The muscles Supraspinatus, Infraspinatus, Teres Minor and Subscapularis form the "Rotator Cuff". Their tendons wrap around the head of the humerus, protecting the joint and ensuring shoulder stability.
The most important muscles in this area is the Triceps Brachii, which consists of two longer heads (1a on the lateral part of the arm; 1b on the inner part) and one shorter head (1c on the inner part).