nerve root contributes to both the axillary nerve, which supplies the deltoid, and the musculocutaneous nerve, which supplies the biceps brachii. Consequently, C5 nerve root dysfunction may produce weakness in both shoulder abduction and elbow flexion despite these muscles being supplied by different peripheral nerves. This pattern can assist in distinguishing C5 nerve root involvement from an isolated axillary or musculocutaneous neuropathy.

Consequently, due to overlapping innervation and anatomical variation, clinical myotome maps should be interpreted as representations of predominant patterns of motor innervation rather than fixed anatomical territories.

ISNCSCI Standardized Key Muscle Functions

The International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI) identifies 10 bilateral key muscle functions for standardised motor examination. Each key muscle function is associated with a spinal cord segment and is tested using a defined movement and muscle group. These key muscle functions are used for neurological classification of spinal cord injury and should not be interpreted as exclusive anatomical myotomal distributions.

Table 1: Key Muscle Functions - ISNCSCI Standard

Spinal Nerve Root	Key Muscle Function	Representative Muscle(s)
C5	Elbow flexion	Biceps brachii Brachialis
C6	Wrist extension	Extensor carpi radialis longus/brevis
C7	Elbow extension	Triceps
C8	Finger flexion	Flexor digitorum profundus
T1	Small finger abductors	Abductor digiti minimi
L2	Hip flexion	Iliopsoas
L3	Knee extension	Quadriceps
L4	Ankle dorsiflexion	Tibialis anterior
L5	Long toe extension	Extensor hallucis longus
S1	Ankle plantarflexion	Gastrocnemius Soleus
S2-S4	Anal sphincter contraction

Clinical note: ISNCSCI key muscle functions provide a standardised method for assessing motor function following spinal cord injury. Because most muscles receive innervation from more than one spinal nerve root, these key muscle functions should not be considered exclusive root-to-muscle relationships.

Clinical Myotomal Distribution

The ISNCSCI key muscle functions provide a standardised approach to motor examination; however, they do not represent the full distribution of motor innervation associated with each spinal nerve root. Studies using clinical observations, electromyography, and direct intraoperative nerve root stimulation demonstrate considerable overlap between adjacent myotomes, with individual spinal nerve roots contributing to multiple muscles and movements.

The following tables summarise commonly recognised and predominant myotomal relationships for the upper and lower limbs. These distributions should be used as a clinical guide rather than interpreted as fixed anatomical boundaries.

Table 2: Cervical and Upper Limb Myotomal Distribution
Spinal Nerve Root	Commonly Associated Movement(s)	Representative Muscle(s)	Peripheral Nerve(s)
C5	Shoulder abduction Elbow flexion	Deltoid Biceps brachii	Axillary Musculocutaneous
C6	Elbow flexion Wrist extension Forearm pronation	Biceps brachii Brachioradialis Exensor carpi radialis longus/brevis Pronator teres	Musculocutaneous Radial Median
C7	Elbow extension wrist flexion Finger extension	Triceps Flexor carpi radialis Finger extensors	Radial Median
C8	Finger flexion Thumb/finger extension	Flexor digitorum profundus Extensor pollicis longus Finger extensors	Median Ulnar Radial
T1	Finger abduction/adduction	Interossei Abductor digiti minimi	Ulnar

Table 3: Lumbosacral and Lower Limb Myotomal Distribution
Spinal Nerve Root	Commonly Associated Movement(s)	Representative Muscle(s)	Peripheral Nerve(s)
L2	Hip flexion	Iliopsoas	Femoral/direct lumbar plexus branches
L3	Knee extension Hip adduction	Quadriceps Adductors	Femoral Obturator
L4	Knee extension Ankle dorsiflexion	Quadriceps Tibialis anterior	Femoral Deep fibular
L5	Great toe extension Ankle dorsiflexion Hip abduction	Extensor hallucis longus Tibialis anterior Gluteus medius	Deep fibular Deep fibular Superior gluteal
S1	Ankle plantarflexion Ankle eversion	Gastrocnemius Soleus Fibularis longus/brevis	Tibial Superficial fibular
S2	Knee flexion	Hamstrings	Sciatic

Clinical interpretation: Weakness involving several muscles supplied by different peripheral nerves but sharing a common spinal nerve root may be more suggestive of a radicular pattern than weakness confined to the distribution of a single peripheral nerve. Significant overlap and individual variation exist; therefore, findings should be interpreted alongside sensory examination, reflexes, and other clinical findings rather than used in isolation.

Assessment

Purpose

Myotomal strength is typically assessed through isometric resisted muscle testing with findings used to assist in identifying the presence and potential level of nerve root involvement. Assessment of multiple muscles can also assist with neurological localisation. Weakness affecting muscles supplied by different peripheral nerves but sharing a common spinal nerve root may support a radicular pattern, whereas weakness limited to muscles supplied by a single peripheral nerve may suggest a more distal peripheral nerve lesion.

Myotomal testing should not be interpreted in isolation. Motor testing has demonstrated variable diagnostic accuracy and generally limited sensitivity for detecting radiculopathy; therefore, normal muscle strength does not necessarily exclude nerve root involvement. Findings should be interpreted alongside the patient's history and other components of the neurological examination, including sensory testing and reflexes. Muscle strength may be documented using a standardised muscle strength grading scale.

Medical Research Council (MRC) Muscle Strength Grading Scale

Table 4: MRC Muscle Strength Scale
Grade	Description
0	No contraction
1	Flicker or trace of contraction
2	Active movement with gravity eliminated
3	Active movement against gravity
4	Active movement against gravity and resistance
5	Normal power

Technique

Myotomal testing is commonly performed using resisted isometric muscle testing. The patient is positioned to isolate the movement being assessed and instructed to hold the position while the examiner applies resistance. One commonly described clinical approach uses resisted isometric contraction maintained for approximately 5 seconds, while assessing for weakness and comparing the response with the contralateral side.

Testing should be performed bilaterally, simultaneously using consistent positioning, stabilisation, and resistance. The examiner should assess for differences in strength between sides and consider whether weakness follows a pattern consistent with involvement of a spinal nerve root. Muscle strength may be documented using the Medical Research Council (MRC) muscle strength scale as listed above in Table 4.

Table 5 includes common Myotomal Testing Techniques used in current literature:

Table 5: Myotomal Testing Techniques (Adapted from )
Root	Test Movement	Patient Testing Position Anti-Gravity	Patient Testing Position Gravity Eliminated	Resistance/Examiner Action (Anti-Gravity)
C5	Shoulder abduction	Sitting	Sidelying	Examiner applies downward force proximal to elbow into adduction
C6	Elbow flexion & wrist extension	Sitting (both)	Elbow flexion: Sidelying Wrist extension: Sitting	Elbow flexion: Examiner applies resistance at wrist to extend the elbow Wrist extension: Examiner applies resistance on dorsum of hand into wrist flexion.
C7	Elbow extension & wrist flexion	Sitting (both)	Elbow extension: Sidelying Wrist flexion: Sitting	Elbow extension: Examiner applies resistance at wrist to flex the elbow. Wrist flexion: Examiner applies resistance on palm of hand into wrist extension.
C8	Finger flexion	Sitting	Sitting	Examiner applies resistance to distal phalanx into extension
T1	Finger abduction	Sitting	Sitting	Examiner applies resistance into finger adduction
L2	Hip flexion	Sitting	Sidelying	Patient flexes hip in sitting while examiner applies resistance into hip extension
L3	Knee extension	Sitting	Sidelying	Patient extends their knee 20 degrees from full extension and examiner applies resistance into flexion
L4	Ankle dorsiflexion	Sitting	Sidelying	Patient dorsiflexes their ankle while examiner applies resistance into plantar flexion
L5	Great toe extension	Sitting or Supine		Examiner applies resistance at distal phalanx into great toe flexion
S1	Ankle plantarflexion and eversion	Ankle plantarflexion: Prone or standing	Sidelying (both)	Ankle plantarflexion: Examiner applies resistance into ankle dorsiflexion
S2	Knee flexion	Prone	Sidelying	Patient flexes the knee to 90 degrees and examiner applies resistance into knee extension
S2-4	Anal sphincter contraction	Prone	Prone	The examiner inserts a gloved finger into the patient's rectum and graded as being present or absent. The instruction to the patient should be “squeeze my finger as if to hold back a bowel movement"

Repeated Testing

A single strength assessment may not identify all motor deficits associated with nerve root dysfunction. Emerging evidence suggests that repeated muscle contractions may reveal fatigable weakness that is not apparent during initial strength testing, potentially providing additional information when nerve root involvement is suspected despite normal or near-normal strength. However, evidence supporting repeated or endurance-based myotomal testing remains limited, and this approach has not been established as a standardised component of the neurological examination.

Diagnostic Accuracy

The diagnostic performance of myotomal testing varies depending on the spinal level assessed, testing procedure, and reference standard. Overall, motor testing appears to demonstrate limited sensitivity, indicating that normal muscle strength does not reliably exclude nerve root involvement.

In patients with surgically verified single-level cervical radiculopathy, objective motor weakness was identified in 48.5% of patients. Among patients demonstrating motor weakness, approximately two-thirds followed the expected standard myotomal pattern, with variation observed between cervical nerve root levels. Notably, greater variability was observed with C5/6 level (C6 root) radiculopathy. These findings demonstrate that motor weakness may assist with localisation but does not always correspond to traditional myotomal distributions.

The diagnostic accuracy of the neurological examination may improve when motor findings are considered alongside other neurological findings. In cervical radiculopathy, combinations of muscle weakness, sensory loss, and tendon reflex abnormalities have demonstrated higher specificity than some individual examination components, although sensitivity remains variable. For lumbosacral radiculopathy, motor testing has demonstrated poor-to-moderate sensitivity, ranging from 0.13 to 0.61 across included studies. Therefore, myotomal findings are best interpreted as one component of the broader neurological examination rather than as an independent diagnostic test.

Clinical relevance

Myotomal assessment contributes to the evaluation of patients with possible neurological involvement and can help clinicians recognise patterns of motor weakness and localise potential dysfunction within the nervous system. When a weakness occurs across muscles innervated by different peripheral nerves but shares a common spinal root, the pattern can help assist clinicians in determining and distinguishing a radiculopathy from a peripheral mononeuropathy.

Myotomal findings may also contribute in identifying the potential level of nerve root involvement and determining the need for further investigation or referral. However, patterns of weakness do not always correspond precisely with traditional myotome maps because of overlapping innervation and individual anatomical variation. As a result, myotomal findings should be interpreted in conjunction with a battery of tests including patient's history, sensory examination, reflexes, and other relevant clinical findings rather than used independently to establish a diagnosis.