Our muscles immediately begin to use energy when we are walking or running. For the first three minutes our muscles will burn glycogen, a special sugar which is stored in the muscles for a quick infusion of energy. Some glycogen is always stored within our muscle tissues. During this period fat is not burned. This process is called anaerobic metabolism. Often during the first few minutes of strenuous activity, especially during anaerobic metabolism, we may experience burning in muscles of our arms, legs or back. This is due to the creation of lactic acid which occurs when glycogen is burned.
In an exercise of more than three minutes the glycogen stored within the muscles will eventually burn up all the glycogen and our muscles will move into aerobic metabolism. This occurs because the glycogen is now being burned in the presence of oxygen which is brought to the muscles by way of the blood stream. As long as we breathe correctly we will bring oxygen to the muscles and this process will continue.
Oxygen is necessary to the muscle`s ability to function properly. With sufficient oxygen the muscles can extract all the energy they need from blood sugar. During the period we exercise the liver and muscles will release their stored carbohydrate so that it can be used as energy by the muscles hence allowing us to keep on exercising.
Once these stores of glycogen are used up which usually occurs after about 20 minutes, the body will start burning its fat stores to produce blood sugar and ultimately glycogen. The longer you exercise the more fat burned. Fat, body fat and not dietary fat, can now be used virtually indefinitely to produce energy to support our exercise program.
While we are walking, we do not directly burn fat but once we stop walking the glycogen which has been burned up must be replaced. This can only happen by what we eat and by the liver draws fat from the bodys fat tissues. If we are on a limited caloric intake then the food we consume will be used basically for feeding our brain and fat stores will have to be drawn on to replace the glycogen used up while exercising.
Researchers find that "fat-burning" may last for anywhere from six to twenty-four hours after a regular exercise program.
There are certain aerobic exercises which are good in nature and simple.