Like engines, muscles do work and require fuel. The body supplies a fuel called ATP to the muscles. This allows the muscles to do their work of contracting. Because it is very important that muscles always have access to this fuel, the body has adapted different ways to make this fuel. It can use:
* an aerobic pathway, which requires oxygen to make fuel, or
* an anaerobic pathway, which makes fuel without oxygen.

During normal activity the blood supply to a muscle can provide all of the fuel (fatty acids and glucose) to a muscle using aerobic metabolism. This holds true under moderate exercise, as well. Moving through your day to day activities you fuel your muscles in this way. This is true during walking, gardening, and bike riding, too. However, when the activity level reaches about 80 percent of the maximum possible, aerobic metabolism can no longer meet the energy needs of the muscles. Now anaerobic metabolism kicks in and begins to provide large amounts of energy to the muscles. Heavy weight lifting, sprinting or any rapid burst of hard exercise will trigger this energy pathway. Anaerobic reactions are not efficient. They produce large amounts of a byproduct called lactic acid. Lactic acid build-up contributes to muscle fatigue and rapid breathing.

In order to help our bodies be their strongest, we should do exercise which requires both kinds of fuel metabolism. Aerobic exercise keeps our hearts healthy. An easy way to detect aerobic exercise is when we have an increased heart rate, but we can still speak without gasping for breath. Hiking, biking, cross country skiing, and dancing are types of aerobic exercise. Think of long and slow exercise when you think aerobic. Most of your exercise should be of this type to develop hear lung fitness and improve your body's fat burning ability.

Anaerobic exercise helps us increase our muscle strength and stay ready for quick bursts of speed. Activities done with enough force to cause shortness of breath and burning muscles are examples. Think of short and fast when you think of anaerobic.

Check with your healthcare provider before you start an exercise program. Choose activities that you enjoy, and that are readily accessible. To learn more, visit your local bookstore or library, or call your local YMCA or YWCA.

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