Shoulder anatomy

Authors: Jaashing He , Hannah Goodings | Reviewer: Darryl Caves | Published: 7 June 2023 | Updated: ~

Posterior view of man's torso with arm raised. Muscles of the rotator cuff (supraspinatus, subscapularis, teres minor, and infraspinatus) are superimposed and labeled.

The shoulder is designed for movement and a large range of motion. Bone shape, muscle coordination and connective tissue all work together to form our most flexible joint. With this large range of movement, we sacrifice some stability. Unlike the hip joint with its “ball in cup” design giving great bony stability, the shoulder has a “ball on small plate” design. The surrounding muscles and connective tissue help keep the upper arm bone (the “ball”) in place on the shoulder blade (the “dish”). There are four muscles responsible for keeping the ball in place during movement. This group of muscles is referred to as the rotator cuff muscle group and includes the supraspinatus (above the spine of the scapula), subscapularis (on the front of the scapula), and infraspinatus (below the spine of the scapula) and teres minor (on the back of the scapula). These four small muscles are responsible for “balancing the ball” but they are not the only muscles found in the shoulder. There are larger muscles that surround the joint that are used to perform movements that involve strength such as lifting, pushing or carrying.

Cartoon of front shoulder skeleton with joints (sternoclavicular, acromioclavicular, glenohumeral, scapulothoracic) labeled.

Though we often think of the shoulder as a singular joint, there are actually four joints and articulations within the shoulder system.

The joints/articulations are:

  • Sternoclavicular joint (sternum and collar bone)
  • Acromioclavicular (shoulder blade and collar bone)
  • Glenohumeral joint (shoulder blade and upper arm)
  • Scapulothoracic (ribcage and shoulder blade)

Shoulders do not work in isolation. The bones, muscles and connective tissue all interact with surrounding regions of the body resulting in the potential for disruptions in movement patterns in the shoulder. For example, the shape of the ribcage can also impact shoulder movement. It is what the shoulder blade slides on to allow for movement of the arm above shoulder height. If the shoulder blade cannot move smoothly over the ribs, reaching above shoulder height may become difficult and painful.