Kinetics of oxygen uptake during supine and upright heavy exercise
Journal of applied physiology
of oxygen uptake during supine and upright heavy exercise. J. Appl. Physiol. 87(1): 253-260, 1999.-It is presently unclear how the fast and slow components of pulmonary oxygen uptake (V O 2 ) kinetics would be altered by body posture during heavy exercise [i.e., above the lactate threshold (LT)]. Nine subjects performed transitions from unloaded cycling to work rates representing moderate (below the estimated LT) and heavy exercise (V O 2 equal to 50% of the difference between LT and peak V O 2
... n LT and peak V O 2 ) under conditions of upright and supine positions. During moderate exercise, the steady-state increase in V O 2 was similar in the two positions, but V O 2 kinetics were slower in the supine position. During heavy exercise, the rate of adjustment of V O 2 to the 6-min value was also slower in the supine position but was characterized by a significant reduction in the amplitude of the fast component of V O 2 , without a significant slowing of the phase 2 time constant. However, the amplitude of the slow component was significantly increased, such that the end-exercise V O 2 was the same in the two positions. The changes in V O 2 kinetics for the supine vs. upright position were paralleled by a blunted response of heart rate at 2 min into exercise during supine compared with upright heavy exercise. Thus the supine position was associated with not only a greater amplitude of the slow component for V O 2 but also, concomitantly, with a reduced amplitude of the fast component; this latter effect may be due, at least in part, to an attenuated early rise in heart rate in the supine position. posture; gas exchange kinetics; oxygen transport; slow component of oxygen uptake PULMONARY OXYGEN UPTAKE (V O 2 ) has been reported to adjust more slowly at the onset of exercise in the supine position for both moderate- (8, 15, 18, 19, 22, 25, 34) and heavy-intensity exercise (8, 9, 23) . Despite greater cardiac output (15, 18, 23) , it appears that effective blood flow to the working leg muscles is less in the supine posture (11, 13, 25, 33) , presumably as a consequence of lower arterial pressure in the legs when the effect of gravity (hydrostatic gradient effect) is removed. These results imply that the slowing of V O 2 kinetics in the supine position may be the result of a blunted cardiovascular response to exercise.