Pulmonary O₂ uptake kinetics during moderate-intensity exercise transitions initiated from low versus elevated metabolic rates: insights from manipulations in cadence.

INTRODUCTION: The rate of adjustment (τ) of phase II pulmonary O₂ uptake (VO₂p) is slower when exercise transitions are initiated from an elevated baseline work rate (WR) and metabolic rate (MR). In this study, combinations of cycling cadence (40 vs. 90 rpm) and external WR were used to examine the effect of prior MR on τVO₂p.

METHODS: Eleven young men completed transitions from 20 W (BSL) to 90% lactate threshold, with transitions performed as two steps of equal ∆WR (LS, lower step; US, upper step), while maintaining a cadence of (1) 40 rpm, (2) 90 rpm, and (3) 40 rpm but with the WRs elevated to match the higher VO₂p associated with 90 rpm cycling (40MATCH); transitions lasted 6 min. VO₂p was measured breath-by-breath using mass spectrometry and turbinometry; vastus lateralis muscle deoxygenation [HHb] was measured using near-infrared spectroscopy. VO₂p and HHb responses were modeled using nonlinear least squares regression analysis.

RESULTS: VO₂p at BSL, LS and US was similar for 90 rpm and 40MATCH, but greater than in 40 rpm. Compared to 90 rpm, τVO₂p at 40 rpm was shorter (p < 0.05) in LS (18 ± 5 vs. 28 ± 8 s) but not in US (26 ± 8 vs. 33 ± 9 s), and at 40MATCH, τVO₂p was lower (p < 0.05) (19 ± 6 s) in LS but not in US (34 ± 13 s) despite differing external WR and ∆WR.

CONCLUSIONS: A similar overall adjustment of [HHb] and VO₂p in LS and US across conditions suggested dynamic matching between microvascular blood flow and O₂ utilization. Prior MR (rather than external WR per se) plays a role in the dynamic adjustment of pulmonary (and muscle) VO₂p.