This principle is also known as SAID: Specific Adaptation to Imposed Demands. Explains Kent Edwards, Master Trainer (NASM) at Crunch Fitness in NYC, "A plateau in your workout is similar to driving a car up a hilly and windy road. The first time, it's difficult to maneuver through inclines and sharp turns; but the more you drive the same road, it gets easier to anticipate each turn or hill in conjunction with using the gas, brake, and steering wheel. Eventually, doing the drive is easy because your mind and body are prepared for the obstacles ahead."
Sometimes, however, the reason for a fitness standstill is less scientific. "A plateau also has a lot to do with your motivation and focus," says Helen Macey, personal trainer at New York Health & Racquet Club. "If your workout is no longer delivering your desired physical transformation, it's discouraging. We call it a plateau, but it's largely a lack of that initial drive we once had."