how to use the pec deck machine properly?

to contract a muscle fully and correctly, we need to know what it does and where the attachments are. when one of the actions of the chest(pectoralis major) is the internal rotation of the shoulder.

it is a bad idea to externally rotate our shoulder and try to contract it. we are not only limiting our range of motion, we are also injuring the muscle and connective tissues while also putting our shoulders in a vulnerable position if we do not have the proper flexibility.