Oxygen deficits and oxygen delivery kinetics during submaximal intensity exercise in humans after 14 days of head-down tilt-bed rest.

Beat-by-beat Q(a)O2 and breath-by-breath VO2 were assessed in ten male subjects (24 +/- 3.5 years; 78 +/- 7.7 kg; 182 +/- 5.6 cm) during cycling exercise at 50 W before and after a 14-day period of head-down tilt-bed rest (HDTBR). O2 deficit (DefO2) was calculated as the difference between the volume of O2 that would have been consumed if a steady state had been immediately attained minus that actually taken up during exercise. Q(a)O2 kinetics was described fitting the data with a non-linear mono-exponential model with time delay. Mean response times (MRT) of VO2 and Q(a)O2 kinetics were then calculated. DefO2 and MRT of VO2 response did not change after HDTBR, whereas MRT of Q(a)O2 kinetics increased. The invariance of VO2 kinetics after HDTBR suggests that, although Q(a)O2 response became slower after HDTBR, it did not affect the kinetics of peripheral gas exchange, which probably remained under the control of local muscular mechanisms.