A popular consumer-oriented fitness program claims to be based on the science of “muscle confusion.” This is simply a marketing term created to describe the physiological effect of periodization, which is a method of organizing exercise programs based on alternating periods of intensity. The concept of periodization was developed by Soviet Union sport scientists who recognized that periods of high-intensity exercise (high stress) should be followed by a period of low-intensity exercise (low stress) to let the body to fully recover from the workouts and allow the time for the physiological adaptations to occur.