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

Beat-by-beat \( \dot{Q} \) _aO₂ and breath-by-breath \( \dot{V} \)O₂ 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. \( \dot{Q} \) _aO₂ kinetics was described fitting the data with a non-linear mono-exponential model with time delay. Mean response times (MRT) of \( \dot{V} \)O₂ and \( \dot{Q} \) _aO₂ kinetics were then calculated. DefO₂ and MRT of \( \dot{V} \)O₂ response did not change after HDTBR, whereas MRT of \( \dot{Q} \) _aO₂ kinetics increased. The invariance of \( \dot{V} \)O₂ kinetics after HDTBR suggests that, although \( \dot{Q} \) _aO₂ 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.