As a personal trainer I frequently get asked, "does muscle turn into fat?"
If you stop exercising your muscles will NOT turn to fat. BUT, why would you ever stop?!
While, it’s simple to think of it that way, it’s more complex than muscles becoming fat. You will indeed lose muscle due to a sudden decrease in physical activity, and you may gain weight, but they aren't necessarily a result of one another because you can lose muscle without gaining fat and you can gain fat without losing muscle.
Your body fat percentage will change, which is a better indicator of the muscle to fat ratio in one’s body. If you stop your resistance training it will affect the body in many ways including the decrease in ones metabolism and the amount of energy expended each day. Muscle tissue is “active tissue”. In short, this means that the more muscle tissue you have the more calories your body burns while at rest. Diet plays a huge part in this equation as well. Does your caloric intake decrease as your physical activity does? Or does it stay the same? In the end, a stop in physical activity will more than often result in a decrease in muscle and an increase in fat, but they aren't equally or strictly inversely related.