Muscle atrophy is the term we use to describe muscle that has decreased in size and function. Atrophy specifically means getting smaller. In most cases, muscle atrophy occurs because of decreased use of the muscle. If you don't get much physical activity or exercise at all, the muscles that you are not using will get smaller. Basically the muscles are signaled to adjust their size to whatever they are being asked to do. We see this most prominently, in people that are bed bound and are not using their leg muscles very much. This phenomenon is not limited only to muscle. Bones also lose their strength and density when they are not used. We see this in astronauts that return from zero gravity for just a week.
I think that you may benefit from an exercise program. Exercise not only prevents your muscles from becoming atrophied, it also helps prevent weight gain and heart disease. Most people would benefit from a half hour a day of aerobic exercise that increases the heart rate.
I suggest that you schedule an appointment with your primary care physician
. He or she can take discuss the benefits of exercise and the specific things you can do to prevent muscle atrophy from occurring to you.