Overview
This lecture covers the major muscles of the anterior thorax and abdominal wall, including their functions, locations, and relationships as synergists or antagonists.
Muscles of the Anterior Thorax
- Pectoralis major flexes, adducts, and medially rotates the arm at the shoulder joint.
- Synergists of pectoralis major: coracobrachialis (flexion, adduction), latissimus dorsi (adduction), teres major, subscapularis (medial rotation).
- Antagonists of pectoralis major: deltoid and supraspinatus (abduction), infraspinatus and teres minor (lateral rotation), teres major and latissimus dorsi (extension).
- External intercostals elevate ribs for inhalation (breathing in).
- Internal intercostals depress ribs for forced exhalation (breathing out); antagonists to external intercostals.
- Serratus anterior protracts (pulls forward) the scapula; synergist is pectoralis minor, antagonists are trapezius and rhomboids.
- Pectoralis minor lies deep to pectoralis major; protracts the scapula; origin: ribs 2–5, insertion: coracoid process.
- Subclavius depresses the clavicle.
Muscles of the Abdominal Wall (Superficial to Deep)
- External oblique flexes the vertebral column, compresses abdomen, and increases intra-abdominal pressure for forced exhalation.
- Internal oblique (beneath external oblique) also flexes vertebral column and compresses abdomen for exhalation.
- Rectus abdominis (“six-pack” muscle) flexes vertebral column and compresses abdomen for exhalation.
- Transversus abdominis (deepest) only compresses abdominal contents to increase intra-abdominal pressure; does not flex the vertebral column.
- Linea alba is a connective tissue structure along the midline where abdominal muscles attach.
- Pyramidalis is a small muscle that tenses the linea alba.
Key Terms & Definitions
- Synergist — muscles that work together to perform the same action.
- Antagonist — muscles that oppose each other's actions.
- Protract — to move a body part forward.
- Elevate/Depress — to raise or lower a body part.
- Flexion — decreasing the angle of a joint.
- Adduction/Abduction — movement toward/away from the midline.
Action Items / Next Steps
- Review and memorize the locations, functions, and antagonistic/synergistic relationships of the anterior thorax and abdominal wall muscles.
- Study the order of abdominal wall muscles from superficial to deep.
- Learn key muscle origins and insertions for better understanding.