This is one of the most damaging myths that ever reared its ugly head. 95% of the pros will tell you that the biggest bodybuilding mistake they ever made was to over-train–and this happened even when they were taking steroids. Imagine how easy it is for the natural athlete to overtrain! When you train your muscles too often for them to heal, the end-result is zero growth and perhaps even losses. Working out every day, if you’re truly using the proper amount of intensity, will lead to gross overtraining. A body part, worked properly, i.e. worked to complete, total muscular failure that recruited as many muscle fibers as physiologically possible, can take 5-10 days to heal.

To take it a step further, even working a different body part in the next few days might constitute overtraining. If you truly work your quads to absolute fiber-tearing failure, doing another power workout the next day that entails heavy bench-presses or deadlifts is going to, in all probability, inhibit gains. After a serious leg workout, your whole system mobilizes to heal and recover from the blow you’ve dealt it. How, then, can the body be expected to heal from an equally brutal workout the next day? It can’t, at least not without using some drugs to help deal with the catabolic processes going on in your body [and even they’re usually not enough .]

Learn to accept rest as a valuable part of your workout. You should probably spend as many days out of the gym as you do in it.