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The scapula, commonly referred to as the shoulder blade, is a flat, triangular bone that sits above the rib cage in the upper back. It connects the clavicle to the humerus, forming the shoulder joint. The scapula can move in six directions, including protraction, retraction, elevation, depression, upward rotation, and downward rotation, and each movement is produced by specific muscles. Protraction of the scapula is accomplished by the serratus anterior, pectoralis major, and pectoralis minor muscles. These muscles, along with the scapula's other intrinsic and extrinsic muscles, provide a wide range of movement and mobility for the upper limb.
| Characteristics | Values |
|---|---|
| Muscles responsible for protraction | Serratus anterior, pectoralis major, and pectoralis minor |
| Muscles responsible for retraction | Trapezius, rhomboids, and latissimus dorsi |
| Muscles responsible for elevation | Trapezius, levator scapulae, and rhomboid muscles |
| Muscles responsible for depression | Latissimus dorsi, serratus anterior, pectoralis major and minor, and trapezius |
| Muscles responsible for upward rotation | Trapezius and serratus anterior |
| Muscles responsible for downward rotation | Latissimus dorsi, levator scapulae, rhomboids, and pectoralis major and minor |
| Muscle synergists for upward rotation | Upper and lower trapezius and the serratus anterior |
| Intrinsic muscles of the scapula | Rotator cuff muscles, teres major, subscapularis, teres minor, and infraspinatus |
| Extrinsic muscles of the scapula | Triceps, biceps, and deltoid |
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What You'll Learn
The serratus anterior muscle
The serratus anterior is a key scapular stabiliser, keeping the shoulder blades against the rib cage during rest and movement. It is the prime mover in scapular protraction and upward rotation, acting in concert with the upper and lower fibres of the trapezius muscle to enable overhead lifting. When the shoulder blade is fixed, the serratus anterior acts as an accessory inspiratory muscle, lifting the rib cage and supporting breathing.
The serratus anterior is occasionally called the "big swing muscle" or the "boxer's muscle" due to its role in protraction of the scapula, the pulling of the scapula forward and around the rib cage that occurs when throwing a punch. It is also involved in shoulder abduction, the lateral movement of the scapulae away from the spine.
Weakness in the serratus anterior can lead to scapula winging, where the medial side of the scapula tilts forward, resembling a wing. This can be caused by damage to the long thoracic nerve, which supplies the serratus anterior and travels inferiorly on its surface. This nerve can be vulnerable during certain types of surgery, such as lymph node clearance from the axilla for breast cancer.
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The pectoralis major muscle
The pectoralis major is divided into three heads: the clavicular, sternal (sternocostal), and abdominal. The clavicular head originates in the medial third to two-thirds of the clavicle and upper sternum, while the sternocostal head originates in the lower sternum, external oblique aponeurosis, and the costal cartilage of ribs 1-6. The tendons of these two heads twist around each other, forming two distinct laminae. The anterior lamina is the tendon of the clavicular head, and the posterior lamina is the tendon of the sternocostal head. The posterior lamina of the sternocostal head turns 90 degrees before inserting into the humerus, while the anterior lamina of the clavicular head inserts into the caudal region.
The pectoralis major is innervated by the medial and lateral pectoral nerves, which originate from the medial and lateral cords, respectively. The medial pectoral nerve innervates the lateral sternocostal head, as well as the pectoralis minor muscle, while the lateral pectoral nerve innervates the clavicular head and medial sternocostal head. Both nerves enter the medial aspect of the pectoralis major deep into the muscle. The primary function of the pectoralis major is the internal rotation and adduction of the humerus. It is also capable of humeral flexion when the arm is in a position of extension posterior to the coronal plane of the thorax.
Injuries to the pectoralis major muscle are uncommon but have become more prevalent in the last 20 years due to increased participation in weight lifting. A classic patient with a pectoralis major injury is a muscular male between 20 and 40 years old, who often recollects a tearing sensation and an audible pop, followed by immediate pain, localized swelling, weakness, and ecchymosis in the axilla and upper arm.
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The pectoralis minor muscle
Furthermore, the pectoralis minor muscle may act as an accessory muscle of inspiration, aiding in respiration. It is also involved in the depression of the pectoral girdle at the acromioclavicular and sternoclavicular joints. Understanding the anatomy and functions of the pectoralis minor muscle is essential for various physiological and clinical contexts.
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The trapezius muscle
The trapezius is one of the superficial extrinsic back muscles, along with the latissimus dorsi, rhomboid major and minor, and levator scapulae muscles. It has several origin points along the midline of the posterior neck and back, extending from the skull and vertebral column to the shoulder girdle. The upper fibres of the trapezius act with the levator scapulae to produce an elevation of the scapula at the scapulothoracic joint.
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The rhomboid muscles
The rhomboid minor is a small, cylindrical muscle that originates from the nuchal ligament and the spinous processes of the seventh cervical (C7) and first thoracic (T1) vertebrae. It extends obliquely and inserts into the base of the medial end of the spine of the scapula. The rhomboid major, on the other hand, is a broad, quadrilateral muscle that originates from the spinous processes of the second to fifth thoracic vertebrae (T2-T5). It also extends obliquely and inserts into the medial border of the scapula, between the inferior angle and the root of the spine of the scapula.
The main function of the rhomboid muscles is scapular retraction, which involves sliding the scapula superiorly and medially along the trunk. This movement helps to stabilize the scapula and reinforce the shoulder. Additionally, the rhomboids also work with other muscles to elevate and protract the scapula, contributing to its overall stability and positioning.
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Frequently asked questions
The scapula, commonly referred to as the shoulder blade, is a bone that sits above the rib cage in the upper back.
The scapula can move in six directions: protraction, retraction, elevation, depression, upward rotation, and downward rotation.
The serratus anterior, pectoralis major, and pectoralis minor muscles are responsible for protraction.
The scapula allows for a full range of motion in the shoulder joint, one of the most mobile and versatile joints in the body.
The intrinsic muscles of the scapula include the rotator cuff muscles (supraspinatus, infraspinatus, subscapularis, and teres minor), teres major, and levator scapulae.
