Logan Steele
The scapula, or shoulder blade, is the bone that sits above the rib cage in the upper back. The scapula moves in six directions, and each movement is produced by specific primary muscles. The scapulothoracic joint enables the scapula to move against the chest wall during movements of the upper limb. The scapula can be elevated or depressed around the sagittal axis that passes through the centre of the acromioclavicular joint. While the elevation is limited by the coracoclavicular ligament, the depression goes as far as the coracoid process of the scapula and touches the clavicle. The pectoralis minor and the lowermost fibres of the serratus anterior muscle can actively depress the scapula.
| Characteristics | Values |
|---|---|
| Scapula | Commonly referred to as the shoulder blade |
| Movement | Can move in six directions |
| Movement | Can move in three degrees of freedom: elevation, depression, and rotation |
| Muscles that depress the scapula | Pectoralis minor and the lowermost fibers of the serratus anterior muscle |
| Muscles that elevate the scapula | Trapezius and levator scapulae |
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What You'll Learn
Pectoralis minor and serratus anterior muscle
The scapula, or shoulder blade, is a bone in the upper back that sits above the rib cage. It can move in six directions, and each movement is produced by specific primary muscles. The serratus anterior (SA) and pectoralis minor (PM) are two of the muscles that act on the scapula.
The serratus anterior is a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax. It inserts along the superior angle, medial border, and inferior angle of the scapula. The main part of the serratus anterior lies deep under the scapula and the pectoral muscles. It acts as a prime mover in scapular protraction and scapular upward rotation. It is also a key stabiliser, keeping the shoulder blades against the ribcage at rest and during movement. The serratus anterior is also known as the "boxer's muscle" because it is responsible for the protraction of the scapula that occurs when throwing a punch.
The pectoralis minor, on the other hand, functions as a synergist of the serratus anterior. It engages in the protraction movement of the scapula, along with downward rotation, depression, and anterior tilting. Overuse of the pectoralis minor can result in adaptive shortening of the muscle.
Both the serratus anterior and pectoralis minor play important roles in shoulder and neck health. Impairments in the activation of the serratus anterior have been linked to shoulder and neck disorders. Exercises that target the serratus anterior are often included in the rehabilitation of shoulder or neck pain, as they help to stabilise the scapula against the thorax. While the pectoralis minor is activated during these exercises, the extent of its activation is not yet clear.
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Trapezius and levator scapulae
The scapula, or shoulder blade, is the bone that sits above the rib cage in the upper back. The scapulae can move in six directions, and each movement is produced by specific primary muscles. Two of these muscles are the trapezius and levator scapulae.
The trapezius is a large, flat muscle that extends from the occipital bone of the skull to the lower thoracic vertebrae and the scapula. It has three functional parts: the superior, middle, and inferior fibres. The superior fibres elevate and rotate the scapula, the middle fibres retract the scapula, and the inferior fibres depress the scapula and stabilize the shoulder joint.
The levator scapulae muscle originates on the posterior tubercle of the transverse process of cervical vertebrae 1 to 4. It inserts onto the vertebral margin of the scapula, between the superior angle and the root of the spine. The levator scapulae elevate the scapula and tilt the glenoid cavity inferiorly by rotating the scapula downward. If the scapula is fixed, a contraction of the levator scapulae leads to the lateral flexion of the cervical vertebral column to the side and stabilizes the vertebral column during rotation.
Trigger points are common in the levator scapulae and can cause pain or discomfort. This can be relieved by various modalities and active movements. Ultrasound-guided hydro-dissection (hydro-release) has emerged as an effective treatment for neck pain and stiff shoulders. This procedure involves injecting a saline solution between tissues to improve tissue gliding and provide therapeutic benefits.
Scapular elevation exercises have been proposed to improve symptoms of neck and shoulder pain and stiffness, which is common in the trapezius and levator scapulae muscles. These exercises induce separate contractions of the trapezius and levator scapulae muscles by modifying the alignment of the spine and upper limbs. However, it is important to note that these exercises alone may not effectively enhance intermuscular gliding due to the simultaneous contraction of both muscles.
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Rhomboids and pectoralis minor
The rhomboids are a group of muscles formed by the rhomboid major and minor. They are located in the upper back and are important for upper limb movement and stability of the shoulder girdle and scapula. The rhomboids work with other muscles to anchor the scapula and prevent winging. They also help to elevate, retract and rotate the scapula, and protract the medial border of the scapula, keeping it in position at the posterior thoracic wall.
The rhomboid minor is a small, cylindrical muscle that originates at the nuchal ligament and C7 and T1 vertebra. It inserts at the medial border of the scapula, near its base. The rhomboid major is a quadrangular muscle located inferior to the rhomboid minor. It is thin and flat and twice as wide as the rhomboid minor.
The pectoralis minor, meanwhile, is a thin, triangular muscle located beneath the pectoralis major in the upper chest. It contributes to several actions, including depression of the scapula. This muscle originates at the third, fourth, and fifth ribs and inserts at the coracoid process of the scapula.
The pectoralis minor is innervated by the medial pectoral nerve, a branch of the brachial plexus. It receives arterial blood supply from the pectoral branch of the thoracoacromial trunk, which arises from the axillary artery.
Together, the rhomboids and pectoralis minor muscles play a crucial role in scapular movement and stability, with the rhomboids focusing on retraction, elevation, and rotation, and the pectoralis minor contributing to depression and other movements.
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Dorsal scapular nerve
The scapula, or shoulder blade, can move in six directions, with each movement produced by specific, primary muscles. One such muscle is the dorsal scapular nerve, a motor nerve that provides innervation to the rhomboid major, rhomboid minor, and levator scapulae muscles. These muscles work dynamically and collectively and are known as periscapular stabilising muscles.
The dorsal scapular nerve is a branch of the brachial plexus, usually derived from the ventral ramus of cervical nerve C5. It arises just above the clavicle, from the proximal part of the anterior ramus of C5. Once the nerve leaves C5, it pierces the middle scalene muscle and continues deep to the levator scapulae muscle and the rhomboids (minor superior to major). The nerve is accompanied by the dorsal scapular artery, which provides blood to the trapezius, levator scapulae, and rhomboid muscles.
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Acromioclavicular joint
The acromioclavicular joint (AC joint) is the articulation between the two bones of the pectoral girdle: the clavicle and scapula. It is a plane synovial joint, with flat articular surfaces that are approximately the same size. The AC joint is a multiaxial joint, allowing movement in three degrees of freedom: protraction-retraction, elevation-depression, and axial rotation. This joint enables the pectoral girdle to follow the movements of the shoulder joint, particularly after the sternoclavicular joint has reached its maximal range of motion.
The AC joint is formed by the junction of the lateral end of the clavicle and the acromion process of the scapula. The articular surfaces of the joint are lined with fibrocartilage and are enclosed by the joint capsule, which consists of a loose layer of fibrous tissue lined internally by a synovial membrane. The joint capsule and the ligaments surrounding the AC joint work together to provide stability and maintain the integrity of the joint. The AC joint is reinforced by three main ligaments: the acromioclavicular ligament, the coracoclavicular ligament (consisting of the conoid and trapezoid ligaments), and the joint capsule ligament. These ligaments contribute to horizontal stability and prevent superior dislocation of the AC joint.
The movements within the AC joint are entirely passive, and the elevation and depression of the acromion occur in a sagittal axis. The elevation of the acromion is sustained by tension in the coracoclavicular ligament, while depression is limited by contact between the coracoid process of the scapula and the inferior surface of the clavicle. The AC joint allows for a gliding movement in the superior/inferior and anteroposterior planes, along with a small amount of axial rotation.
The AC joint is crucial for integrating shoulder movements with those of the pectoral girdle, enabling the flexed arm to be fully elevated. It also allows for the transmission of forces from the upper limb to the clavicle. The scapulothoracic joint, which includes the AC joint, enables the mobility of the scapula around the fulcrum of the AC joint and is important for the proper positioning of the shoulder joint in space. The protraction and retraction of the scapula occur around a vertical axis passing through the lateral end of the clavicle.
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Frequently asked questions
The pectoralis minor and the lowermost fibres of the serratus anterior muscle actively depress the scapula.
The scapula, commonly referred to as the shoulder blade, is the bone that sits above the rib cage in the upper back.
The scapula moves in six directions, with each movement produced by specific primary muscles. The scapula moves against the chest wall in three degrees of freedom: elevation, depression, and rotation.
