Action:  Shoulder abduction

Nerves:  Suprascapular, axillary, CN XI (spinal accessory), and long thoracic

Skeletal muscles:  Supraspinatus, deltoid, trapezius, and serratus anterior

Cutaneous distribution:  None except for the axillary nerve

Neuromuscular deficit:  Weakness/paralysis when abducting at the shoulder under resistance. In normal subjects the supraspinatus initiates the first 15 degrees of abduction along the vertical plane. The deltoid functions from 15 to 90 degrees, while synergistic actions of the trapezius and serratus anterior abduct from 90 to 180 degrees by rotating the scapula laterally. Denervation is accompanied by muscular atrophy, shoulder adduction, ‘winged’ scapula, and cutaneous deficit along the distribution of the axillary (superior lateral brachial cutaneous) nerve.

Differential diagnosis:  Abductor weakness/paralysis plus cutaneous deficit along the superior and lateral arm are diagnostic of axillary nerve impairment. A “winged” scapula indicates a deficit of the serratus anterior/long thoracic nerve.