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The clavicle, also known as the collar bone, is a long, horizontally positioned bone that connects the shoulder blade to the sternum. It is a crucial part of the shoulder complex, providing stability and protection to the neurovascular structures of the thorax. While there are no muscles that act directly on the sternoclavicular joint, several muscles are attached to the clavicle and influence its movement. These muscles include the trapezius, deltoid, sternocleidomastoid, pectoralis major, subclavius, sternohyoid, and levator claviculae. The contraction and relaxation of these muscles enable the clavicle to rotate, tilt, and swing during movements such as abduction, adduction, and flexion of the scapula.
| Characteristics | Values |
|---|---|
| Number of muscles attached to the clavicle | 5 or 6 |
| Muscles attached to the lateral third of the clavicle | The trapezius muscle, The deltoid muscle |
| Muscles attached to the medial third of the clavicle | The sternocleidomastoid muscle, The pectoralis major muscle, The subclavius muscle, The sternohyoid muscle |
| Clavicle joint | Sternoclavicular joint, Acromioclavicular joint |
| Clavicle structure | Sigmoid-shaped long bone |
| Clavicle function | Connection between the axial skeleton and the pectoral girdle, Acts as a brace for the shoulder |
| Clavicle rotation | Upward rotation during elevation, Passive rotation during flexion |
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What You'll Learn
The trapezius muscle
The actions of the trapezius muscle largely depend on the direction of the fibres that are contracting. The descending (upper) fibres act with the levator scapulae muscle to produce an elevation of the scapula at the scapulothoracic joint. In the same way, they also maintain the level of the shoulders against gravity, for example, when a weight is being carried in the hand. When the muscle is acting unilaterally, the descending fibres produce an ipsilateral lateral flexion of the head and neck by acting on the atlanto-occipital joint and upper cervical vertebrae, respectively.
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The deltoid muscle
The deltoid's primary functions include arm abduction, flexion, internal rotation, and horizontal adduction. Arm abduction involves raising the arm out to the side of the body. Flexion is moving the arm forward, towards an overhead position, and extension is moving the arm backward, behind the body. The deltoid muscle also helps to compensate for lost arm strength due to injury, such as a rotator cuff tear. Additionally, it stabilises the shoulder joint to prevent dislocations as you lift your arm or carry weight with your arms at your side.
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The sternocleidomastoid muscle
The SCM is closely related to certain neurovascular structures that pass through the neck, either towards the head or the periphery of the body. The muscle is innervated by the accessory nerve (cranial nerve XI) and direct branches of the cervical plexus (C2-C3). The SCM also has inspiratory muscle action, as it can lift the sternum and clavicles when taking a fixed point on the temporal bone, thus expanding the thoracic cavity.
The SCM's name comes from its attachments to the manubrium of the sternum (sterno-), the clavicle (-cleido-), and the mastoid process of the temporal bone (-mastoid). The sternal head originates from the manubrium of the sternum, while the clavicular head originates from the medial third of the clavicle. These two heads join into one muscle belly, which inserts on the lateral surface of the mastoid process of the temporal bone and the lateral half of the superior nuchal line of the occipital bone.
Injuries, tension, sprains, strains, atrophy, and tumours can all damage the SCM muscle. Sternocleidomastoid syndrome is a condition involving neck stiffness, pain, and other symptoms that can arise from trigger points developing in the SCM muscle. Treatment options for SCM issues include massage, osteopathic manipulation, physical therapy, and surgery in severe cases. Maintaining good posture and managing stress are also important for SCM health.
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The pectoralis major muscle
The pectoralis major is a thick, fan-shaped or triangular convergent muscle of the human chest. It is the largest and most superficial muscle in the chest area, lying under the breast. The pectoralis major arises from the anterior surface of the sternal half of the clavicle, the breadth of the half of the anterior surface of the sternum, the costal cartilages of the true ribs, and the aponeurosis of the abdominal external oblique muscle. It is attached to the anterior surface of the medial third of the clavicle.
The pectoralis major has four primary actions, which are responsible for the movement of the shoulder joint. The first action is flexion of the humerus, as in throwing a ball underhand or lifting a child. The second action is adduction, the third is internal rotation of the humerus, and the fourth is keeping the arm attached to the trunk of the body. The muscle also contributes to horizontal adduction and inward rotation of the humerus. When at an approximately 110-degree angle, it contributes to adduction of the humerus.
The pectoralis major is innervated by the medial (C8-T1) and lateral (C5-C7) pectoral nerves, which originate from the medial and lateral cords, respectively. It receives double motor innervation from the medial pectoral nerve and the lateral pectoral nerve. The muscle maintains its blood supply via the pectoral branch of the thoracoacromial artery.
Pectoralis major tears are uncommon injuries that have become more prevalent over the past 20 years due to increased participation in weight lifting. Tears typically affect otherwise healthy individuals and are known to affect athletes, especially in high-impact contact sports such as powerlifting.
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The subclavius muscle
The main function of the subclavius muscle is to actively stabilise the clavicle at the sternoclavicular joint during movements of the shoulder and arm. It also helps to prevent dislocation of the clavicle at this joint. Additionally, the subclavius protects the underlying neurovascular structures, including the brachial plexus and subclavian vessels, in the event of a clavicular fracture.
The subclavius may also play a role in controlling the position of the scapula in relation to the thorax through its lateral aponeurotic extension and fascial connections. Contraction of the subclavius is hypothesised to increase tension at the tendinous border and the aponeurosis, potentially regulating the output of kinetic energy during the upper extremity's movement.
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Frequently asked questions
The medial third of the clavicle has attachment sites for the sternocleidomastoid (SCM) muscle and the subclavius muscle.
When the SCM contracts, the head rotates to the opposite side and the head bends laterally. When both SCMs contract, this causes head flexion.
The trapezius muscle and the deltoid muscle.
The sternohyoid muscle, the pectoralis major muscle, and the levator claviculae muscle.
