Which muscle is the prime mover for abduction of the arm at the shoulder (glenohumeral) joint?

• A. Deltoid (middle fibers)
• B. Pectoralis major
• C. Teres major
• D. Latissimus dorsi

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the term 'prime mover' (agonist) — it refers to the muscle primarily responsible for producing a specific movement at a joint.
2. Step 2: Identify the movement in question, which is abduction of the arm at the shoulder (glenohumeral) joint. Abduction means moving the arm away from the midline of the body.
3. Step 3: Recall the main muscles involved in shoulder movements: the deltoid, pectoralis major, teres major, and latissimus dorsi, and understand their primary actions.
4. Step 4: Analyze each muscle's function: the deltoid (especially its middle fibers) is known for abducting the arm; pectoralis major primarily adducts and medially rotates the arm; teres major and latissimus dorsi mainly adduct and medially rotate the arm.
5. Step 5: Conclude that the deltoid muscle, specifically its middle fibers, is the prime mover for abduction of the arm at the shoulder joint.