One of the secrets to continued muscle growth is to prevent your muscles from adapting too quickly. Doing repetitive movements over and over habituates your muscles fairly quickly and they adapt and stop growing. The P90X workout calls this strategy muscle confusion. You want to be sure that you engage fast twitch fibers which stimulate growth and burn fat. Explosive movements, such as those used in the Insanity workout activate fast twitch fibers which makes muscles stronger and grown even though it is mainly a cardio fitness program.Both workout also work your legs a lot. Many people. especially guys go top heavy and train just the muscles that look good in t-shirts. By training your legs more, you will become a better athlete and over all physical specimen.