Extensor Pollicis Longus and Extensor Pollicis Brevis, Both Supplied by the Posterior Interosseous Nerve, a Branch of the Radial Nerve, C7
Have the patient put the ulnar edge of his hand and forearm on a flat surface, with the hand midway between prone and supine and the thumb extended vertically. If resistance is applied to the distal phalanx, the longus is being tested. If resistance is applied to the proximal phalanx, the brevis is being tested.
The tendons of these two muscles plus the tendon of the abductor pollicis longus make up the sides of the "snuff-box" on the radial edge of the wrist.
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