Which movement is most likely an action of the pectoralis major muscle?
a. Abduction of humerus
b. Retraction of scapula
c. Downward depression of scapula
d. Adduction of humerus
The correct answer and explanation is:
Correct Answer: d. Adduction of humerus
Explanation:
The pectoralis major is a large, fan-shaped muscle located on the front of the chest. It plays a major role in moving the humerus (upper arm bone) and contributes to several important motions of the shoulder joint.
Anatomy of the Pectoralis Major:
- Origin: Clavicle (collarbone), sternum (breastbone), and the cartilage of the first six ribs.
- Insertion: Lateral lip of the bicipital groove of the humerus.
- Innervation: Medial and lateral pectoral nerves.
Main Actions of the Pectoralis Major:
- Adduction of the Humerus: The pectoralis major pulls the arm toward the body’s midline. This is called adduction. For example, when you bring your arms down to your sides from an outstretched position, this muscle is heavily involved.
- Medial Rotation of the Humerus: It helps rotate the arm inward.
- Flexion of the Humerus: Especially from an extended position (e.g., lifting your arm in front of your body).
- Horizontal Adduction: Such as when pushing something forward like in a bench press.
Why the Other Options Are Incorrect:
- a. Abduction of humerus: This is the opposite of adduction and is mainly performed by the deltoid and supraspinatus muscles.
- b. Retraction of scapula: Controlled by muscles like the rhomboids and trapezius.
- c. Downward depression of scapula: Done by muscles like the lower trapezius and pectoralis minor (not major).
Summary:
The pectoralis major muscle adducts the humerus and brings it toward the body’s centerline. Among the choices, adduction of the humerus (option d) best represents the function of this muscle.