Deltoid muscles, situated in the shoulders, are responsible for the majority of arm movements, including arm abduction, arm flexion and medial rotation, and arm extension and lateral rotation. These movements involve the deltoids working with the rotator cuff muscles, supraspinatus, pectoralis major, infraspinatus, terres minor and terres major.
The deltoids have three key fibers: anterior, middle and lateral, each of which is responsible for different movements of the arm. The lateral muscle is responsible for internal rotation and flexion of the shoulder, while the anterior muscle is responsible for the external rotation and transverse flexion. Arm abduction is the work of all three fibers of the deltoid muscles working together.