National Certification Board for Therapeutic Massage & Bodywork (NCBTMB) Practice Exam

The deltoid muscle is primarily responsible for shoulder flexion due to its specific anatomical placement and function. It is a large, triangular muscle that covers the shoulder joint and has three distinct parts: anterior, lateral, and posterior. The anterior fibers of the deltoid are particularly active during shoulder flexion, which occurs when the arm is raised forward in front of the body.

During this movement, the deltoid works synergistically with other muscles such as the pectoralis major, but its primary role in initiating and controlling the motion of lifting the arm straight up and forward makes it the key muscle involved in shoulder flexion. The deltoid's well-developed structure around the shoulder joint is optimally designed to facilitate this action.

While other muscle groups like the trapezius, latissimus dorsi, and serratus anterior do play roles in shoulder movements, they are not primarily associated with shoulder flexion. The trapezius contributes to various movements of the shoulder and upper back, the latissimus dorsi is more involved in shoulder extension and adduction, and the serratus anterior primarily aids in scapular protraction and rotation. Therefore, the deltoid is the most relevant muscle group for shoulder flexion.