Influence of muscle metabolic heterogeneity in determining the VO2p kinetic response to ramp-incremental exercise

The pulmonary O₂ uptake (Vo_2p) response to ramp-incremental (RI) exercise increases linearly with work rate (WR) after an early exponential phase, implying that a single time constant () and gain (G) describe the response. However, variability in and G of Vo_2p kinetics to different step increments in WR is documented. We hypothesized that the "linear" Vo_2p-WR relationship during RI exercise results from the conflation between WR-dependent changes in and G. Nine men performed three or four repeats of RI exercise (30 W/min) and two step-incremental protocols consisting of four 60-W increments beginning from 20 W or 50 W. During testing, breath-by-breath Vo_2p was measured by mass spectrometry and volume turbine. For each individual, the Vo_2p RI response was characterized with exponential functions containing either constant or variable and G values. A relationship between and G vs. WR was determined from the step-incremental protocols to derive the variable model parameters. and G increased from 21 ± 5 to 98 ± 20 s and from 8.7 ± 0.6 to 12.0 ± 1.9 ml·min^–1·W^–1 for WRs of 20-230 W, respectively, and were best described by a second-order () and a first-order (G) polynomial function of WR (lowest Akaike information criterion score). The sum of squared residuals was not different (P > 0.05) when the Vo_2p RI response was characterized with either the constant or variable models, indicating that they described the response equally well. Results suggest that and G increase progressively with WR during RI exercise. Importantly, these relationships may conflate to produce a linear Vo_2p-WR response, emphasizing the influence of metabolic heterogeneity in determining the apparent Vo_2p-WR relationship during RI exercise.

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