Humans are very dynamic in nature. We are not static, and our physiology is adapting to the imposed demands of life on the planet on a minute-to-minute basis. This being the case, if we are not adapting, then we need to understand why. The body requires certain critical parameters - much like a dosage of medicine - to stimulate an adaptation process. If we are not adapting, and changes are not occurring, then we are either not applying the correct loads OR there is some form of limiting factor present.