The shoulder has about eight muscles that attach to the scapula, humerus, and clavicle. These muscles form the outer shape of the shoulder and underarm. The muscles in the shoulder aid in a wide range of movement and help protect and maintain the main shoulder joint, known as the glenohumeral joint.

The largest of these shoulder muscles is the deltoid. This large triangular muscle covers the glenohumeral joint and gives the shoulder its rounded-off shape. It stretches across the top of the shoulder from the clavicle in the front to the scapula in the back. It then stretches downward to near the center of the humerus bone. Different fibers of the muscle are responsible for different actions, including raising the arm and assisting the pectoralis muscle in the chest. One important function of the deltoid is preventing joint dislocation when a person carries heavy objects.

Other muscles that aid in shoulder movement include:

  • Infraspinatus: This rotator cuff muscle helps with the raising and lowering of the upper arm.
  • Triceps brachii: This large muscle in the back of the upper arm helps straighten the arm.
  • Pectoralis major: This large fan-shaped muscle stretches from the armpit up to the collarbone and down across the lower chest region. It connects to the sternum (breastbone).
  • Pectoralis minor: The smaller of the pectoralis muscles, this muscle fans out from the upper ribs up to the shoulder area.
  • Teres major: This muscle helps rotate the upper arm.
  • Biceps brachii: Commonly known as the bicep muscle, this muscle rests on top of the humerus bone. It rotates the forearm and also flexes the elbow.
  • Latissimus dorsi: This flat rectangular muscle of the back helps the arms rotate as well as move away and closer to the body.
  • Subscapularis: This is a large triangular muscle near the humerus and collarbone. It helps rotate the humerus.
  • Supraspinatus: This small muscle is located at the top of the shoulder and helps raise the arm away from the body.

Four muscles—the supraspinatus, infraspinatus, teres minor, and subscapularis—make up the rotator cuff. It stabilizes the shoulder and holds the head of the humerus into the glenoid cavity to maintain the principal shoulder joint.

Because these muscles are used in a wide range of motion and are responsible for bearing heavy loads, shoulder muscle pain is a common ailment. The most common cause of shoulder pain is overexertion of a muscle or injury to it. Twisting, pulling, or falling are common ways muscles in the shoulders become painful. Repetitive use injuries primarily affect the deep muscles; however, pain and soreness as a result of pulled muscles from heavy lifting or overexertion usually subsides in a few days.

Minor shoulder muscle pain can usually be healed with a combination of rest, ice, elevation, and compression of the impacted region.