O₂ uptake kinetics above and below the lactic acidosis threshold during sinusoidal exercise

O₂ uptake (V̇O₂) kinetics at the onset of a constant work rate exercise are difficult to describe for work rates above the lactic acidosis threshold (LAT), because the steady-state level of V̇O₂ response can usually not be identified. To describe the ability of the O₂ transport system to deliver and the cells to utilize O₂ above the LAT relative to that below the LAT, we applied a fluctuating (sinusoidal) variation of work rate. After 4 min of constant work at the midpoint of the sinusoidal work rate, a fluctuating work rate, at a period of 4 min, was applied below the LAT for the next 16 min. This was repeated in a range of work rates above the LAT with the same sine- wave amplitude. V̇O₂ response appeared to follow a sinusoidal pattern similar to that of work rate for below- and above-LAT exercise. However, the amplitude of the V̇O₂ response was significantly reduced (5.4 ± 2.6 vs. 7.6 ± 1.9 ml · min^-1 · W^-1, P < 0.01), and the phase lag increased above- compared with below-LAT work rate. V̇O₂/heart rate fluctuations were dramatically reduced, whereas heart rate amplitude decreased and phase lag increased, for above-LAT sinusoidal work rate changes. These results suggest that V̇O₂ kinetics are slowed in the work rate domain above the LAT relative to that below the LAT and that V̇O₂ kinetics could be limited by the O₂ transport mechanisms to the exercising muscle.