How does the shoulder work?

Our shoulders connect our upper arms to our torso. The shoulder joint is the body’s most flexible joint, as well as its most complex. Three bones, nearly a dozen different muscles and many ligaments and tendons meet in the shoulder. The way that they are connected allows us to move our arms in just about any direction we want. But this flexibility comes at a price: The shoulder is prone to injury. And it’s not always easy to find the cause of shoulder pain.

 

Illustration: Muscles, tendons, bursa and bones in the shoulder areaMuscles, tendons, bursa and bones in the shoulder area

 

The bones of the shoulder

Three bones meet in the shoulder:

  • the collarbone (clavicle)
  • the shoulder blade (scapula)
  • the upper arm bone (humerus)

Together, the ends of these bones make up the joints of the shoulder girdle (pectoral girdle).

Illustration: The bones in the shoulder (front view)The bones in the shoulder (front view)

 

The shoulder blade: Form and function

The shoulder blade makes up the back part of the shoulder girdle (see illustration). It is a flat, triangular bone. The back muscles that connect to the head of the upper arm bone are attached here.

The shoulder blade is divided into a small upper and larger lower surface by a ridge called the scapular spine. This ridge runs straight across the shoulder blade, forming the "roof" of the shoulder (the acromion) at one end.

The shoulder socket (glenoid) is located on the side edge of the shoulder blade. Together with the head of the upper arm bone (humerus), it makes up the main shoulder joint. Another part of the shoulder blade is known as the coracoid process.

Illustration: The bones in the shoulder (back view)The bones in the shoulder (back view)

 

The joints of the shoulder

Together, the shoulder blade, collarbone and the upper arm bone form the three joints of the shoulder girdle (see illustration):

  • The shoulder joint connects the upper arm to the shoulder blade. This is where the round head of the upper arm bone (head of the humerus) rests in the shoulder socket (glenoid) at the side of the shoulder blade.
  • The sternoclavicular (SC) joint connects the breastbone (sternum) to the collarbone (clavicle). As a ball-and-socket joint, this joint can be moved in all directions, although its movement is restricted by tight ligaments. We use it to raise and lower our collarbone when we shrug our shoulders, move our shoulder blades closer together, or cross our arms.
  • The acromioclavicular (AC) joint connects the bone at the top of the shoulder (acromion) to the collarbone (clavicle). We use this joint to lift our arms above shoulder height. Because the joint needs to cope with many different forces, it has very stable ligaments and, as a result, can't move very freely.

Most movements of the arm or shoulder involve more than one of these joints.

Illustration: The muscles in the shoulder region (front view)The muscles in the shoulder region (front view)

 

The muscles of the shoulder

The shoulder joint is the body’s most flexible joint: A group of four muscles holds the head of the upper arm bone in the relatively flat shoulder socket (glenoid). Each of these muscles has a tendon that attaches it to the bone. Together, this group of muscles and tendons is known as the rotator cuff.

The name "rotator cuff" comes from the function and position of the muscles: They make it possible for us to move our arms in various ways (raising and lowering them, swiveling them to the front or back and turning them inwards or outwards). These four muscles surround the upper arm bone (humerus) like a cuff. All of them are attached to the shoulder blade at one end, and to the upper arm bone at the other (see illustration).

The deltoid muscle, which we use to raise our arm, covers the shoulder joint. Doctors often use this muscle when giving injections into muscle tissue, such as some vaccines.

 

Illustration: The muscles of the rotator cuff (side view of the right shoulder)The muscles of the rotator cuff (side view of the right shoulder)


What else is part of the shoulder joint?

Besides the bones, muscles and tendons, the shoulder joint contains:

  • The protective shoulder joint capsule: The capsule is strengthened by individual ligaments from the front and the back.
  • The shoulder labrum: This is a ring of cartilage that helps to keep the head of the upper arm bone in the shoulder socket. Also, one of the two biceps tendons is attached at one end to the upper edge of the shoulder labrum.
  • Bursa: Bursas are fluid-filled sacs that reduce friction between muscles, tendons and bones. The largest bursa is found under the acromion. Overuse or "wear and tear" can cause inflammation in the bursas.
  • Ligaments: These strong bands of tissue connect the shoulder blade, collarbone and upper arm bone to each other at various points.