To begin with, muscle can never turn into fat and fat can never turn into muscle. It is impossible.

Lack of usage to a muscle causes the muscle to become small and weak. Lack of movement to the body causes more fat to store on the body.

When we have weak muscles it is harder to perform our daily tasks as well as any extra activities we like to do. As we carry more fat on our bodies (especially combined with weak muscles) we add more stress to our joints and bones. This is a frequent cause of back and joint aches.

Fat is a body tissue, which requires very few calories to remain on the body. Muscle is an active tissue that is constantly burning calories even at rest. The less fat and more muscle on the body, the more calories the body will burn in daily activities, including sitting and watching television.