Name the primary muscle involved in shoulder abduction.

The primary muscle involved in shoulder abduction is indeed the deltoid. This muscle has three distinct parts—anterior, lateral, and posterior—that work together to perform shoulder movements. The lateral portion of the deltoid is particularly crucial for initiating and carrying out shoulder abduction, which is the movement of the arm away from the body in the frontal plane.

While the supraspinatus also plays a role in shoulder abduction, particularly at the beginning of the movement, it acts as a synergist rather than the primary mover. The trapezius and latissimus dorsi respectively have different primary functions related to scapular movement and shoulder adduction or extension. The deltoid, by contrast, is designed specifically for the abduction movement, making it the strongest and most significant muscle for this action.