The purpose of this experiment is to measure oxygen consumption and ventilation at rest and during incremental exercise.

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Nicholas Wright

Physiology Laboratory – Analysis of Expired Air


The stress that is imposed on the human body by strenuous physical effort varies with the level of the maximal capacity of the body to perform exercise. If an individual has a physical working capacity of a certain amount and does exercise at a level of 50 per cent of his capacity, he is under much less stress than another person doing the same activity using 70 per cent of his capacity. The larger the level of the physical working capacity of an individual, the more strenuous exercise he will be able to take and the less stress he will have imposed on him by any particular exercise level relative to someone of a lower exercise capacity.

The most useful measurement is that of the maximal oxygen uptake (VO2 max) by the body, because this indicates the maximal capabilities of the respiratory and cardiovascular systems to supply oxygen (and therefore release energy) to the working muscles. As an example, two men of the same body size may have a VO2: max 3 l/min in the one case and 2 I/min in the other. If they each have to do work that requires an oxygen consumption 1.5 l/min (approx. 7-8 kcal/min (29-33 KJ)), this work level is 50 per cent of the maximal capability of the first man but 75 per cent that of the second. (Astrand, P.O.1956)

Vo2 max is often expressed, not as l/min but as ml/kg of body mass/mint Very approximate levels for people of varying degrees of fitness might be 3545 ml/kg/min for an average young man and 50 ml or more for a young man of higher levels of fitness. Extremely fit men may have values of 70-80 ml/kg/min or even higher, but these are exceptional values. There is also a difference between the sexes, with women having about 15-20 per cent lower values. Mc Ardle, Katch, Katch (1994).

When O2 max is being measured on an individual, it can be measured either directly by having the person do exercise up to the extremes of his or her capability, or else by sub-maximal tests where the results are extrapolated to supposedly maximal values. Both approaches have their drawbacks. The direct measurement, to be a valid reflection of the real maximal working capacity of the individual, requires considerable motivation: it is a somewhat unpleasant experience for those not accustomed to it to exercise to complete exhaustion. Unless repeated measurements are made on each individual, it is unlikely that a real VO2 max will have been obtained.

The drawback to the sub-maximal approach concerns the accuracy of the extrapolation. As the exercise is sub maximal, motivation becomes less relevant. The extrapolation requires heart-rate to be measured (sometimes in conjunction with oxygen uptake) during three or four different levels of standardized exercise, up to a moderately strenuous degree.

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However, two sources of error arise. First, sub maximal heart-rates can vary within the individual independently of the exercise, because of such influences as emotion, time of day, effects of eating or drinking, smoking, temperature, etc. And second, a direct extrapolation from sub-maximal heart-rates to maximal will not represent the normal situation, because at near maximal values the relationship of heart-rate to oxygen uptake is different from that at lower levels of exercise. However, the error is seldom large, and a sub-maximal test, especially in a nutritional context, is the method of choice. With experience and especially under laboratory conditions, ...

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