How Muscles Work
When you exercise, your body does not actually create new muscles. Instead, your existing muscle cells grow larger and stronger, and the number of capillaries — the networked blood vessels between arterioles and venules — increases. With regular exercise, muscles also develop more mitochondria — this is where biochemical processes of respiration and energy production occur in the cell. The result is larger, more defined muscle mass, not newly created muscle tissue.
From Exercise to Inactivity
Adopting a sedentary — or inactive — lifestyle has the opposite effect on your muscles. The increased blood flow previously needed to fuel your cells during exercise is no longer required, and your body begins to contract and reduce the size of your capillaries. Muscles don’t disappear or turn to fat, but rather shrink and decrease in mass. Fat may be produced if your diet provides your body with more calories than you require for the level of activity you maintain.
While stopping exercise may decrease the size of your muscles, extremely poor nutrition, starvation and disease can cause muscle atrophy, where muscles can completely waste away. Without the calories, vitamins and nutrients of healthy food, your body is thrust into a state of malnutrition. Not only does this cause permanent damage to other organs, but it can also lead to death.
Maintaining Your Muscles
Keeping active and following a healthy diet means maintaining your muscle mass and preventing the accumulation of body fat. A few pushups, situps and stretches each day help keep your muscles active and your body limber. Outside of weightlifting tournaments or training for a triathlon, devoting a couple hours a day, just a few days a week, means having the rest of the week off. Spend just 10 to 20 minutes during your off days to keep those muscles awake.