title: Influence of muscle metabolic heterogeneity in determining the V̇o2p kinetic response to ramp-incremental exercise.
abstract: The pulmonary O2 uptake (V̇o2p) response to ramp-incremental (RI) exercise increases linearly with work rate (WR) after an early exponential phase, implying that a single time constant (τ) and gain (G) describe the response. However, variability in τ and G of V̇o2p kinetics to different step increments in WR is documented. We hypothesized that the "linear" V̇o2p-WR relationship during RI exercise results from the conflation between WR-dependent changes in τ 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 V̇o2p was measured by mass spectrometry and volume turbine. For each individual, the V̇o2p RI response was characterized with exponential functions containing either constant or variable τ and G values. A relationship between τ and G vs. WR was determined from the step-incremental protocols to derive the variable model parameters. τ 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 (τ) 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 V̇o2p RI response was characterized with either the constant or variable models, indicating that they described the response equally well. Results suggest that τ and G increase progressively with WR during RI exercise. Importantly, these relationships may conflate to produce a linear V̇o2p-WR response, emphasizing the influence of metabolic heterogeneity in determining the apparent V̇o2p-WR relationship during RI exercise.
authors.affiliation: Canadian Centre for Activity and Aging, The University of Western Ontario, London, Ontario, Canada; School of Kinesiology, The University of Western Ontario, London, Ontario, Canada;
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Keir
authors.affiliation: School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom.
authors.forename: Alan P
authors.initials: AP
authors.lastname: Benson
authors.affiliation: Canadian Centre for Activity and Aging, The University of Western Ontario, London, Ontario, Canada; School of Kinesiology, The University of Western Ontario, London, Ontario, Canada;
authors.forename: Lorenzo K
authors.initials: LK
authors.lastname: Love
authors.affiliation: Canadian Centre for Activity and Aging, The University of Western Ontario, London, Ontario, Canada; School of Kinesiology, The University of Western Ontario, London, Ontario, Canada;
authors.forename: Taylor C
authors.initials: TC
authors.lastname: Robertson
authors.affiliation: Rehabilitation Clinical Trials Center, Division of Respiratory and Critical Care Physiology and Medicine, Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, California; and School of Biomedical Sciences, University of Leeds, Leeds, United Kingdom.
authors.forename: Harry B
authors.identifier: http://orcid.org/0000-0002-7884-0726
authors.initials: HB
authors.lastname: Rossiter
authors.affiliation: Canadian Centre for Activity and Aging, The University of Western Ontario, London, Ontario, Canada; School of Kinesiology, The University of Western Ontario, London, Ontario, Canada; Department of Physiology and Pharmacology, The University of Western Ontario, London, Ontario, Canada; jkowalch@uwo.ca.
authors.forename: John M
authors.initials: JM
authors.lastname: Kowalchuk
mesh_terms: D000328:Adult; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test / Q000379:methods; D006801:Humans; D007700:Kinetics; D008168:Lung / Q000502:physiology; D008297:Male; D018482:Muscle, Skeletal / Q000502:physiology*; D010101:Oxygen Consumption / Q000502:physiology*; D005082:Physical Exertion / Q000502:physiology; D011659:Pulmonary Gas Exchange / Q000502:physiology; D012119:Respiration
keywords: O2 uptake kinetics; dynamic linearity; muscle fibers; ramp-incremental exercise; time constant; work rate