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Most types of cells—including those of the heart, nerves, and brain—can produce energy only aerobically. That is why a constant supply of oxygen to these cells is necessary. For example, if the delivery of oxygen to a portion of the heart is stopped, that area of the heart suffers a heart attack or myocardial infarction. If the brain stops getting oxygen, a stroke occurs in the deprived area. Nearly all cells in the body require a constant supply of oxygen. On the other hand, the muscles of your body can create energy with (aerobic) or without (anaerobic) oxygen. However, the muscles recover from anaerobic work such as strength training or a hard sprint, only when enough oxygen (aerobic energy production) is available during recovery after the hard effort. Oxygen must be available so that the depleted anaerobic energy stores such as adenosine triphosphate (ATP) and creatine phosphate (CP), can be replenished. So you see, even anaerobic exercise is ultimately dependent on the ability of your body to deliver and use oxygen. When you say you're "aerobic" or "anaerobic," these words actually refer to the muscles being used for the activity. This is true since the energy for muscle contraction and movement is produced inside the muscle cells. In reality, your body is always aerobic, until your last gasp! From this perspective you probably prefer to remain aerobic for a long time! And yes, you're aerobic at rest. However, because your energy output is so low, this is not a very effective way to burn calories and fat and get into shape. Aerobic exercise is the key to building a stronger heart that can reduce your chances of heart disease, burn lots of fat, and deliver oxygen to the cells of your body for continuous activity or recovery from hard effort. Aerobic exercise is any activity that you can keep at for about two minutes or longer that raises your heart rate and generally involves the large muscle groups of the hips, thighs, and buttocks. Aerobic exercise means that your muscles are getting enough oxygen to make energy by aerobic energy production. When you produce energy aerobically, generally you can carry on a conversation while exercising and you feel like you could continue exercising indefinitely. (On the other hand, if you can recite the Gettysburg address with theatrics and gusto, maybe you should work a little harder!)
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