It's not a fitness myth: There really is such a thing as muscle memory, although it may not mean exactly what you think it does.
Although people colloquially refer to never forgetting how to ride a bike, throw a baseball or serve a tennis ball as examples of "muscle memory," those comeback skills actually stem from a motor learning process. In other words, they are "motor memory." When you learn how to perform these movements well and can do them automatically without conscious thought, that information becomes encoded in the brain, so in the future, "you still have the fundamental coordination that you learned," explained David Behm, a university research professor in the School of Human Kinetics and Recreation at the Memorial University of Newfoundland in Canada.