Which muscle contributes mainly to the horizontal abduction of the humerus?

The posterior deltoid muscle plays a significant role in the horizontal abduction of the humerus. This movement occurs when the arm is raised to the side and then moved backward, away from the midline of the body. The posterior fibers of the deltoid are specifically oriented to facilitate this action, providing the necessary force to pull the humerus in a horizontal direction.

In contrast, the medial deltoid primarily assists with shoulder abduction rather than horizontal abduction. The rectus abdominis is involved in flexing the trunk and stabilizing the pelvis but does not have a direct role in shoulder movements. Similarly, the erector spinae is primarily responsible for extending the spine and maintaining an upright posture, with no influence on the horizontal abduction of the humerus. Therefore, the contribution of the posterior deltoid to horizontal abduction is crucial in understanding shoulder mechanics and exercise movements that involve this action.