Exercise is a stressor to the body and your body will respond with the us of energy. Your muscles will need to use and replace energy. It is important to eat high protein and carbohydrate food to replace the energy used. If exercise is done over a period of time the body will adjust accordingly. A new swimmer will use more energy at first once they are in shape and the body adjusts to swimming their energy usage goes down. The effect of training will reduce the amount of lactic acid building up and the expenditure of energy.
The human body is basically a muscular machine designed for movement. When you exercise the body systems go into work to increase energy to the muscles. The higher your heart rate the higher your metabolic rate. Oxygen consumption, heart rate and respiration respond to the increase in energy used. The same is true for walking and running. The more you do it the less energy you use but you burn calories. Home exercise equipment will also provide a good aerobic work out. Working out on elliptical, stair climbers, and treadmills provide a good way to burn calories and build up energy.