1. A progressive exercise test was performed on forty-four male and twenty-nine female healthy Europeans, aged between 20 and 64 years. Values for cardiac frequency (fH) and ventilation (V̇) were interpolated to standard (submaximal) oxygen uptakes (V̇o2) of 0·751/min and 1·01/min. The tidal volume (Vt) at a ventilation of 20 and 301 BTPS/min was also determined (Vt 20 and Vt 30 respectively).

2. The slope of the linear relationship between cardiac frequency or ventilation and oxygen uptake (SfH and SV̇ respectively) can be used as a measure of the fitness of an individual, as it indicates the increase in fH or V̇ that is obligatory for an increase in energy expenditure equivalent to an additional oxygen uptake of 1·0 1/min (about the increase necessary for walking on the level at a normal speed). By analogy with the responses of an athlete, a ‘fit’ subject is one in whom responses are economically low, i.e. SfH and SV̇ are lower than in sedentary individuals. Measures of SfH and SV̇ can also be used to indicate the demands of everyday activities on fH and V̇.

3. When SfH and SV̇ are related to the individual's capacity to adapt fH and V̇ from resting to predicted maximum values (‘adaptation capacity’ ACfH and ACV̇ respectively), the resulting index (SfH × 100/ACfH or SV̇ × 100/ACV̇) expresses the percentage of the adaptation capacity used for an additional energy expenditure equivalent to a V̇o2 of 1·01/min, and can be considered a measure of the ‘physiological strain’ of exercise. The effects on exercise responses of differences in body muscle can be allowed for by multiplying this index by lean body mass (LBM). The lower the (size-adjusted) physiological strain index, the fitter the individual subject.

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