When you first begin exercising - no matter whether its weight training or cardio - your muscles immediately begin to use energy to allow them to work. For the first three minutes of activity, your muscles burn glycogen, the form of sugar stored in the muscles for a quick infusion of energy. During this period fat is not burned. This process is called anaerobic metabolism. During the first few minutes of strenuous activity, especially during anaerobic metabolism, you may experience a burning sensation in the muscles of your arms, legs, or back. This is caused by the accumulation of lactic acid, which is the result of burned glycogen. This burning sensation will soon pass and should not disrupt your workout. Anaerobic metabolism is the main form of energy for activities that require short bursts of power such as sprinting, football, and bodybuilding.

As your exercising continues, your body will eventually burn up all the glycogen stored within the muscles. Your muscles will then move into the aerobic metabolism phase. When this occurs, lactic acid production stops. This occurs because the glycogen is now being burned in the presence of oxygen, which is brought to the muscles by the blood stream. Provided you continue to breath correctly, your system will continually bring oxygen to the muscles. These stores of glycogen are expended after about 20 minutes. The body will then start burning its fat stores as an energy source. Thus, the longer you exercise, the more fat your body will burn.

What this means for you

For optimum health and fat burning, your bodybuilding workouts should consist of both anaerobic training and aerobic training. Weight training will strengthen and build the muscles, as well as boost your metabolism so youll be burning fat even after you leave the gym. Aerobic exercise such as cardio machines or aerobic classes will both stimulate your cardiovascular systems as well as burn body fat while your doing it.

Bob Howard 5/5/2006