Generating energy in the body without oxygen is known as anaerobic metabolism. When the lungs can’t get enough oxygen into the bloodstream to keep up with the demands for energy from the muscles, your body switches to anaerobic metabolism. This happens during vigorous physical activity, such as sprinting or heavy weight lifting, when you’re called upon to summon up an immediate burst of energy. Anaerobic metabolism can provide energy to the body more quickly from stored carbohydrates than the cardiovascular system can produce aerobically. But it is less efficient than aerobic metabolism because it produces larger amounts of its byproduct, lactic acid. A build up of lactic acid waste in the muscles causes your muscles to ache and makes your breathing more rapid. Your body needs extra oxygen to break down the lactic acid, which is what makes you gasp for air after a sudden burst of activity.
Although most of the exercise you take should be aerobic to keep the heart and lungs healthy, it’s also important to do some vigorous exercise to trigger the production of lactic acid. This type of exercise keeps your body ready for quick bursts of speed (so next time you need to run for the bus, you’ll be able to catch it). It also helps build up muscle strength and endurance, which are two key elements of muscular fitness. Exercise programmes designed to build strength steadily increase the weight that the muscle has to resist, while endurance training involves more repetitions of an action. This steady increase in stimulus to the muscles is known as progressive overload.