Which of the following muscles is the prime mover for flexion of the arm at the shoulder?

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

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the concept of 'prime mover' in muscle actions. The prime mover, also known as the agonist, is the muscle primarily responsible for producing a specific movement. In this case, the movement is flexion of the arm at the shoulder.
2. Step 2: Recall the anatomy of the shoulder joint and the muscles involved in arm flexion. Flexion refers to moving the arm forward and upward, decreasing the angle between the arm and the torso.
3. Step 3: Analyze the function of each muscle listed in the options: - Latissimus dorsi: This muscle is primarily involved in extension, adduction, and medial rotation of the arm, not flexion. - Deltoid (posterior fibers): These fibers are responsible for extension and lateral rotation of the arm, not flexion. - Trapezius: This muscle primarily stabilizes and moves the scapula, not directly involved in arm flexion. - Pectoralis major: This muscle is responsible for flexion, adduction, and medial rotation of the arm at the shoulder joint.
4. Step 4: Focus on the pectoralis major. This muscle originates from the clavicle, sternum, and ribs, and inserts into the humerus. Its contraction pulls the arm forward and upward, making it the prime mover for flexion of the arm at the shoulder.
5. Step 5: Conclude that the pectoralis major is the correct answer based on its anatomical function and role as the prime mover for arm flexion at the shoulder joint.