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The scapula, commonly referred to as the shoulder blade, is a sturdy, flat, triangular bone that sits above the rib cage in the upper back. It forms the posterior of the shoulder girdle, connecting the clavicle to the humerus and creating the shoulder joint. The scapula has six directions of movement, each produced by specific primary muscles. These muscles include the latissimus dorsi, serratus anterior, pectoralis major and minor, and the trapezius muscles, which are responsible for depressing the scapula.
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What You'll Learn
Latissimus dorsi
The latissimus dorsi is a large, flat muscle that covers the back and stretches to the sides, behind the arm. The name "latissimus dorsi" comes from Latin and means "broadest of the back". The muscle is also referred to as "lats", especially among bodybuilders.
The latissimus dorsi is the widest muscle in the human body and is relatively thin, covering almost all back muscles at the posterior trunk, except the trapezius. It is considered an extrinsic muscle of the back, meaning it originates from the myogenic cells in the developing upper limb buds rather than from the myotomal dorsal epaxial division of the somite where the intrinsic (deep back) muscles take origin. Embryologically, the latissimus dorsi and teres major muscles are closely related as they both originate from the pre-muscle sheath of the arm. However, unlike the teres major, the latissimus dorsi spreads across the posterior aspect of the thorax and trunk as the myoblasts migrate out of the limb bud into the axial mesenchyme posteriorly.
The latissimus dorsi is responsible for several functions, including extension, adduction, transverse extension (also known as horizontal abduction or horizontal extension), flexion from an extended position, and medial internal rotation of the shoulder joint. It also plays a role in the extension and lateral flexion of the lumbar spine. Additionally, it assists in depression of the arm, working alongside the teres major and pectoralis major muscles.
The latissimus dorsi can influence the movement of the scapulae, particularly their downward rotation. It crosses the inferior angle of the scapula and is surrounded by two major muscles, the teres major and pectoralis major, as it inserts into the floor of the intertubercular groove of the humerus. The latissimus dorsi is supplied by the thoracodorsal artery and receives additional blood supply from the dorsal perforating branches of the inferior three posterior intercostal arteries and the superior three lumbar arteries.
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Serratus anterior
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. Its main part lies deep under the scapula and the pectoral muscles. It is easy to palpate between the pectoralis major and latissimus dorsi muscles. In athletic bodies, the muscle may even be visible to the naked eye along the ribs underneath the axilla. The serratus anterior muscle originates from the 1st to 8th or 9th ribs and inserts at the anterior surface of the scapula. Due to its course, it has a ""serrated" or "saw-toothed" appearance.
The serratus anterior is a key scapular stabiliser, keeping the shoulder blades against the rib cage when at rest and during movement. It acts on the scapula and is the prime mover in both scapular protraction and scapular upward rotation. The contraction of the entire serratus anterior leads to an anterolateral movement of the scapula along the ribs. Due to the pull of the inferior part at the lower scapula, the shoulder joint is shifted superiorly. This shifting now enables the lift of the arm above 90° (elevation). In contrast, the superior part depresses the scapula and thus acts antagonistically.
Another function of the serratus anterior is the active stabilization of the scapula within the shoulder. Finally, in a fixed scapula, the muscle lifts the ribs and acts as an accessory inspiratory muscle. The vascular supply to the serratus anterior comes from the superior and lateral thoracic arteries (branches of the axillary artery) as well as branches from the thoracodorsal artery (branch of subscapular artery).
The serratus anterior is also known as the "boxer's muscle," as it is largely responsible for the protraction of the scapula which occurs when throwing a punch.
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Pectoralis major and minor
The pectoralis minor muscle is one of the most superficial muscles on the anterior aspect of the chest or thoracic wall. It is located under the pectoralis major muscle, and together they form the anterior wall of the axilla. The pectoralis minor is triangular in shape and originates from the margins of the third to fifth ribs adjacent to the costochondral junction. The fibres extend superolaterally to form a flat tendon, which inserts into the medial border and coracoid process of the scapula.
The pectoralis minor muscle is crucial for the stabilization of the scapula, also known as the shoulder blade, by pulling it downward and anteriorly against the thoracic wall. It also rotates the glenoid cavity inferiorly and causes protraction of the scapula. When the pectoralis minor muscle is shortened, it can restrict flexion of the shoulder joint by limiting its rotation and preventing the glenoid cavity from attaining the cranial orientation necessary for complete flexion. This can lead to clinical conditions such as thoracic outlet syndrome, where the muscle impinges on the brachial plexus and axillary blood vessels, causing shoulder impingement.
The pectoralis major muscle is also involved in the movement of the scapula. It is one of the anterior axioappendicular (thoracoappendicular) muscles, along with the pectoralis minor, subclavius, and serratus anterior. Together, these muscles produce various movements of the scapula and can be used as accessory muscles of respiration. The pectoralis major also plays a role in protraction, which is the forward movement of the scapula, and depression, which is the downward movement of the scapula.
Overall, the pectoralis major and minor muscles are important for the stabilization and movement of the scapula, particularly in protraction and depression. Their functions contribute to the full range of motion of the shoulder joint, allowing for abduction, external and internal rotation, and flexion of the arm.
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Trapezius
The trapezius muscle is a large, paired trapezoid-shaped muscle that extends from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula. There is one trapezius muscle on either side of the upper back, extending from the base of the neck across the shoulders and down to the middle of the back. The trapezius muscle is named after its shape, resembling a trapezium or diamond-shaped quadrilateral.
The trapezius muscle has three sections: upper, middle, and lower. These sections connect to different parts of the skull, spine, ribs, and bones in the shoulders. The upper section connects to the skull and neck (cervical spine), while the middle and lower sections attach to the bones in the thoracic spine. The trapezius muscle also connects to the lateral sides of the scapula (shoulder blade) and collarbone (clavicle).
The trapezius muscle plays a crucial role in maintaining posture and moving the upper back, neck, and head. It is involved in various motions, including turning the head, adjusting posture, twisting the torso, shrugging or pulling back the shoulders, and moving the shoulders during arm movements. The upper fibres of the trapezius elevate the scapulae, the middle fibres retract the scapulae, and the lower fibres depress the scapulae.
The trapezius muscle works in conjunction with other muscles to produce coordinated movements, particularly involving the scapula. For example, the rhomboids (major and minor) work with the trapezius to retract and stabilise the scapula, contributing to shoulder movement and posture. The trapezius also assists in abduction of the shoulder by rotating the glenoid upward.
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Levator scapulae
The levator scapulae is a posterior axio-appenducular muscle that connects the upper limb to the vertebral column. It is located in the posterior triangle of the neck, with its superior aspect covered by sternocleidomastoid and its inferior aspect by the trapezius. The levator scapulae originates on the posterior tubercle of the transverse process of cervical vertebrae 1 to 4 and inserts onto the vertebral margin of the scapula between the superior angle and the root of the spine. It is innervated by the cervical nerve (C3-C4) and the dorsal scapular nerve (C5), and supplied by the dorsal scapular artery.
The levator scapulae is responsible for elevating the scapula and rotating it downward. When the scapula is fixed, contraction of the levator scapulae leads to lateral flexion of the cervical vertebral column to the same side. It also stabilizes the vertebral column during rotation. Additionally, when the cervical spine is fixed, the levator scapulae elevates and rotates the scapula, with the lateral angle moving inferiorly. Bilateral contraction of the levator scapulae helps in the extension of the cervical spine.
The levator scapulae is one of the muscles that make up the axioscapular group, which also includes the rhomboid major, rhomboid minor, serratus anterior, and trapezius muscles. These muscles work together to produce downward rotation of the scapula. The levator scapulae is also considered an extrinsic muscle of the scapula, along with the trapezius, rhomboids, and serratus anterior muscles.
The levator scapulae can be implicated in various pathologies, such as levator scapulae syndrome, which presents as tenderness over the upper medial angle of the scapula. Treatment options for this condition include physical therapy and local corticosteroid injections. Trigger points are common in this muscle and can cause pain or discomfort, which can be relieved through various modalities and active movements.
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Frequently asked questions
The scapula, or shoulder blade, is depressed by the latissimus dorsi, serratus anterior, pectoralis major and minor, and the trapezius muscles.
The scapula is a sturdy, flat, triangular bone that sits above the rib cage in the upper back. It connects the clavicle to the humerus and forms the posterior of the shoulder girdle.
The scapula can move in six directions: protraction, retraction, elevation, depression, upward rotation, and downward rotation. Each movement is produced by specific muscles.
The scapula allows for the full function of the shoulder joint, one of the most mobile and versatile joints in the human body. It provides attachment points for muscles, ligaments, and other soft-tissue structures.
Yes, the scapula can become injured through trauma or during surgical procedures. Winging of the scapula may occur after an injury to the long thoracic or spinal accessory nerves. Injury to the dorsal scapular nerve can cause paralysis of the rhomboid muscles.
