The clavicle, commonly known as the collarbone, serves as the only bony connection between the trunk (axial skeleton) and the arm (appendicular skeleton). This slender, S-shaped bone acts as a strut, holding the shoulder joint laterally away from the body to allow for maximum range of motion. Its position makes it a significant attachment point for numerous muscles that control movements of the arm, shoulder, neck, and throat. These attachments transfer force from the torso to the limb and maintain the proper alignment of the upper body.
Muscles That Move the Arm
The anterior fibers of the Deltoid muscle, which give the shoulder its rounded contour, arise from the lateral third of the clavicle. Contraction of these fibers is responsible for shoulder flexion (lifting the arm straight out in front of the body). This muscle group also contributes to the medial, or internal, rotation of the arm at the shoulder joint.
Another major muscle utilizing the clavicle is the Pectoralis Major, a large fan-shaped muscle of the upper chest. Its clavicular head originates from the anterior surface of the medial half of the clavicle. This superior portion acts to flex the extended arm, such as when raising the arm above shoulder height. The Pectoralis Major also works with the Deltoid to produce adduction, pulling the arm back toward the center of the body.
The collaborative action of the anterior Deltoid and the clavicular Pectoralis Major is central to the dynamic mobility of the shoulder. They are antagonistic in some movements but work together to stabilize the glenohumeral joint during strenuous activity.
Muscles That Support the Neck and Head
Beyond arm movement, the clavicle anchors large muscles that control the position and movement of the neck and head. The Sternocleidomastoid (SCM) muscle, a prominent muscle on the side of the neck, has one of its two heads originating from the medial third of the clavicle. When only one side of the SCM contracts, it rotates the head to the opposite side and laterally flexes the neck toward the contracting muscle.
If both SCM muscles contract together, their combined force results in the flexion of the neck, such as nodding the chin toward the chest. The SCM also acts as an accessory muscle of forced inhalation, helping to elevate the clavicle and sternum to expand the chest cavity.
The Trapezius muscle, a large, triangular muscle covering the upper back and neck, also inserts onto the clavicle. Its upper fibers attach to the posterior border of the lateral one-third of the clavicle. This portion of the Trapezius is responsible for elevating the scapula, or shrugging the shoulders, and helps to support the weight of the arm. Additionally, the upper Trapezius fibers assist in the extension of the neck, working to pull the head backward.
Muscles That Anchor and Stabilize the Clavicle
Several smaller, deeper muscles attach to the clavicle, focusing on stabilization and protection rather than major movements. The Subclavius muscle is a small, triangular muscle located entirely beneath the clavicle. It originates from the first rib and its costal cartilage, inserting into a groove on the inferior surface of the middle third of the clavicle.
The primary functions of the Subclavius are to anchor and depress the clavicle, holding it steady against the sternum during movements of the shoulder. This action helps to resist separation of the sternoclavicular joint. A secondary but important role is to provide a protective cushion for the underlying neurovascular structures, such as the brachial plexus and subclavian vessels, in the event of a clavicle fracture.
The infrahyoid strap muscles also interact with the clavicle. The Sternohyoid muscle, a thin, ribbon-like muscle, originates from the posterior surface of the medial end of the clavicle and the sternum. This muscle courses upward to insert onto the hyoid bone in the neck. The Sternohyoid acts to depress the hyoid bone after it has been elevated during swallowing.
The clavicle serves as a complex anatomical nexus, providing origin or insertion points for a wide array of muscles. These attachments allow the bone to function as a lever for the forceful movements of the arm, a scaffold for the supportive muscles of the neck and head, and an anchor for deeper stabilizing muscles. The collarbone is integral to the integrated function of the upper body.