Respiratory compensation and blood pH regulation during variable intensity exercise in trained versus untrained subjects

To determine whether endurance-trained cyclists (T; n = 10) have a superior blood-respiratory buffering for metabolic acidosis relative to untrained subjects (UT; n = 10) during variable intensity exercise (VAR). On three occasions, T and UT pedaled for 24 min alternating high- and low-intensities as percentage of their second ventilatory threshold (VT₂): VAR_LOW 87.5–37.5% VT₂, VAR_MODERATE 125–25% VT₂, and VAR_HIGH 162.5–12.5% VT₂ to complete the same amount of work. Before and just after each VAR trial, maximal cycling power (P_MAX) was assessed. For each trial, the respiratory compensation for exercise acidosis (ventilatory equivalent for CO₂) and the final blood pH, lactate and bicarbonate concentrations were similar for T and UT subjects. However, after VAR_HIGH, UT reduced P_MAX (−14 ± 1%; P < 0.05) while T did not. Our data suggest that endurance training confers adaptations to withstand the low pH provoked by VAR without losing cycling power, although this response is not due to differences in blood-respiratory buffering.