Energy balance analysis suggests that lactate is not a direct cause of the slow component of oxygen uptake kinetics

Purpose: The mechanisms of oxygen uptake (V˙O2) slow component in the severe exercise intensity domain are still a matter of debate. We tested the hypothesis that the rate of blood lactate ([La]) accumulation above maximal lactate steady state (MLSS) is a major cause of V˙O2 slow component. Methods: On 13 males exercising on a cycle-ergometer, we measured gas exchanges, heart rate, and [La] during maximal incremental exercise test to determine maximal aerobic power (w.max) and at constant power exercise tests at 60%, 65%, 70%, and 80% of w.max. Results: Maximal V˙O2 was 3.19 ± 0.37 l·min−1, w.max was 283 ± 28 W. At 60% w.max all variables attained steady state in all subjects. Power at MLSS was 177 ± 21 W. At 80% w.max a clear V˙O2 slow component was observed in all subjects, exercise lasted 11.3 ± 3.1 min and [La] was 7.4 ± 2.2 mmol at 5 min and 11.5 ± 3.6 mmol at 10 min. The energy balance computed at 80% w.max resulted compatible with the principles of the energetics of muscular exercise, if we assume linear [La] increase, and thus constant metabolic power provided by [La] accumulation. Conversely, the metabolic power provided by V˙O2 slow component increases with time. This contrast is incompatible with the tested hypothesis that consequently must be rejected. Conclusion: This study excluded [La] accumulation as a main cause of V˙O2 slow component.