Overview
This lecture introduces the major muscles of the pectoral girdle and shoulder joint, including their origins, insertions, and primary actions on the scapula and humerus.
Muscles of the Pectoral Girdle
- Pectoral girdle consists of the scapula and clavicles; the scapula provides the glenoid cavity for the humerus.
- Muscles that move the pectoral girdle must insert on the scapula.
- Key muscles: rhomboids (major and minor), pectoralis minor, and trapezius.
- Rhomboids originate from thoracic vertebrae and insert on the medial border of the scapula; they adduct and elevate the scapula.
- Pectoralis minor originates on ribs and inserts on the coracoid process; it abducts the scapula.
- Trapezius has multiple origins (occipital bone, cervical and thoracic vertebrae) and inserts on the spine of the scapula; it moves the scapula in all ways except abduction.
Reverse Action Concept
- Reverse action occurs when the fixed insertion allows the muscle to pull the origin.
- Trapezius reverse action can extend the neck by pulling on the occipital protuberance.
Muscles That Move the Shoulder Joint (Humerus)
- Muscles moving the humerus must insert on the humerus.
- Pectoralis major: origin on ribs, sternum, clavicle, inserts on greater tubercle; flexes, adducts, and medially rotates the shoulder.
- Latissimus dorsi: origin on iliac crest, inserts on intertubercular groove; extends and medially rotates the shoulder.
- Teres major: origin on lateral border of scapula, inserts on intertubercular groove; extends and medially rotates the shoulder.
- Teres minor: origin on lateral border of scapula, inserts on greater tubercle; laterally rotates the shoulder.
- Infraspinatus: origin on infraspinous fossa, inserts on greater tubercle; laterally rotates the shoulder.
- Supraspinatus: origin on supraspinous fossa, inserts on greater tubercle; abducts the shoulder.
- Subscapularis: origin on subscapular fossa, inserts on lesser tubercle; medially rotates the shoulder.
- Deltoid: origin on spine of scapula, acromion, and clavicle, inserts on deltoid tuberosity; moves shoulder in all ways except adduction.
Key Terms & Definitions
- Pectoral girdle — Shoulder region made of scapula and clavicle.
- Glenoid cavity — Socket in the scapula for the humerus.
- Adduction — Movement towards the midline of the body.
- Abduction — Movement away from the midline of the body.
- Reverse action — Muscle action where origin moves toward insertion.
Action Items / Next Steps
- Review the origins, insertions, and actions of all muscles covered.
- Practice identifying muscle actions based on attachments.
- Memorize key definitions and terms for upcoming assessments.