Kinetics of oxygen uptake at the onset of exercise near or above peak oxygen uptake

We tested the hypothesis that kinetics of O-2 uptake ((V) over dot O-2) mea sured in the transition to exercise near or above peak (V) over dot O-2 ((V ) over dot O-2peak) would be slower than those for subventilatory threshold exercise. Eight healthy young men exercised at similar to 57, similar to 9 6, and similar to 125% (V) over dot O-2peak. Data were fit by a two- or thr ee-component exponential model and with a semilogarithmic transformation th at tested the difference between required (V) over dot O-2 and measured (V) over dot O-2. With the exponential model, phase 2 kinetics appeared to be faster at 125% (V) over dot O-2peak [time constant (tau(2)) = 16.3 +/- 8.8 (SE) s] than at 57% (V) over dot O-2peak (tau(2) = 29.4 +/- 4.0 s) but were not different from that at 96% (V) over dot O-2peak exercise (tau(2) = 22. 1 +/- 2.1 s). (V) over dot O-2 at the completion of phase 2 was 77 and 80% (V) over dot O-2peak in tests predicted to require 96 and 125% (V) over dot O-2peak. When (V) over dot O-2 kinetics were calculated with the semilogar ithmic model, the estimated tau(2) at 96% (V) over dot O-2peak (49.7 +/- 5. 1 s) and 125% (V) over dot O-2peak (40.2 +/- 5.1 s) were slower than with t he exponential model. These results are consistent with our hypothesis and with a model in which the cardiovascular system is compromised during very heavy exercise.