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)over dot(a)O(2) and breath-by-breath (V)over dotO(2) 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 tiltbed rest (HDTBR). O(2) deficit (DefO(2)) was calculated as the difference between the volume of O(2) that would have been consumed if a steady state had been immediately attained minus that actually taken up during exercise. (Q)over dot(a)O(2) kinetics was described fitting the data with a non-linear mono-exponential model with time delay. Mean response times (MRT) of (V)over dotO(2) and (Q)over dot(a)O(2) kinetics were then calculated. DefO(2) and MRT of (V)over dotO(2) response did not change after HDTBR, whereas MRT of (Q)over dot(a)O(2) kinetics increased. The invariance of (V)over dotO(2) kinetics after HDTBR suggests that, although (Q)over dot(a)O(2) 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.