The process of muscle wasting after surgery or casting is called disuse atrophy. Muscles require a balance of enzymes and hormones that is constantly adjusted according to how much the muscle is being used. Unused muscles shrink because this balance is thrown off. The more a muscle is stressed or used, the more muscle-building enzymes are active. When a muscle is not used regularly, muscle-degrading enzymes take over and cause the muscle to shrink. In other words, muscles work on the "use it or lose it" principle.