Adaptation is the way the body 'programs' muscles to remember particular activities, movements or skills. By repeating that skill or activity, the body adapts to the stress and the skill becomes easier to perform. The Principle of Adaptation explains why a beginning exercisers are often sore after starting a new routine, but after doing the same exercise for weeks and months the athlete has little, if any, muscle soreness. This also explains the need to vary the routine and continue to apply the Overload Principle if continued improvement is desired.
Also see: The Principles of Sports Conditioning