The fact that the pectoralis major can adduct the arm at the shoulder joint when the arm is in anatomic position but abduct the arm if it is already abducted above 100 degrees is an example of the:

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Quizplus · Accepted Answer

The changing line of pull of the muscle relative to the joint is the best choice to explain this phenomenon. As the arm is abducted above 100 degrees, the line of pull of the pectoralis major becomes more horizontal, allowing it to abduct the arm instead of adducting it. This is an example of how the direction of a muscle's pull can change depending on the position of the joint.

The Fact That the Pectoralis Major Can Adduct the Arm