The pec deck, or butterfly machine, trains the pectorals through a controlled arm-adduction motion. Sitting upright with the chest against the pad, users bring the arms together along a fixed arc. This movement emphasizes the inner chest and helps develop the sternal line. It is a low-risk isolation exercise suitable for beginners and advanced lifters seeking chest definition. The machine removes momentum and allows strict form. It can be used as a pre-exhaust exercise before pressing or as a finisher to pump blood into the chest muscles.