Arm muscles are grouped in compartments, each with its own nerve. Knowing which nerve supplies which group lets you find a nerve injury with a few simple tests.
Upper arm
The biceps brachii and brachialis bend the elbow, and the biceps also turns the palm up. The triceps on the back of the arm straightens the elbow.
Forearm
Flexors on the palm side bend the wrist and fingers, and extensors on the back straighten them. Most flexors are supplied by the median nerve and all extensors by the radial nerve.
Hand
Small intrinsic muscles do the fine work. The thenar muscles move the thumb (median nerve), while the interossei spread and close the fingers (ulnar nerve).
| Nerve | Key muscles | Quick test |
|---|---|---|
| Radial | Triceps, wrist and finger extensors | Extend wrist and fingers |
| Median | Forearm flexors, thenar muscles | Touch thumb to little finger, lift thumb up |
| Ulnar | Interossei, hypothenar muscles | Spread fingers against resistance |
At the bedside
Test these motions in every arm injury, before and after splinting, as part of the neurovascular check. Wrist drop suggests a radial nerve injury, often from a humerus fracture or pressure on the arm.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).