Hawaii Massage License Practice Exam

Which muscle is primarily responsible for arm abduction?

1. Deltoid

2. Pectoralis major

3. Triceps

4. Biceps

The correct answer is: Deltoid

The deltoid muscle is primarily responsible for arm abduction due to its anatomical position and structure. This muscle is located on the outer aspect of the shoulder and has three distinct parts: the anterior (front), lateral (middle), and posterior (back) fibers. The lateral part of the deltoid is particularly important for the movement of lifting the arm away from the body, which is known as abduction. When the deltoid contracts, it pulls the humerus (the bone of the upper arm) away from the torso, allowing the arm to move laterally. This action is crucial in various activities, such as raising the arm to the side or reaching upward. The effectiveness of the deltoid muscle in abduction is further enhanced by the rotator cuff muscles, which stabilize the shoulder joint during this movement. In contrast, the other muscles mentioned have different primary functions. The pectoralis major is mainly involved in arm flexion, adduction, and rotation, while the triceps and biceps primarily focus on extending and flexing the elbow joint, respectively. These distinct roles highlight the specialized function of the deltoid in arm abduction.