The purpose of any form of exercise is to stimulate muscles to contract. All muscles do is contract and relax. They are stupid. They don't know what the stimulus is. The cardiovascular system responds to provide energy to those muscles. Muscles are slaves to the stimulus. The heart is a slave to the muscles.
The Best Bang for your buck is exercise that which stimulates the most muscles in the shortest time. This is Intensity. Running, Zumba, spinning etc will not stimulate anywhere near the quantity of muscle fibers that hard weight training does and in a much shorter time.
However high intensity training is hard, but brief. That's why so many people 'cop out' from the hard stuff and dumb down to exercise protocols that are easier but pretty useless.
