Myth: Muscle turns to fat when you stop training (just had to throw this in the mix).
Truth: This is quite impossible. Muscle and fat are two different things altogether. What is more likely, is that when you stop training, you gain fat from your diet. This happens because the calories consumed is now greater than the calories burnt, due to lack of exercise. Also when you stop exercising you tend to stop paying attention to your diet and consume larger amounts of calories. The result is that muscles start to atrophy and fat covers whatever is left. This gives the illusion of muscle turning to fat. Muscle cannot turn into fat and fat cannot become muscle.
P.S. The healthy goat was last seen at Hogwarts looking for a good magic wand to convert fat into muscle.