Peak performance by Andres Ericsson

This is how the body’s desire for homeostasis can be harnessed to drive changes: push it hard enough and for long enough, and it will respond by changing in ways that make that push easier to do. You will have gotten a little stronger, built a little more endurance,developed a little more coordination. But there is a catch ; once the compensator changes have occurred - new muscle fibers have grown and become more efficient , new capillaries have grown , and so on - the body can handle the physical activity that had previously stressed it. It is comfortable again.

The changes stop.
 So to keep the changes happening, you have to keep upping the ante: run farther, run faster , run uphill. If you don’t keep pushing and pushing and pushing some more, the body will settle in homeostasis, albeit at a different level than before,

And you will stop improving.