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Deep Muscles

  • Front (anterior) muscles of the shoulder:
    • The pectoralis minor muscle is a small triangular shaped muscle that lies deep to pectoralis major muscle and passes as three muscular slips from the thoracic wall (ribs III to V) to the coracoid process of the scapula. Pectoralis minor draws the scapula forward and downward, and raises the ribs in forced inspiration.
    • The subclavius muscle is a small muscle that lies deep to pectoralis major muscle. It passes from rib I at the junction between the rib and its costal cartilage to a groove on the inferior (lower) surface of the clavicle. It depresses the clavicle, draws the shoulder forward and downward, and steadies the clavicle during movements of the shoulder girdle.
    • The levator scapulae muscle originates from the transverse processes of the cervical vertebra and descends to attach to the superior (uppermost) and medial (near the midline) corner of the scapula. This muscle elevates the scapula.
    • Rhomboid major and minor muscles attach from the vertebral column to the medial border of the scapula, slightly below the levator scapulae muscles. These muscles retract and elevate the scapula.
    • Teres major muscle originates from posterior surface of the inferior angle of the scapula and attaches the medial lip of the intertubercular sulcus which lies on the anterior surface of the humerus. This muscle extends and medially rotates the humerus.

  • Side (lateral) muscles of the shoulder:
    • Serratus anterior muscle originates as a number of muscular slips from the outer surfaces and superior borders of the first eight or nine ribs, and fascia covering the first intercostal spaces (spaces between each rib). They then form a flattened sheet which passes around the thoracic wall and attaches to the anterior (costal surface, that glides over the ribs) of the medial border of the scapula. The serratus anterior pulls the scapula forward of the the thoracic wall and rotates the scapula for abduction and flexion of the arm.

  • The rotator cuff tendons attach to the deep rotator cuff muscles. These 4 muscles are involved in raising the arm from the side and rotating the shoulder in the many directions. The rotator cuff mechanism also helps keep the shoulder joint stable by holding the humeral head in the glenoid socket.
    • The supraspinatus muscle originates and sits in the supraspinatus fossa of the scapular. The muscle passes as a tendon under the acromion, where its is separated from the bone by a subacromial bursa and then passes over the glenohumeral joint and inserts on the superior facet of the greater tubercle. The supraspinatus muscle weakly flexes the arm, aids deltoid in abduction of the arm and draws the humerus toward the glenoid fossa. It also prevents deltoid from forcing the humerus up against the acromion.
    • Infraspinatus muscle originates and sits in the infraspinous fossa of the scapula. The tendon of the infraspinatus passes posteriorly on to the glenohumeral join and inserts on to the middle facet of the greater tuberosity of the humerus and capsule of the shoulder joint. As a rotator cuff muscle, infraspinatus draws the humerus towards the glenoid fossa, thus resisting posterior dislocation of the arm. It also laterally rotates and abducts the arm.
    • The teres minor muscle originates from the posterior axillary boarder of the scapula and inserts through the capsule of the shoulder joint, and the lower facet of the greater tuberosity of the humerus. This muscle laterally rotates the arm, but only weakly adducts the arm. 

    • Subscapularis forms the largest component of the posterior wall of the axilla (area on the body directly under the joint where the arm connects to the shoulder). It originates from and fills the subcapular fossa on the anterior surface of the scapula and inserts on the lesser tuberosity of the humerus, and part of the capsule of the shoulder joint. This muscle medially rotates the arm, and stabilizes the glenohumeral joint.

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