Adaptation to Exercise
The Principle of Adaptation states that when we undergo the stress of physical exercise, our body adapts and becomes more efficient. It’s just like learning any new skill; at first it’s difficult, but over time it becomes second-nature. Once you adapt to a given stress, you require additional stress to continue to make progress.
There are limits to how much stress the body can tolerate before it breaks down and risks injury. Doing too much work too quickly will result in injury or muscle damage, but doing too little, too slowly will not result in any improvement. This is why personal trainers set up specific training programs that increase time and intensity at a planned rate and allow rest days throughout the program.