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The primary mover for shoulder extension is the Latissimus Dorsi. This muscle originates from several areas, including the lower six thoracic vertebrae, the lumbar vertebrae, and the iliac crest, and it inserts into the humerus. Its primary function is to extend, adduct, and medially rotate the arm. When the arm is raised above the head and then moved backward, the Latissimus Dorsi is heavily engaged, making it the key muscle involved in the movement of shoulder extension.

The other muscles listed do not serve as primary movers in shoulder extension. The Middle Deltoid primarily assists in shoulder abduction. The Internal Oblique mainly functions in trunk rotation and lateral flexion rather than shoulder movements. The Erector Spinae, a group of muscles located along the spine, is primarily involved in spinal extension and stability and does not play a significant role in shoulder movements. Thus, the Latissimus Dorsi is rightly recognized as the most significant muscle during shoulder extension activities.