Anaerobic means "without oxygen." An anaerobic exercise is any exercise that requires short bursts of power, such as all-out sprinting or very heavy weightlifting, which do not require a significant increase in oxygen delivery to the muscle.

The ability to perform this type of work is dependent instead on energy sources stored in the muscle. Because this energy supply is limited, anaerobic exercise can be sustained for only short periods of time. In reality, most exercise is a combination of aerobic and anaerobic exercise. The amount of each is dependent on how hard and fast the exercise proceeds. With a primarily anaerobic or resistance exercise such as sprinting, after about 90 seconds, you begin gasping for air and feel a burning sensation in your lungs - your body is forcing you to stop exercising.

Anaerobic training will help your sports performance, but it won't provide the health benefits of aerobic exercise. When you push it during an anaerobic workout, all you're really doing is tiring yourself out early and cutting short your aerobic training. You'll do better by slowing down your pace and stretching the length of your workout.

During an aerobic workout, the point at which your oxygen supply runs out and you slip into using stored energy is called your anaerobic threshold. If you are out of shape, your body is not very efficient at taking in oxygen, and you hit your anaerobic threshold while exercising at very low levels of intensity. In other words, any mismatch between oxygen demands and your ability to supply oxygen is met by anaerobic sources until they run out.

As you become more physically fit, you are able to go farther faster and yet still supply oxygen to your muscles, with less need to use limited amounts of stored energy.