Does peak inspiratory flow contribute to setting V̇_O2max?. A test of symmorphosis

Symmorphosis predicts that animal design is optimized in such a way that structure 'satisfies but does not exceed' functional requirements. To provide one test of this hypothesis, we examined peak inspiratory flow and its relation to maximum oxygen uptake in humans. We measured maximal forced (peak) inspiratory flow (V̇i_max) and maximum oxygen uptake (V̇_O2max) via cycle ergometry in well trained (V̇_O2max>65 ml O₂·kg^-1·min^-1) and untrained (V̇_O2max<45 ml O₂·kg^-1·min^-1) male subjects. Tests of V̇i_max and peak oxygen uptake (V̇_O2peak) were made while the subjects were breathing through inspiratory orifices differing in area. V̇i_max varied as an identical function of orifice diameter in both groups of subjects. However, V̇_O2peak was more sensitive to decreasing orifice diameter in trained endurance athletes than it was in untrained individuals. The diameter of the largest orifice that caused a reduction in oxygen uptake was over two times larger for trained than for untrained subjects, corresponding to about a four-fold difference in resistance at any flow rate. These results suggest that the structures setting V̇i_max (airway resistance and inspiratory muscle strength) are not matched to oxygen demand (V̇_O2max) in humans. While these structures seem to be 'over-built' and hence do not likely contribute to setting the limits to aerobic performance in most. humans, they may be among the primary limiting factors in the most elite endurance athletes.