title: A raised metabolic rate slows pulmonary O(2) uptake kinetics on transition to moderate-intensity exercise in humans independently of work rate.
abstract: During exercise below the lactate threshold (LT), the rate of adjustment (τ) of pulmonary VO(2) uptake (τ) is slowed when initiated from a raised work rate. Whether this is consequent to the intrinsic properties of newly recruited muscle fibres, slowed circulatory dynamics or the effects of a raised metabolism is not clear. We aimed to determine the influence of these factors on τV(O(2)) using combined in vivo and in silico approaches. Fifteen healthy men performed repeated 6 min bouts on a cycle ergometer with work rates residing between 20 W and 90% LT, consisting of the following: (1) two step increments in work rate (S1 and S2), one followed immediately by the other, equally bisecting 20 W to 90% LT; (2) two 20 W to 90% LT bouts separated by 30 s at 20 W to raise muscle oxygenation and pretransition metabolism (R1 and R2); and (3) two 20 W to 90% LT bouts separated by 12 min at 20 W allowing full recovery (F1 and F2). Pulmonary O(2) uptake was measured breath by breath by mass spectrometry and turbinometry, and quadriceps oxygenation using near-infrared spectroscopy. The influence of circulatory dynamics on the coupling of muscle and τV(O(2)) lung was assessed by computer simulations. The τV(O(2)) in R2 (32 ± 9 s) was not different (P > 0.05) from S2 (30 ± 10 s), but both were greater (P < 0.05) than S1 (20 ± 10 s) and the F control bouts (26 ± 10 s). The slowed V(O(2)) kinetics in R2 occurred despite muscle oxygenation being raised throughout, and could not be explained by slowed circulatory dynamics (τV(O(2)) predicted by simulations: S1 = R2 < S2). These data therefore suggest that the dynamics of muscle O(2) consumption are slowed when exercise is initiated from a less favourable energetic state.
authors.affiliation: Institute of Membrane and Systems Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK.
authors.forename: T Scott
authors.initials: TS
authors.lastname: Bowen
authors.forename: Scott R
authors.initials: SR
authors.lastname: Murgatroyd
authors.forename: Daniel T
authors.initials: DT
authors.lastname: Cannon
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Cuff
authors.forename: Allison F
authors.initials: AF
authors.lastname: Lainey
authors.forename: Andrea D
authors.initials: AD
authors.lastname: Marjerrison
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Spencer
authors.forename: Alan P
authors.initials: AP
authors.lastname: Benson
authors.forename: Donald H
authors.initials: DH
authors.lastname: Paterson
authors.forename: John M
authors.initials: JM
authors.lastname: Kowalchuk
authors.forename: Harry B
authors.initials: HB
authors.lastname: Rossiter
mesh_terms: D000328:Adult; D003198:Computer Simulation; D004734:Energy Metabolism / Q000502:physiology*; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D006801:Humans; D007700:Kinetics; D019344:Lactic Acid / Q000378:metabolism*; D008168:Lung / Q000378:metabolism*; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology*; D052097:Quadriceps Muscle / Q000378:metabolism* / Q000502:physiology
chemical_list: D019344:Lactic Acid