Why is the Behind the Neck Pulldown believed to place the shoulder in a closed-pack position?

The belief that the Behind the Neck Pulldown places the shoulder in a closed-pack position is primarily due to the anatomical and biomechanical characteristics of the shoulder joint during this movement. In a closed-pack position, the joint surfaces are maximally congruent, and the joint is stabilized by both surrounding muscles and passive structures, such as the joint capsule and ligaments.

When the arms are positioned behind the neck during the pulldown, there is a significant amount of tension placed on the shoulder joint due to the positioning of the scapulae and the alignment of the humerus. This positioning increases the stability of the joint, as the capsule and ligaments are under greater tension, which reduces their ability to lengthen and allows for a more secure fit of the joint surfaces. This is critical for maintaining stability during movements that may place high demands on the shoulder.

Thus, the answer highlights how the shoulder, when in a closed-packed position during the Behind the Neck Pulldown, is quite stable due to the anatomical postures of the surrounding structures.