Pectoralis Major Muscle The Pectoralis Major, commonly referred to as the Pecks, is the largest of the chest muscles. It purpose is to adduct and medially rotate the humerus and has two starting points. The first starting point of the Pectoralis Major is from the collar bone, known as the Clavicular head. The second starting point of the Pectoralis Major is from the center of the rib cage called the Sternocostal head. The Clavicular head moves in a downward fashion, as the Sternocostal moves across and up.   The two muscle heads meet to form a large muscle that ends at the top...