Aim: 

V'O₂ kinetics at the onset of supra maximal (SM) exercise has been described to be slower than during exercise of moderate (M) intensity. This has been attributed to the slower kinetics of cardiovascular oxygen delivery at the onset of SM. To test this hypothesis, we assessed in parallel the kinetics of breath-by-breath oxygen uptake (B-by-B V'O₂) and of beat-by-beat cardiac output (b-by-b Q') at the onset of M and SM intensity exercise in humans.

Methods: 

14 male subjects (26 5 yr; 175 6 cm; 73 5.7 Kg; V'O_2max 4.13 0.36 L*min^-1) were studied while pedalling at M, (approximately 80% of maximal aerobic work rate, WR_aer,max) and SM, (approximately 120% WR_aer,max) exercise intensities. B-by-B V'O₂ and b-by-b Q' were measured continuously. The latter was estimated by means of the pulse contour analysis applied to the arteriolar pulse pressure profile. V'O₂ and Q' kinetics were then described by using two- or three-component exponential models.

Results: 

Time constant (t₂) of Phase II and mean response times (MRT) of V'O₂ kinetics were not significantly different between M (t₂ 27.8 10.4 s; MRT 38.6 10 s) and SM (t ₂ 26.8 8.5 s; MRT 36.9 9.5 s). t ₂ of b-by-b Q' was significantly different between M and SM (M = 37.9 23.4; SM = 20.5 13.7; P < 0.05). MRT decreased significantly with increasing WR (M = 33.2 17.7; SP = 18.5 7.1; P < 0.01). V'O₂ during M corresponded to 67.8% of V'O_2max, whereas at the end of SM it equalled 95.4% of V'O_2max.

Conclusion: 

V'O₂ kinetics was not affected by exercise intensity, whereas that of Q' was significantly faster at SM. This seems to disprove the hypothesis that V'O₂ kinetics is limited by the inertia of oxygen cardiovascular transport. However, V'O₂ during SM did not attain V'O_2max implying that: i) the exercise was likely interrupted by the occurrence of peripheral fatigue and; ii) the anaerobic lactacid component formed a substantial fraction of the overall oxygen deficit in SM.