Limitations to Endurance Exercise

Our body's ability to aerobically at a very high level may be limited by the supply of oxygen through our lungs or the ability of the muscles to utilize the oxygen supplied to it.

A high maximum level of oxygen uptake (VO2max) is a vital component of successful endurance performance. The factors limiting VO2max are of considerable interest to exercise physiologists, and have for a long time been a topic of discussion in the literature. The transport of O2 from the lungs to the tissues involves many steps including the diffusion of O2 from air to blood in the alveoli, its transport via the arterial vasculature, to the uptake of O2 by the body's cells. Many of the steps in these processes have been postulated as being rate limiting.

Aerobic RunnerTwo separate schools of thought have emerged on the factors limiting O2 transport, and therefore VO2max, based on the limitation being 'central' or 'peripheral’ (Sutton, 1992). The central mechanisms encompass the steps involved in the O2 delivery from the air to the muscles, consisting of the pulmonary and arterial systems. Peripheral mechanisms involve the extraction and utilization of O2 by the exercising muscles. There a evidence which supports each of these areas of limitation, though it may not be simply a single step that limits the total system. The site of limitation may vary depending on the level of specific factors. A change in one parameter can cause another to become limited. For example, if blood flow to the muscles is inadequate, then the diffusion gradient may become less and therefore become the limiting factor (Sutton, 1992). Wagner (1992) believes all the steps in the O2 pathway interact to determine VO2max, and increasing and decreasing the transport capacity of any one step leads to an increase or decrease in VO2max respectively.

The complexity of the processes involved, and the equivocal results of the many studies in this area, would indicate that the site of limitation varies between individuals, and possibly at any one time depend on the state of associated parameters.

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