Brachial Plexus Practice Test

Shoulder abduction beyond the initial range relies on the deltoid muscle. The deltoid is innervated by the axillary nerve, so when this nerve is damaged, the deltoid cannot contract to lift the arm away from the body at the shoulder. The first roughly 15 degrees of abduction is done by the supraspinatus, which is spared if only the axillary nerve is injured, so some initial movement might occur but cannot be continued without deltoid function. The radial, median, and ulnar nerves do not innervate the deltoid, so injury to them wouldn’t specifically abolish shoulder abduction via the deltoid.