The shoulder girdle (also called the pectoral girdle) refers to the structure of the shoulder that is supported by three bones: the humerus (the upper arm bone), the scapula (the shoulder blade), and the clavicle (the collar bone). The bones form four joints held together by a number of muscles and ligaments.
Shoulder Girdle Joints
The most familiar of the four joints of the shoulder girdle is the glenohumeral joint, often called the shoulder joint. It's a ball and socket joint where the upper arm bone meets the scapula in the glenoid fossa. The glenoid fossa is a shallow socket and the ligaments connecting the bones are relatively loose, meaning the shoulder joint is very mobile but lacks stability.
The acromioclavicular (AC) joint is where the clavicle and scapula connect at the acromion, a pointy projection on the upper outer point of the scapula. The AC joint lets the clavicle glide and rotate on the scapula during movements such as shrugging or raising the arm overhead. The sternoclavicular (SC) joint is where the clavicle and sternum (breastbone) connect and moves only slightly.
The fourth joint, called the scapulothoracic joint, is not really a joint; it is where the shoulder blade glides against the rib cage. This scapulothoracic region is important because the muscles surrounding the shoulder blade work together to keep the socket lined up during shoulder movements. The major muscles that support this area are the rhomboids and trapezius, with some help from the levator scapula, serratus anterior, and pectoralis minor.
Shoulder (Glenohumeral) Joint Ligaments and Muscles
Four ligaments connect the humerus to the scapula at the glenohumeral joint:
- The coracoacromial ligament helps prevent the head of the humerus from moving too far up.
- The coracohumeral ligament strengthens the top part of the joint capsule and provides some support when the shoulder and arm move in towards your body.
- The transverse humeral ligament holds the long head of the biceps muscle in a groove on the head of humerus.
- The glenohumeral ligaments strengthen the front part of the joint capsule.
The shoulder muscles that attach the scapula to the humerus and support the shoulder joint are the deltoid, teres major, and rotator cuff muscles. The deltoid, which lifts the arm up and away from the body with help from the teres major, is the strongest shoulder muscle.
Rotator Cuff Muscles
Four rotator cuff muscles connect the scapula to the upper arm bone. These four muscles are important in moving (rotating) the arm:
- The supraspinatus is at the top part of the shoulder blade and helps abduct (move out to the side) your arm.
- The infraspinatus is on the lower part of the shoulder blade and helps laterally rotate (turn inward) your arm.
- The teres minor is located next to the infraspinatus and also helps laterally rotate the arm.
- The subscapularis is on the underside of the shoulder blade and helps adduct (move inward) and medially rotate (turn outward) the arm.
Shoulder Injury and Care
Rotator cuff injuries, such as tendonitis and tears, and other shoulder problems are common. According to the American Academy of Orthopaedic Surgeons, the warning signs of shoulder problems are stiffness, inability to rotate the arm in all the normal positions, the feeling the shoulder joint could move out of the socket, or lack of shoulder strength to do daily activities.
The best way to help prevent shoulder injuries is stretching and strengthening the muscles of the shoulder girdle and avoiding overuse. A little care goes a long way.
Carol Wiley - Carol Wiley is a technical writer turned licensed massage therapist turned freelance writer. Visit her freelance writing site or Massage ...
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