Aerobic exercise refers to exercise which is of moderate intensity, undertaken for a long duration. Aerobic means "with oxygen", and refers to the use of oxygen in a muscle's energy-generating process. Many types of exercise are aerobic, and by definition are performed at moderate levels of intensity for extended periods of time.

Aerobic exercise and fitness can be contrasted with anaerobic exercise, of which strength training and weight training are the most salient examples. The two types of exercise differ by the duration and intensity of muscular contractions involved, as well as by how energy is generated within the muscle.

Initially during aerobic exercise, glycogen is broken down to produce glucose, but in its absence, fat metabolism is initiated instead. The latter is a slow process, and is accompanied by a decline in performance level. The switch to fat as fuel is a major cause of what marathon runners call "hitting the wall".

Anaerobic exercise, in contrast, refers to the initial phase of exercise, or any short burst of intense exertion, in which the glycogen or sugar is consumed without oxygen, and is a far less efficient process. Operating anaerobically, an untrained 400 meter sprinter may "hit the wall" short of the full distance.


There are various types of aerobic exercise. In general, aerobic exercise is one performed at a low to moderate level of intensity over a long period of time. For example, running a long distance at a moderate pace is an aerobic exercise, but sprinting is not.

Among the recognised benefits of doing regular aerobic exercise are:

• Strengthening the muscles involved in respiration, to facilitate the flow of air in and out of the lungs

• Strengthening and enlarging the heart muscle, to improve its pumping efficiency and reduce the resting heart rate

• Toning muscles throughout the body, which can improve overall circulation and reduce blood pressure

• Increasing the total number of red blood cells in the body, to facilitate transport of oxygen throughout the body

As a result, aerobic exercise can reduce the risk of death due to cardiovascular problems. In addition, high-impact aerobic activities (such as jogging or jumping rope) can stimulate bone growth, as well as reducing the risk of osteoporosis for both men and women.

In addition to the health benefits of aerobic exercise, there are numerous performance benefits:

• Increased storage of energy molecules such as fats and carbohydrates within the muscles, allowing for increased endurance

• Neovascularisation (Abnormal formation of new blood vessels, usually in or under the retina or on the iris surface.)of the muscle sarcomeres to increase blood flow through the muscles

• Increasing speed at which aerobic metabolism is activated within muscles, allowing a greater portion of energy for intense exercise to be generated aerobically

• Improving the ability of muscles to use fats during exercise, preserving intramuscular glycogen

• Enhancing the speed at which muscles recover from high intensity exercise

Anaerobic exercise is typically used by athletes in non-endurance sports to build power and by body builders to build muscle mass. Muscles that are trained under anaerobic conditions develop biologically differently giving them greater performance in short duration-high intensity activities.

There are two types of anaerobic energy system, the ATP-PCr energy system, which uses creatine phosphate as the main energy source, and the lactic-acid (or anaerobic glycolysis) system that uses glucose (or glycogen) in the absence of oxygen. The latter is an inefficient use of glucose and produces by-products that are thought to be detrimental to muscle function. The lactic-acid system is the dominant energy system during high to maximal intensity exercise over short durations (up to about 1 min), but the lactic acid system can still provide a proportion of the required energy during aerobic exercise, as the body has the capacity to get rid of the anaerobic by-products at a certain rate. The efficiency of by-product removal by muscles can improve through training.

Anaerobic threshold

The anaerobic threshold (AT) is the exercise intensity at which lactate starts to accumulate in the blood stream. This happens when it is produced faster than it can be removed (metabolised). This point is sometimes referred to as the lactate threshold, or the onset of blood lactate accumulation (OBLA). When exercising below the AT intensity any lactate produced by the muscles is removed by the body without it building up.

The anaerobic threshold is a useful measure for deciding exercise intensity for training and racing in endurance sports (e.g. distance running, cycling, rowing, swimming and cross country skiing), and can be increased greatly with training.

Fartlek (speed-play) training and interval training take advantage of the body being able to temporarily exceed the anaerobic threshold, and then recover (reduce blood-lactate) while operating at below the threshold, but still doing physical activity. Fartlek and interval training are similar, the main difference being the relative intensities of the exercise, best illustrated in a real-world example: Fartlek training would involve constantly running, for a period time running just above the anaerobic threshold, and then running at just below it, while interval training would be running quite high above the anaerobic threshold, but then slowing to a walk during the rest periods.

Fartlek would be used by people who are constantly moving, with occasional bouts of speed, such as basketballers, while interval training is more suited to sprinters, who exert maximum effort and then can stop exerting completely. With both styles of training, you can exert more effort before fatiguing and burn more calories than exercising at a constant pace (continuous training), but will emphasise training the anaerobic system rather than the aerobic system. Long duration training below the anaerobic threshold is recommended to primarily work the aerobic system.

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