Which muscle is the prime mover of arm abduction when all its fibers are used?

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

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the concept of a 'prime mover' in muscle actions. A prime mover, also known as an agonist, is the muscle primarily responsible for generating a specific movement.
2. Step 2: Review the anatomy and function of the deltoid muscle. The deltoid is a triangular-shaped muscle located on the shoulder. It has three distinct sets of fibers: anterior, middle, and posterior. When all fibers contract together, the deltoid is responsible for arm abduction, which is the movement of the arm away from the midline of the body.
3. Step 3: Compare the deltoid's function to the other muscles listed in the problem. The biceps brachii is primarily involved in elbow flexion and forearm supination, not arm abduction. The pectoralis major is responsible for arm flexion, adduction, and medial rotation, while the latissimus dorsi is involved in arm extension, adduction, and medial rotation.
4. Step 4: Recognize that arm abduction requires a muscle that can lift the arm away from the body laterally. The deltoid, specifically its middle fibers, is uniquely suited for this action when all fibers are engaged.
5. Step 5: Conclude that the deltoid is the prime mover for arm abduction when all its fibers are used, as it is anatomically and functionally designed for this movement.