I purchased a book, Maximum Muscle Plan, and it continuously urges that you can't gain muscle and do extensive cardio at the same time. Basically, you have to devote more of your time to either one at a given point in your training. my question is that in the book it states that a really good alternative to cardio is to do circuit training: choose 10 random weight training exercises, reduce the weight to a weight u can do at least 20 reps of. Then, you go to the first routine, do 10 reps, go to the next routine and do 10 reps, and continue through the whole 10 exercises; then, once you are done, repeat the process 3 or 4 times. The book says that this is a good way of getting cardio and building muscle. Is it as effective as normal cardio, and if so, should i incorporate circuit training into my workout instead of cardio?