P90X - Chest and Back: Superset two of the body's largest muscle groups for a lactic acid bath like no other! You'll be fully pumped five minutes into the program, and your muscles will beg for mercy the rest of the way. Your body will learn how to keep it together under duress as you struggle to meet your target number of reps during round two. And if you feel a little queasy after your set, at least you'll know you're not alone.