Electromyographic evaluation of an accessory hand muscle

Accessory hand muscles consistent with extensor digitorum brevis manus muscles were noted in a young woman. Well-defined compound muscle action potentials were elicited in these muscles with stimulation of the radial nerve. Needle electromyography showed poor recruitment and large motor unit potentials with frequent cramp discharges only in the anomalous muscles. Therefore, the extensor digitorum brevis manus is innervated by the posterior interosseous nerve, and symptoms may be due to chronic denervation and cramping.