Which of the following actions does not involve the shoulder joint directly?

The action of flexion of the spine does not involve the shoulder joint directly. This movement pertains specifically to the bending of the spine forward, which is primarily facilitated by the muscles supporting the vertebral column, such as the rectus abdominis and the erector spinae. It focuses on the spinal movement rather than influencing the shoulder joint.

In contrast, elevation of the scapula, horizontal adduction of the humerus, and adduction of the humerus directly involve the shoulder joint. Elevation of the scapula involves the muscles such as the trapezius and elevator scapulae lifting the shoulder blades upwards, indirectly affecting the shoulder region. Horizontal adduction and adduction of the humerus involve movements of the upper arm towards the center of the body and are directly associated with the shoulder joint's range of motion and muscle actions within that area.