Journal
JOURNAL OF APPLIED PHYSIOLOGY
Volume 120, Issue 5, Pages 503-513Publisher
AMER PHYSIOLOGICAL SOC
DOI: 10.1152/japplphysiol.00804.2015
Keywords
O-2 uptake kinetics; ramp-incremental exercise; work rate; muscle fibers; dynamic linearity; time constant
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Funding
- National Science and Engineering Research Council of Canada (NSERC) [RGPGP-2015-00084]
- NSERC
- BBSRC [BB/I024798/1, BB/I00162X/1] Funding Source: UKRI
- Biotechnology and Biological Sciences Research Council [BB/I00162X/1, BB/I024798/1] Funding Source: researchfish
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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.
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