A 51-year-old woman comes to the physician because of progressively worsening lower back pain. The pain radiates down the right leg to the lateral side of the foot. She has had no trauma, urinary incontinence, or fever. An MRI of the lumbar spine shows disc degeneration and herniation at the level of L5–S1. Which of the following is the most likely finding on physical examination?

Exaggerated patellar tendon reflex

Diminished sensation of the anus and genitalia

Diminished sensation of the anterior lateral thigh

A 29-year-old woman presents to the primary care office for a recent history of falls. She has fallen 5 times over the last year. These falls are not associated with any preceding symptoms; she specifically denies dizziness, lightheadedness, or visual changes. However, she has started noticing that both of her legs feel weak. She's also noticed that her carpet feels strange beneath her bare feet. Her mother and grandmother have a history of similar problems. On physical exam, she has notable leg and foot muscular atrophy and 4/5 strength throughout her bilateral lower extremities. Sensation to light touch and pinprick is decreased up to the mid-calf. Ankle jerk reflex is absent bilaterally. Which of the following is the next best diagnostic test for this patient?

Electromyography (including nerve conduction studies)

A 48-year-old male presents to his primary care provider with a two-week history of low back pain and left leg pain. He reports that his symptoms started while he was working at his job as a construction worker. He has since experienced intermittent achy pain over his lumbar spine. He has also noticed pain radiating into his left leg and weakness in left ankle dorsiflexion. On exam, he demonstrates the following findings on strength testing of the left leg: 5/5 in knee extension, 4/5 in ankle dorsiflexion, 4/5 in great toe extension, 5/5 in ankle plantarflexion, and 5/5 in great toe flexion. The patellar reflexes are 5/5 bilaterally. He is able to toe walk but has difficulty with heel walking. Weakness in which of the following compartments of the leg is most likely causing this patient’s foot drop?

Superficial posterior compartment

A 35-year-old man is brought to the trauma bay by ambulance after sustaining a gunshot wound to the right arm. The patient is in excruciating pain and states that he can't move or feel his hand. The patient states that he has no other medical conditions. On exam, the patient's temperature is 98.4°F (36.9°C), blood pressure is 140/86 mmHg, pulse is 112/min, and respirations are 14/min. The patient is alert and his Glasgow coma scale is 15. On exam, he has a single wound on his right forearm without continued bleeding. The patient has preserved motor and sensation in his right elbow; however, he is unable to extend his wrist or extend his fingers further. He is able to clench his hand, but this is limited by pain. On sensory exam, the patient has no sensation to the first dorsal web space but has preserved sensation on most of the volar surface. Which of the following structures is most likely injured?

Recurrent motor branch of the median nerve

Motor testing of myotomes for USMLE Step 1: High-Yield Anatomy Lessons

Myotomes - Muscle Maps

Myotome Chart with Motor Actions and Spinal Nerve Roots

Upper Limb
- C5: Shoulder abduction (Deltoid)
- C6: Elbow flexion & Wrist extension (Biceps)
- C7: Elbow extension & Wrist flexion (Triceps)
- C8: Finger flexion (FDP)
- T1: Finger abduction (Interossei)
Lower Limb
- L2: Hip flexion (Iliopsoas)
- L3: Knee extension (Quadriceps)
- L4: Ankle dorsiflexion (Tibialis anterior)
- L5: Great toe extension (EHL)
- S1: Ankle plantarflexion (Gastrocnemius)

⭐ Foot drop (weak dorsiflexion) classically points to an L4/L5 lesion, while difficulty walking on tiptoes suggests an S1 lesion.

Upper Limb Testing - Arm Actions

Motor tests for upper limb myotomes help localize nerve root lesions. Each level corresponds to a primary muscle action, crucial for clinical examination.

Nerve Root	Action Tested	Key Muscles Involved
C5	Shoulder Abduction	Deltoid, Supraspinatus
C6	Elbow Flexion, Wrist Extension	Biceps, Brachioradialis
C7	Elbow Extension, Wrist Flexion	Triceps, FCR/ECR
C8	Finger Flexion (Grip)	FDS, FDP
T1	Finger Abduction/Adduction	Interossei

⭐ A C5-C6 root injury (Erb's Palsy) causes a "waiter's tip" hand. In contrast, a C8-T1 root injury (Klumpke's Palsy) results in a "claw hand" from paralysis of intrinsic hand muscles.

📌 Mnemonic: Use your fingers to count down the nerve roots: 5 (arm up), 6 (flex bicep), 7 (push down), 8 (grip), T1 (spread fingers).

Lower Limb Testing - Leg Levers

Myotome Chart: Spinal Nerve Roots and Muscle Movements

This table outlines key muscle tests for lower limb myotomes, often conceptualized as levers of the leg.

Nerve Root	Primary Muscle(s)	Action	Test Maneuver
L1, L2, L3	Iliopsoas	Hip Flexion	Patient lifts thigh against resistance.
L3, L4	Quadriceps Femoris	Knee Extension	Patient extends knee against resistance.
L4	Tibialis Anterior	Foot Dorsiflexion & Inversion	Patient dorsiflexes and inverts foot.
L5	Ext. Hallucis Longus	Great Toe Extension	Patient extends big toe against resistance.
S1	Gastrocnemius, Soleus	Foot Plantarflexion	Patient walks on toes or plantarflexes.

⭐ The L5 nerve root is the most commonly affected in lumbar disc herniations. Weakness in great toe extension (Extensor Hallucis Longus) is a key clinical sign for L5 radiculopathy, often presenting with foot drop.

High‑Yield Points - ⚡ Biggest Takeaways

C5 (deltoid) is key for shoulder abduction; test by resisting downward arm pressure.

C6 governs elbow flexion (biceps) and wrist extension.

C7 controls elbow extension (triceps) and wrist flexion.

L4 weakness causes foot drop (failed ankle dorsiflexion).

L5 is tested by great toe extension (extensor hallucis longus).

S1 is crucial for ankle plantarflexion; assess with toe-walking.

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