I first heard about this concept from Joe DeFranco a couple of years ago. He brought up an interesting concept about how you should mix up cardio exercises constantly in an attempt to actually reduce your neurological efficiency. What did he mean by that? Well, as you train you get better at doing an exercise and as you get better at doing an exercise you become more and more efficient at using your muscles (not wasting energy). Now, this seems like a good thing, but if you are simply using the exercise as a means to get your heart rate up, then being efficient is actually detrimental. If you become efficient, you theoretically will burn fewer calories for a given rep and have a lower heart rate response… So if the goal is to increase your heart rate it starts to make sense that neurological efficiency becomes a bad thing. This is where Coach Joe Defranco proposed the idea of mixing up cardio. This way, the body never really gets better at doing the exercise, heart rate response remains high and you actually have a larger metabolic demand per rep.