http://www.8weeksout.com/2011/03/01/...-of-intensity/
Getting caught up in all the details of training, it can be easy to lose sight of the big picture and forget that your body doesn’t care if you have big or small muscles, a high or low bodyfat percentage, or how many pull ups or kettlebell swings you can do. The only thing your body really cares about is its own survival.
On the most fundamental level, it is this innate drive of the body to stay alive and maintain equilibrium within all of its many systems that creates the opportunity for performance and/or physique improvements to be made. Adaptation to various training stimuli is nothing more than the body’s defense mechanisms at work, plain and simple.
You place a stress on the body by lifting weights, doing some form of cardiovascular exercise, etc., and the body in turn responds to this stress by adapting to it so that it is better suited to handle this stress the next time. The exact adaptations that take place, of course, depend on the specific type of training stress that you put your body under, but the end result is the same. The next time you train, the body is now better equipped to meet the physical demands you are placing on it.
Because of the body’s increased preparation, the same level of training now presents a relatively lower level of physical stress on the body and the body’s defense mechanisms will not be called into action like before. In order to continue to see results, you inevitably have to place a greater stress on the body to once again force it to respond by adapting.
This process of repetitive stress and adaptation provides the underlying foundation for improved performance as well as general health and fitness. And it is this incredibly adaptability of the human body in response to physical stressors that keeps us all alive in the first place.