Which muscle acts as the prime mover for arm flexion at the shoulder joint?

• A. Deltoid (posterior fibers)
• B. Latissimus dorsi
• C. Pectoralis major
• D. Trapezius

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the concept of a prime mover (agonist) in muscle actions. The prime mover is the muscle primarily responsible for producing a specific movement at a joint.
2. Step 2: Identify the movement described in the problem: arm flexion at the shoulder joint. Arm flexion involves moving the arm forward and upward in the sagittal plane.
3. Step 3: Review the anatomy and function of the muscles listed in the problem. For example: - Pectoralis major: A large chest muscle responsible for flexion, adduction, and medial rotation of the arm at the shoulder joint. - Deltoid (posterior fibers): These fibers are involved in extension and lateral rotation, not flexion. - Latissimus dorsi: This muscle is responsible for extension, adduction, and medial rotation of the arm, not flexion. - Trapezius: Primarily involved in scapular movements, such as elevation, depression, and retraction, not arm flexion.
4. Step 4: Match the muscle's function to the movement described. Since arm flexion at the shoulder joint is the focus, the pectoralis major is the muscle that acts as the prime mover for this action.
5. Step 5: Conclude that the pectoralis major is the correct answer based on its anatomical role in arm flexion at the shoulder joint.