The Pec is not really an internal rotator.

A degree of internal rotation may be part of the arm path depending on the humeral orientation, but any internal rotational force of the pec is only a faction of the the main force pulling its insertion towards the origin, which is best described as varying degrees/angles of adduction and flexion, but more importantly understanding these movement beyond their nomenclature is the key to understanding functional anatomy, which is what is needed to understand biomechanics and training.
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