Which of the following is the major abductor muscle of the upper arm?

• A. Biceps brachii
• B. Deltoid
• C. Latissimus dorsi
• D. Pectoralis major

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the term 'abductor muscle'—an abductor muscle is responsible for moving a limb away from the midline of the body. In this case, the upper arm is being moved away from the torso.
2. Step 2: Review the anatomy of the upper arm and shoulder region. The major muscles involved in arm movement include the deltoid, biceps brachii, latissimus dorsi, and pectoralis major.
3. Step 3: Analyze the function of each muscle: - The deltoid muscle is primarily responsible for abduction of the arm at the shoulder joint. - The biceps brachii is involved in flexion and supination of the forearm, not abduction. - The latissimus dorsi is responsible for adduction, extension, and internal rotation of the arm. - The pectoralis major is involved in adduction and flexion of the arm.
4. Step 4: Identify the muscle that matches the function of abduction. Based on the analysis, the deltoid muscle is the major abductor of the upper arm.
5. Step 5: Confirm the answer by recalling that the deltoid muscle has three distinct parts (anterior, middle, and posterior fibers), with the middle fibers being primarily responsible for arm abduction.