The pectoral girdle is the anatomical system that attaches the arms to the central skeleton. Its primary role is to create a stable, yet highly mobile, base that provides the structural foundation for the shoulder and facilitates movement for the upper limbs. Each side of the body has its own pectoral girdle, which functions independently. This arrangement supports the wide array of actions performed by the arms, from lifting objects to reaching overhead.
Anatomical Components
The pectoral girdle is composed of two bones on each side of the body: the clavicle and the scapula. The clavicle, commonly known as the collarbone, is a slender, S-shaped bone that extends horizontally across the upper chest. It acts as a strut, holding the shoulder joint away from the trunk to maximize the arm’s range of motion.
The clavicle forms two joints. The sternoclavicular joint is the connection between the medial, or central, end of the clavicle and the sternum (breastbone). This is the only direct bony link between the pectoral girdle and the axial skeleton. At its lateral, or outer, end, the clavicle connects with the scapula at the acromioclavicular joint, which forms the bony tip of the shoulder.
The scapula, or shoulder blade, is a large, flat, triangular bone situated on the posterior side of the rib cage. The scapula features the glenoid cavity, a shallow socket on its lateral edge that articulates with the head of the humerus (upper arm bone) to form the main shoulder joint. Above the glenoid cavity is the acromion, a projection that meets the clavicle, and the coracoid process, a hook-like structure for muscle attachment. The scapula is not attached to the rib cage with bone; instead, it glides across the back of the thorax, allowing for extensive mobility.
Range of Motion and Function
The primary function of the pectoral girdle is to position the shoulder joint in space, enabling the arm to move freely in multiple directions. This is accomplished through a series of coordinated movements of the scapula across the back of the thorax. These movements are necessary for achieving the full range of upper limb motion.
There are six principal movements of the scapula. Elevation is the upward motion of the shoulder, similar to the action of shrugging, while depression is the opposing downward movement. Protraction involves pulling the scapula forward and away from the spine, as when reaching for an object in front of you. Its counterpart, retraction, is the act of pulling the shoulder blades backward, toward the midline of the body.
The final pair of movements involves rotation of the scapula. Upward rotation occurs when the arm is lifted overhead, causing the glenoid cavity to tilt upwards to maintain joint stability. Conversely, downward rotation happens as the arm is lowered back to the side.
Key Supporting Musculature
The movements of the pectoral girdle are driven by a network of muscles on the anterior (front) and posterior (back) sides of the thorax. These muscles anchor the scapula and clavicle to the trunk, working in coordinated groups to produce movement and provide stability.
For elevation, the upper fibers of the trapezius muscle and the levator scapulae are the primary movers. When reaching forward for protraction, the serratus anterior muscle is used. This muscle, sometimes called the “boxer’s muscle,” holds the scapula against the rib cage and pulls it forward.
For retraction, the middle fibers of the trapezius and the rhomboid muscles are engaged. The rhomboids lie deep to the trapezius and contribute to scapular retraction and rotation. The coordinated action of these opposing muscle groups ensures the scapula is a stable platform for the arm during dynamic movements.
Common Pectoral Girdle Injuries
The bones and joints of the pectoral girdle are susceptible to injury from direct trauma or repetitive stress. A clavicle fracture, or broken collarbone, is one of the most frequent fractures in the body. These injuries often result from a fall directly onto the shoulder or an outstretched hand.
Another common injury is an acromioclavicular (AC) joint separation, often called a separated shoulder. This is not a dislocation of the main shoulder joint, but an injury to the ligaments connecting the clavicle to the acromion of the scapula. AC separations are frequently seen in athletes in contact sports.
Scapular dyskinesis describes the abnormal movement or positioning of the shoulder blade. This condition, sometimes visible as “winging” of the scapula where the medial border lifts away from the rib cage, is often due to weakness or dysfunction in the serratus anterior muscle. Scapular dyskinesis can lead to pain and impaired shoulder function.