title: Thermoregulatory responses to exercise at a fixed rate of heat production are not altered by acute hypoxia.
abstract: This study sought to assess the within-subject influence of acute hypoxia on exercise-induced changes in core temperature and sweating. Eight participants [1.75 (0.06) m, 70.2 (6.8) kg, 25 (4) yr, 54 (8) ml·kg-1·min-1] completed 45 min of cycling, once in normoxia (NORM; [Formula: see text] = 0.21) and twice in hypoxia (HYP1/HYP2; [Formula: see text]= 0.13) at 34.4(0.2)°C, 46(3)% RH. These trials were designed to elicit1) two distinctly different %V̇o2peak[NORM: 45 (8)% and HYP1: 62 (7)%] at the same heat production (Hprod) [NORM: 6.7 (0.6) W/kg and HYP1: 7.0 (0.5) W/kg]; and2) the same %V̇o2peak[NORM: 45 (8)% and HYP2: 48 (5)%] with different Hprod[NORM: 6.7 (0.6) W/kg and HYP2: 5.5 (0.6) W/kg]. At a fixed %V̇o2peak, changes in rectal temperature (ΔTre) and changes in esophageal temperature (ΔTes) were greater at end-exercise in NORM [ΔTre: 0.76 (0.19)°C; ΔTes: 0.64 (0.22)°C] compared with HYP2 [ΔTre: 0.56 (0.22)°C,P< 0.01; ΔTes: 0.42 (0.21)°C,P< 0.01]. As a result of a greater Hprod(P< 0.01) in normoxia, and therefore evaporative heat balance requirements, to maintain a similar %V̇o2peakcompared with hypoxia, mean local sweat rates (LSR) from the forearm, upper back, and forehead were greater (allP< 0.01) in NORM [1.10 (0.20) mg·cm-2·min-1] compared with HYP2 [0.71 (0.19) mg·cm-2·min-1]. However, at a fixed Hprod, ΔTre[0.75 (0.24)°C;P= 0.77] and ΔTes[0.63 (0.29)°C;P= 0.69] were not different in HYP1, compared with NORM. Likewise, mean LSR [1.11 (0.20) mg·cm-2·min-1] was not different (P= 0.84) in HYP1 compared with NORM. These data demonstrate, using a within-subjects design, that hypoxia does not independently influence thermoregulatory responses. Additionally, further evidence is provided to support that metabolic heat production, irrespective of %V̇o2peak, determines changes in core temperature and sweating during exercise.NEW & NOTEWORTHYUsing a within-subject design, hypoxia does not independently alter core temperature and sweating during exercise at a fixed rate of heat production. These findings also further contribute to the development of a methodological framework for assessing differences in thermoregulatory responses to exercise between various populations and individuals. Using the combined environmental stressors of heat and hypoxia we conclusively demonstrate that exercise intensity relative to aerobic capacity (i.e., %V̇o2max) does not influence changes in thermoregulatory responses.
authors.affiliation: School of Human Kinetics, Faculty of Health Sciences, University of Ottawa, Ontario, Canada.
authors.forename: Geoff B
authors.initials: GB
authors.lastname: Coombs
authors.affiliation: School of Human Kinetics, Faculty of Health Sciences, University of Ottawa, Ontario, Canada.|Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas and the University of Texas Southwestern Medical Center, Dallas, Texas; and.
authors.forename: Matthew N
authors.identifier: http://orcid.org/0000-0001-7400-8353
authors.initials: MN
authors.lastname: Cramer
authors.affiliation: School of Human Kinetics, Faculty of Health Sciences, University of Ottawa, Ontario, Canada.|Thermal Ergonomics Laboratory, University of Sydney, Lidcombe, New South Wales, Australia.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Ravanelli
authors.affiliation: School of Human Kinetics, Faculty of Health Sciences, University of Ottawa, Ontario, Canada.
authors.forename: Pascal
authors.initials: P
authors.lastname: Imbeault
authors.affiliation: School of Human Kinetics, Faculty of Health Sciences, University of Ottawa, Ontario, Canada; ollie.jay@sydney.edu.au.|Thermal Ergonomics Laboratory, University of Sydney, Lidcombe, New South Wales, Australia.
authors.forename: Ollie
authors.identifier: http://orcid.org/0000-0002-6076-6337
authors.initials: O
authors.lastname: Jay
mesh_terms: D001831:Body Temperature / Q000502:physiology; D001833:Body Temperature Regulation / Q000502:physiology*; D015444:Exercise / Q000502:physiology*; D005260:Female; D006358:Hot Temperature; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D013546:Sweating / Q000502:physiology; D022722:Thermogenesis / Q000502:physiology*
keywords: core temperature; cutaneous vascular conductance; heat stress; hypoxemia; skin blood flow; sweating; thermoregulation