Which muscle is primarily affected during shoulder abduction?

Shoulder abduction primarily involves the deltoid muscle, which is the main muscle responsible for lifting the arm away from the body. The deltoid is a large, triangular muscle that covers the shoulder joint and functions to raise the arm in a lateral direction. It has three distinct parts: the anterior (front), lateral (middle), and posterior (back) fibers, with the lateral fibers being the most active during abduction.

While other muscles such as the trapezius might assist in shoulder movement and stabilizing the shoulder girdle, they do not perform abduction in the same direct manner as the deltoid. The biceps brachii primarily acts in elbow flexion and supination and is not involved in shoulder abduction. The pectoralis major, although it contributes to several shoulder movements, primarily functions in flexion, adduction, and internal rotation rather than abduction. Therefore, the deltoid is the muscle that is primarily engaged during shoulder abduction, making it the correct answer.