Influence of muscle metabolic heterogeneity in determining the (V)over dotO2p kinetic response to ramp-incremental exercise

The pulmonary O-2 uptake (VO2p) response to ramp-incremental (RI) exercise increases linearly with work rate (WR) after an early exponential phase, implying that a single time constant (tau) and gain (G) describe the response. However, variability in tau and G of VO2p kinetics to different step increments in WR is documented. We hypothesized that the linear VO2p-WR relationship during RI exercise results from the conflation between WR-dependent changes in tau 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 VO2p was measured by mass spectrometry and volume turbine. For each individual, the VO2p RI response was characterized with exponential functions containing either constant or variable tau and G values. A relationship between tau and G vs. WR was determined from the step-incremental protocols to derive the variable model parameters tau 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 (tau) 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 VO2p RI response was characterized with either the constant or variable models, indicating that they described the response equally well. Results suggest that tau and G increase progressively with WR during RI exercise. Importantly, these relationships may conflate to produce a linear VO2p-WR response, emphasizing the influence of metabolic heterogeneity in determining the apparent VO2p-WR relationship during RI exercise.