If you are out of shape, you know it. Even climbing a flight of stairs can leave you out of breath! If you haven't been exercising regularly, even a little exercise, like going up steps, can put a strain on your body.

John Foxx/Stockbyte/Getty Images Marathons require strength, power, and endurance.

Your body responds to exercise with a coordinated effort from many systems, including working muscles, the heart, lungs, blood vessels, skin, brain and others. Here are what the different parts of your body do when you exercise:

• The muscles provide the strength, power, and endurance to do the exercise.
• Enzymes within the muscles mobilize various fuels to provide ATP to meet the energy demands of working muscle.
• The heart and blood vessels increase the blood flow to deliver more oxygen to the working muscle.
• The lungs increase the rate of breathing to deliver more oxygen to the muscle.

All of these systems can be improved by training. Lack of training causes them to atrophy.

Let's compare the couch potato with the trained athlete. The couch potato has these problems:

• weakened or atrophied muscles
• weakened or atrophied heart muscle
• decreased metabolic enzymes in the muscles
• diminished lung capacity
• the wrong metabolic fuels

How strong are you?You'd probably be amazed to learn how much force your body's joints and muscles actually support on a daily basis. This interactive segment from Discovery takes you inside the body and explains how much strength your bones and muscles really possess.

If muscles are not used regularly, then their mass decreases. The muscle proteins and fibers that develop the force diminish. Therefore, the couch potato simply cannot generate the force required for the physical activity involved in running a marathon.

The heart is a muscle like skeletal muscle. It also adapts to a less active state by losing muscle mass. While this does not affect the couch potato's ability to pump blood to his/her tissues, it limits the ability to increase in blood flow during exercise. The heart will not be able to stretch as far or develop the pressure required to increase the cardiac output. That's why doctors tell you that you need to exercise regularly to keep your heart in shape.

The enzymes involved in anaerobic (without oxygen) and aerobic (with oxygen) metabolism help provide the energy to working muscle. In the couch potato, however, the levels of these enzymes go down. So the inactive person's body cannot metabolize fuel as well.

The fuels used in the body during exercise mainly come from ­carbohydrates (glucose, muscle glycogen) and some fat. However, the couch potato has mostly fat and probably little glycogen. It takes longer to mobilize fat as a fuel than glycogen. That means that the breakdown of fat cannot keep pace with the energy demands of working muscle.

All of these factors combine to limit the ability of the couch potato to exercise. The good news is that, with a moderate exercise program, the couch potato or anyone else can improve their fitness and their body's response to exercise. At least enough to do a 10 km run!

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