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

Beat-by-beat Q'aO2 and breath-by-breath V'O2were assessed in ten male subjects (24 ± 3.5 years;78 ± 7.7 kg; 182 ± 5.6 cm) during cycling exercise at50 W before and after a 14-day period of head-down tiltbedrest (HDTBR). O2 deficit (DefO2) was calculated asthe difference between the volume of O2 that would havebeen consumed if a steady state had been immediatelyattained minus that actually taken up during exercise. Q'aO2kinetics was described fitting the data with a non-linearmono-exponential model with time delay. Mean responsetimes (MRT) of V'O2 and Q'aO2 kinetics were then calculated.DefO2 and MRT of V'O2 response did not changeafter HDTBR, whereas MRT of Q'aO2 kinetics increased.The invariance of V'O2 kinetics after HDTBR suggests that,although Q'aO2 response became slower after HDTBR, itdid not affect the kinetics of peripheral gas exchange,which probably remained under the control of local muscularmechanisms.