A 45 year old man came to his physician complaining of pain an weakness in his right shoulder. The pain began after a fall on his outstretched hand approximately 6 months previously. the patient recalled having some minor shoulder tenderness but no other specific symptoms. He was otherwise fit and we'll.

On examination of the shoulder, there was marked wasting of the muscles in the supraspinous and infraspinous fosse. the patient found initiation of abduction difficult and there was a weakness of lateral rotation of the humorous.

Identify the wasted muscle and explain your diagnosis.