Myotomes Complete Guide: Motor Testing by Spinal Nerve Level

A myotome is the group of muscles innervated by a single spinal nerve root. Myotomes are to motor function what dermatomes are to sensation — a map linking spinal cord levels to specific movements. A focused myotome examination takes 60 seconds and can localize lesions to specific spinal segments with clinical precision that far exceeds what strength testing of individual muscles provides. Clinical value: when a patient presents with motor weakness, pinpointing which myotome is affected narrows the lesion location. Weakness of elbow flexion (C5-C6) with preserved elbow extension (C7) and finger abduction (T1) suggests an upper trunk brachial plexus injury or a C5-C6 radiculopathy, not a distal nerve or muscle problem. A lesion at C7 would weaken elbow extension while sparing C5-C6 function above and T1 function below. Myotome vs individual nerve: a single muscle often receives contributions from multiple spinal levels because peripheral nerves (like the median, ulnar, sciatic) carry fibers from several roots. The myotome groups muscles by their primary spinal level. This is why myotome testing can localize root lesions when isolated nerve testing might be misleading. This content is for educational purposes only and does not constitute medical advice.

The upper limb has 6 testable myotome levels. Each has a characteristic signature movement. C5 — Shoulder abduction - Test: patient abducts arm to 90 degrees against resistance. Examiner pushes down on the mid-humerus. - Key muscle: deltoid (middle fibers). Also supraspinatus (first 15 degrees). - Weak C5: shoulder abduction weakness. Patient may shrug the shoulder to compensate. - Clinical relevance: classic Erb palsy (C5-C6 injury from obstetric shoulder dystocia or traumatic stretching) produces the 'waiter's tip' posture — shoulder adducted, elbow extended, forearm pronated. C6 — Elbow flexion and wrist extension - Test (flexion): patient flexes elbow to 90 degrees against resistance; examiner pulls down on wrist. - Test (wrist extension): patient extends wrist against resistance. - Key muscles: biceps brachii (flexion, with brachialis), extensor carpi radialis longus and brevis (wrist extension). - C6 signature: 'make a muscle' movement plus wrist extension. - Clinical relevance: C6 radiculopathy produces biceps weakness and absent biceps reflex plus weakened wrist extension. C7 — Elbow extension, wrist flexion, finger extension - Test (elbow extension): patient extends elbow against resistance. - Test (wrist flexion): patient flexes wrist against resistance. - Test (finger extension): patient extends MCP joints against resistance. - Key muscles: triceps (elbow extension), flexor carpi radialis (wrist flexion), extensor digitorum (finger extension). - Clinical relevance: C7 is the most common cervical radiculopathy. Produces triceps weakness, absent triceps reflex, and finger drop if severe. C8 — Finger flexion - Test: patient flexes DIP joints of index and middle fingers against resistance (try to pull them straight). - Key muscle: flexor digitorum profundus (FDP). Also flexor digitorum superficialis. - Clinical relevance: C8 lesions produce weak grip. Klumpke palsy (C8-T1 avulsion from excessive arm abduction) includes finger flexion weakness plus intrinsic hand weakness. T1 — Finger abduction - Test: patient spreads fingers against resistance. Examiner squeezes fingers together. - Key muscles: dorsal interossei (abduct fingers). Also palmar interossei (adduct fingers) and abductor digiti minimi. - Clinical relevance: T1 is the classic intrinsic hand myotome. Klumpke palsy disables intrinsic function, producing claw hand deformity. Mnemonic for upper limb myotomes — 'Shoulder 5 flex 6 ex 7 grip 8 one' (C5 shoulder, C6 flex elbow, C7 extend elbow, C8 grip/flex fingers, T1 splay fingers).

The lower limb has 5 testable myotome levels. Each has a characteristic signature movement. L2 — Hip flexion - Test: patient lifts thigh off exam table against resistance. - Key muscle: iliopsoas (iliacus + psoas major). - Weak L2: difficulty climbing stairs, Trendelenburg-like gait abnormality. - Clinical relevance: L2 radiculopathy produces proximal leg weakness and decreased or absent cremasteric reflex in males. L3 — Knee extension - Test: patient extends knee from flexed position against resistance; examiner tries to flex the knee. - Key muscle: quadriceps femoris (rectus femoris, vastus medialis/intermedius/lateralis). - Weak L3: difficulty rising from chair or climbing stairs; knee buckles. - Clinical relevance: L3-L4 radiculopathy produces quadriceps weakness and decreased patellar reflex (L4 more commonly than L3). L4 — Ankle dorsiflexion - Test: patient pulls toes toward shin against resistance. - Key muscle: tibialis anterior. Also peroneus tertius, extensor hallucis longus (although EHL is primarily L5). - Weak L4: foot drop when walking. - Clinical relevance: L4 radiculopathy produces ankle dorsiflexion weakness and decreased patellar reflex. Distinguish from L5 radiculopathy by the specific muscle involved — L4 is tibialis anterior (main dorsiflexor); L5 is EHL (big toe extension). L5 — Great toe extension - Test: patient extends big toe against resistance. - Key muscle: extensor hallucis longus (EHL). Also tibialis anterior (L4-L5 overlap) and peroneus longus (lateral eversion). - Weak L5: classic sign of L5 radiculopathy. 'Can you hold your big toe up against my pressure?' - Clinical relevance: L5 is the most common lumbar radiculopathy. Produces weak EHL, weak ankle inversion, and sensory loss over the first webspace. S1 — Ankle plantar flexion - Test: patient pushes foot downward against resistance, or more sensitively, walks on tiptoe or hops on one leg. - Key muscle: gastrocnemius and soleus (triceps surae). - Weak S1: patient cannot stand on tiptoes on affected side. - Clinical relevance: S1 radiculopathy produces plantar flexion weakness, absent or decreased Achilles reflex, and sensory loss over posterior calf and lateral foot. S2 — Knee flexion - Test: patient flexes knee against resistance (examiner tries to straighten the knee). - Key muscle: hamstrings (biceps femoris, semitendinosus, semimembranosus). - Less commonly tested because weakness is subtle and other levels dominate. - Clinical relevance: S2 is also important for toe flexion and anal sphincter tone. Mnemonic for lower limb myotomes — 'L2 kick L3 kick L4 toe L5 toe S1 push S2 flex' (L2 hip kick/flex, L3 knee kick/extend, L4 ankle toe-to-shin, L5 big toe up, S1 push foot down, S2 knee heel-to-butt). For board exams, the most frequently-tested lower limb myotomes are L4 (dorsiflexion — tibialis anterior), L5 (big toe extension — EHL), and S1 (plantar flexion — gastrocnemius).

Deep tendon reflexes correlate with specific myotome levels. A diminished or absent reflex supports the diagnosis of a radiculopathy at the corresponding level. Upper limb reflexes: - Biceps reflex: C5-C6 (especially C5) - Brachioradialis reflex: C5-C6 - Triceps reflex: C7-C8 (especially C7) - Finger jerk (Hoffman sign): C7-T1 Lower limb reflexes: - Cremasteric reflex: L1-L2 - Patellar (quadriceps) reflex: L2-L4 (especially L4) - Medial hamstring reflex: L5-S1 - Achilles (ankle) reflex: S1-S2 (especially S1) - Babinski: cortical/pyramidal — not a myotome reflex specifically, but upgoing toe suggests upper motor neuron pathology Clinical correlation: if a patient has absent biceps reflex with weakness in shoulder abduction and elbow flexion, this is highly suggestive of a C5 radiculopathy. If the patient has absent Achilles reflex with plantar flexion weakness, S1 radiculopathy is likely. Reflex findings complement motor testing for localization.

Understanding myotomes unlocks the ability to distinguish different types of motor weakness. Radiculopathy (single nerve root compression): - Weakness confined to one myotome - Absent reflex at that level - Sensory loss in corresponding dermatome - Common causes: disc herniation, foraminal stenosis, nerve root inflammation - Example: L5 radiculopathy = EHL weakness, sensory loss in first webspace, reflex usually normal (L4 and S1 reflexes preserved) Plexopathy (brachial or lumbosacral plexus injury): - Weakness across multiple myotomes - Distribution depends on which part of plexus is affected - Example: upper trunk brachial plexus injury (Erb palsy) affects C5-C6 → shoulder abduction weak, elbow flexion weak, wrist extension weak - Example: lower trunk brachial plexus injury (Klumpke) affects C8-T1 → finger flexion weak, intrinsic hand muscles weak - Sensory loss follows the affected plexus distribution Myelopathy (spinal cord itself): - Produces long-tract signs: upper motor neuron weakness below the lesion (hyperreflexia, spasticity, Babinski) - Plus local radiculopathy at the lesion level (LMN signs, weakness in that myotome) - Sensory level: characteristic finding where sensation is normal above a certain dermatome and impaired below - Example: C7 myelopathy = weakness in C7 muscles (triceps, etc.), absent C7 reflex (triceps), but hyperreflexia in hamstrings, quads, gastrocnemius below the lesion Peripheral neuropathy: - Weakness in distribution of specific peripheral nerve (not myotome) - Distal-predominant pattern typical - Example: ulnar neuropathy at elbow → weakness in intrinsic hand muscles (interossei, hypothenar), sensory loss in ulnar distribution. Crosses multiple myotomes but respects peripheral nerve. Distinguishing a brachial plexus injury from a root lesion: plexopathy affects multiple roots (usually clustered) and typically has a specific distribution; radiculopathy affects one root. Electrodiagnostic studies (EMG/NCS) are the gold standard for distinguishing these when clinical examination is ambiguous.

For a focused motor screen that covers all major myotomes: Step 1 (C5): 'Lift your arms out to the side like you're flying, and hold them there as I try to push them down.' Test both sides simultaneously. Step 2 (C6): 'Bend your elbows to 90 degrees like you're going to make a muscle, and don't let me straighten them.' Then 'pull your wrists up toward the ceiling.' Step 3 (C7): 'Push my hands away from your body.' (triceps). 'Spread your fingers apart.' (partially tests T1 too). Step 4 (C8): 'Make a tight fist and squeeze my fingers as hard as you can.' Step 5 (T1): 'Spread your fingers wide apart and hold them while I try to squeeze them together.' Step 6 (L2): 'Lift your thigh off the table without using your hands.' Step 7 (L3): 'Straighten your knee against my resistance.' Step 8 (L4): 'Pull your toes up toward your shin.' Or test by walking on heels — L4 weakness causes heel-walking difficulty. Step 9 (L5): 'Lift your big toe up against my pressure.' — The most sensitive test for L5 radiculopathy. Step 10 (S1): 'Push your foot down against my hand.' Or test by walking on tiptoes. The entire screen takes 60 seconds for a cooperative patient and provides a snapshot that covers every major spinal level. A focused examination based on history can then dig deeper into specific areas. Asymmetric weakness is usually more clinically significant than symmetric weakness. Bilateral, symmetric weakness has a broader differential (neuromuscular disease, systemic illness, generalized weakness) compared to asymmetric weakness which points toward a focal lesion.

Pitfall 1: isolated muscle testing missing myotome logic. Testing only biceps strength tells you the muscle is weak but not the root. Testing C5 (deltoid), C6 (biceps), and C7 (triceps) in sequence quickly localizes. Think in myotomes, not individual muscles. Pitfall 2: ignoring pain on effort. A patient who gives inconsistent strength due to pain may have a radiculopathy with pain-related inhibition, not true weakness. Distinguish by asking about pain location and timing. Pitfall 3: forgetting bilateral comparison. Always test both sides. Subtle unilateral weakness is often missed without direct comparison. Pitfall 4: confusing wrist drop (radial nerve) with C7 radiculopathy. Both present with impaired wrist and finger extension. Distinguish by: radial nerve palsy has weak triceps, brachioradialis spared (reflex intact) if injury is distal, and specific distribution. C7 radiculopathy has weak triceps, brachioradialis intact, but also involves elbow extension and finger extension in myotome pattern. Pitfall 5: foot drop source ambiguity. Foot drop can be L4-L5 radiculopathy (weak tibialis anterior + EHL) or common peroneal neuropathy (trauma at fibular neck). Distinguish by: L5 radiculopathy has back pain + sensory loss along medial leg + possible S1 involvement; peroneal neuropathy has isolated foot drop without back pain and sensory loss over dorsum of foot. Pitfall 6: assuming normal reflexes rule out radiculopathy. Not all radiculopathies show absent reflexes. C5, T1, L5, and S2 radiculopathies may have preserved reflexes because those segments don't correspond to a well-tested deep tendon reflex. Pitfall 7: mistaking general deconditioning for myotome weakness. A deconditioned patient may have diffuse mild weakness. True myotome weakness is focal, specific, and disproportionate compared to surrounding muscle groups. Pearl 1: the three highest-yield myotomes to test efficiently are C7 (triceps — most common cervical radiculopathy), L5 (EHL — most common lumbar radiculopathy), and C5 (deltoid — most visible proximal upper limb weakness). Pearl 2: symmetric weakness of a specific myotome should raise suspicion for myelopathy or neuromuscular disease. A single myotome affected bilaterally suggests central spinal cord involvement. Pearl 3: subtle weakness is often revealed by functional testing. Ask a patient to walk on heels (L4), walk on tiptoes (S1), rise from a squat (L3-L4), or climb stairs (L2-L4) rather than relying only on resistance testing. Pearl 4: unmyelinated dermatome sensory loss without motor findings is common in radiculopathy. Sensory findings often lead motor findings by weeks to months. Conversely, motor weakness without sensory loss is unusual for radiculopathy — consider alternative diagnoses (motor neuron disease, neuromuscular junction disorders).