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

O2 uptake (VO2) 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 VO2 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. VO2 response appeared to follow a sinusoidal pattern similar to that of work rate for below- and above-LAT exercise. However, the amplitude of the VO2 response was significantly reduced (5.4 +/- 2.6 vs. 7.6 +/- 1.9 ml.min-1 x W-1, P < 0.01), and the phase lag increased above- compared with below-LAT work rate. VO2/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 VO2 kinetics are slowed in the work rate domain above the LAT relative to that below the LAT and that VO2 kinetics could be limited by the O2 transport mechanisms to the exercising muscle.