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AN13.1-8 | General Features, Joints, radiographs & surface marking (Upper Limb) — Part 4

Muscle Testing of the Upper Limb

Testing muscles is a fundamental clinical skill. You test a muscle by asking the patient to perform its primary action against resistance, while you palpate the muscle belly to confirm it's contracting.

Here are the key muscles to test:

Trapezius (spinal accessory nerve, CN XI):
• Action: Elevation and retraction of the scapula
• Test: Ask the patient to shrug their shoulders against your downward pressure. Palpate the muscle between the neck and shoulder.

Pectoralis major (medial and lateral pectoral nerves):
• Action: Adduction and medial rotation of the arm
• Test: Ask the patient to press their palms together in front of the chest ("prayer position with force"). Palpate the anterior axillary fold.

Serratus anterior (long thoracic nerve):
• Action: Protracts the scapula and holds it against the chest wall
• Test: Ask the patient to push against a wall with outstretched arms. If the nerve is damaged, the medial border of the scapula sticks out — called "winging of the scapula".

Latissimus dorsi (thoracodorsal nerve):
• Action: Extension, adduction, and medial rotation of the arm
• Test: Ask the patient to cough while you palpate the posterior axillary fold. Latissimus dorsi contracts forcefully during coughing.

Deltoid (axillary nerve):
• Action: Abduction of the arm (especially 15-90°)
• Test: Ask the patient to abduct the arm to 90° against your resistance. Palpate the rounded contour of the shoulder.

Biceps brachii (musculocutaneous nerve):
• Action: Flexion of the elbow and supination of the forearm
• Test: Ask the patient to flex the elbow against resistance with the forearm supinated. The biceps tendon is easily visible and palpable in the cubital fossa.

Brachioradialis (radial nerve):
• Action: Flexion of the elbow (especially in mid-pronation)
• Test: Ask the patient to flex the elbow with the forearm in mid-prone position against resistance. Palpate the lateral forearm just below the elbow.

Development of the Upper Limb

The upper limb develops from a small bud of tissue that appears during the 4th week of embryonic development. Understanding this process explains many congenital anomalies you may see in clinical practice.

Timeline of development:

1. Week 4: Upper limb buds appear as small elevations on the lateral body wall. The upper limb bud appears slightly before the lower limb bud (about 2 days earlier). Each bud has a core of mesenchyme (embryonic connective tissue from lateral plate mesoderm) covered by ectoderm.

1. Week 5: The tip of the limb bud develops a thickened ridge of ectoderm called the apical ectodermal ridge (AER). This ridge acts like a "growth engine" — it sends signals (mainly FGF proteins) that keep the underlying mesenchyme proliferating. If the AER is damaged, limb growth stops at that point.

1. Week 6: The hand plate forms as a flattened expansion at the tip. Finger rays appear as condensations of mesenchyme.

1. Week 7: Apoptosis (programmed cell death) between the finger rays separates the digits — like removing the webbing between fingers. If this process is incomplete, the result is syndactyly (webbed or fused fingers), one of the commonest congenital hand anomalies.

1. Week 8: The limb rotates laterally by 90° so that the elbow points posteriorly and the palm faces anteriorly. (The lower limb rotates medially — elbow back, knee front. This opposite rotation is why the dermatome pattern spirals differently in each limb.)

Key signalling centres:
• AER: Controls proximal-distal growth (shoulder → hand direction)
• Zone of polarising activity (ZPA): Controls anterior-posterior patterning (thumb → little finger). Produces Sonic Hedgehog (SHH) protein — too much SHH can cause extra digits (polydactyly)
• Wnt and BMP pathways: Control dorsal-ventral patterning (back of hand → palm)

Common congenital anomalies:
• Polydactyly: Extra digits (most common in the little finger, may run in families)
• Syndactyly: Fused digits (failure of apoptosis between finger rays)
• Amelia: Complete absence of a limb (failure of limb bud formation)
• Meromelia/Phocomelia: Partial absence — historically associated with thalidomide exposure
• Cleft hand (lobster-claw deformity): Absence of central rays