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Endurance is, simply put, the ability to endure, or an object or person's lasting quality. Thus, the longer a thing lasts, the greater the endurance. Endurance may refer to short-term--high intensity, anaerobic exercise such as sprinting--or long term, which may last hours or even days in duration, as in the case of marathons, triathlons, and ultramarathons.
In terms of fitness, endurance may be broken down into several types: aerobic endurance (cardiorespiratory endurance), anaerobic endurance, speed-endurance and strength-endurance. It is most commonly broken up into cardiorespiratory endurance and muscular endurance.
Well-trained endurance athletes are able to generate blood lactate levels that are 20-30% higher than those of untrained individuals under similar conditions. This produces significantly enhanced endurance as their muscles are better equipped to utilize it to fuel further muscular energy.
Cardiorespiratory endurance refers to the efficiency with which the body delivers oxygen and nutrients needed for muscular activity and transports waste products from the cells. It is also sometimes referred to as aerobic endurance or aerobic fitness. Improving aerobic endurance enables the heart, lungs, and muscles to do work over a longer period of time. Cardiorespiratory conditioning can decrease risk factors associated with heart disease, increase vitality, increase maximum oxygen uptake, and can aid weight loss or maintenance.
In addition to this, training cardiorespiratory endurance improves aerobic capacity caused by fibre adaptation, more specifically an increase in the size of mitochondria, which enhances the ability of the fibres to generate aerobic energy. It also facilitates an increase in capillary density, which enhances the fibres' capacity to transport oxygen, and thus to create energy. Finally, endurance training increases the number of enzymes relevant to the Krebs cycle, a chemical process within muscles that allows the regeneration of ATP under aerobic conditions. The enzymes involved in this process may actually increase by a factor of two to three after a sustained period of endurance training.