Adaptation is your body's physiological response to training. According to sharecare.com, the principle of adaptation refers to "the process of the body getting accustomed to a particular exercise or training program through repeated exposure. As the body adapts to the stress of the new exercise or training program, the program becomes easier to perform and explains why beginning exercisers are often sore after starting a new routine, but, after doing the same exercise for weeks and months at the same intensity, the exerciser experiences little, if any, muscle soreness. This reinforces the need to constantly vary the exercise and training routine if you want to maximize your results."
When you do new exercises or load your body in a different way, your body reacts by increasing its ability to cope with that new load.
It typically takes anywhere from four to six weeks for your body to adapt to a training method, so it's a good idea to change the frequency, intensity, time and type of exercise you do periodically to avoid plateaus and continue challenging your body.
Click here to watch a video and learn more about Adaptation Cycles.