The time course of adaptations in thermoneutral maximal oxygen consumption following heat acclimation

European Journal of Applied Physiology (2019) 119:2391–2399 https://doi.org/10.1007/s00421-019-04218-2 ORIGINAL ARTICLE The time course of adaptations in thermoneutral maximal oxygen consumption following heat acclimation Mark Waldron1,2 · O. Jeffries3 · J. Tallent4 · S. Patterson4 · V. Nevola5 Received: 22 March 2019 / Accepted: 24 August 2019 / Published online: 12 September 2019 © The Author(s) 2019 Abstract Purpose This study investigated the effects of a 10-day heat acclimation (HA) programme on the time course of changes in thermoneutral maximal oxygen uptake (V̇ O2max) during and up to 10 days post-HA. Methods Twenty-two male cyclists were assigned to a HA or control (Con) training group following baseline ramp tests of thermoneutral V̇ O2max. Ten days of fixed-intensity (50% baseline V̇ O2max) indoor cycling was performed in either ~ 38.0 °C (HA) or ~ 20 °C (Con). V̇ O2max was re-tested on HA days 5, 10 and post-HA days 1, 2, 3, 4, 5 and 10. Results V̇ O2max initially declined across time in both groups during training (P < 0.05), before increasing in the post-HA period in both groups (P < 0.05). However, V̇ O2max was higher than control by post-HA day 4 in the HA group (P = 0.046). Conclusions The non-linear time course of V̇ O2max adaptation suggests that post-testing should be performed 96-h posttraining to identify the maximal change for most individuals. In preparation for training or testing, athletes can augment their aerobic power in thermoneutral environments by performing 10 days HA, but the full effects will manifest at varying stages of the post-HA period. Keywords Thermal · Training · Cycling · Endurance · Heat acclimation · Maximal aerobic power Abbreviations CO2 Carbon dioxide Con Control Tc Core body temperature HA Heat acclimation HR Heart rate Hct Hematocrit Hb Haemoglobin V̇ O2max Maximal oxygen uptake PV Plasma volume Communicated by Narihiko Kondo. * Mark Waldron 1 College of Engineering, Swansea University, Swansea, UK 2 School of Science and Technology, University of New England, Armidale, NSW, Australia 3 School of Biomedical Sciences, Newcastle University, Newcastle Upon Tyne, UK 4 Sport, Health and Applied Sciences, St Mary’s University, London, UK 5 Defence Science and Technology Laboratory (Dstl), Fareham, Hampshire, UK RH Relative humidity RPE Rating of perceived exertion Ts Thermal sensation Introduction Heat acclimation (HA) describes a systematic process, whereby serial exposures to artificially heated environments, often in combination with exercise, can lead to rapid adaptations that enhance the capacity to thermoregulate in the heat (Gibson et al. 2015; Taylor and Cotter 2006). These adaptations improve heat tolerance (Sawka et al. 2011), characterised by increased sudomotor function (Fox et al. 1963), reduced heart rate (HR) (Senay et al. 1976), hypervolemia (Nielsen et al. 1993), increased cardiac output (Lorenzo et al. 2010) and a reduced core body temperature (Tc) for a given heat exposure or exercise intensity (Nielsen et al. 1993). This combination of physiological adaptations can enhance endurance performance in hot (Racinais et al. 2015) and thermoneutral environments (Lorenzo et al. 2010). The maximal rate of oxygen uptake (V̇ O 2max) is an important determinant of endurance performance, explaining ~ 20–60% of the variation in performances of different 13 Vol.:(0123456789) 2392 mode and distance (Coyle et al. 1988; Schabort et al. 2000; Jacobs et al. 2011). V̇ O2max is chiefly limited by central cardiovascular factors, such as O2 transport (Bassett and Howley 2000). Heat acclimation can improve V̇ O2max, and is thought to occur owing to heat-induced cardiovascular changes (Périard et al. 2016). Indeed, there have been historical observations of a relationship between V̇ O2max and heat tolerance (Shvartz et al. 1978; Pandolf 1979; Havenith and van Middendorp 1990). However, evidence of the efficacy of HA on V̇ O2max is equivocal, with a number of studies demonstrating 4–13% changes (Nadel et al. 1974; Shvartz et al. 1977; Sawka et al. 1985; Pivarnik et al. 1987; Lorenzo et al. 2010; James et al. 2017) and others reporting no change or a reduction following a range of HA protocols (Houmard et al. 1990; Gore et al. 1997; Chen et al. 2013; Karlsen et al. 2015; Keiser et al. 2015; Neal et al. 2016a, b; Rendell et al. 2017; Sotiridis et al. 2018). There are various explanations for discrepancies in the aforementioned findings. First, not all studies have evaluated adaptations in V̇ O2max in thermoneutral conditions, thus limiting the understanding of a transfer between heat-induced adaptation and aerobic capacity in temperate environments, which is a topic of ongoing debate (Nybo and Lundby 2016). This could be related to limited V̇ O2max trainability (Bouchard et al. 2011) or inter-individual and inter-system differences in adaptation reported after 11 days of isothermal acclimation (Corbett et al. 2018). It is therefore possible that V̇ O2max increases more substantially among HA-responsive individuals compared to their less responsive counterparts (Minson and Cotter 2016). Daanen et al. (2011) also demonstrated that the acute stress imposed by HA acted, initially, to suppress the adaptation in Tc, before adaptation beyond baseline in the days following acclimation. Thus, a period of recovery (from heat, exercise or both) might be necessary to fully realise adaptation in multi-system physiological measurements, such as V̇ O2max. This reasoning is consistent with the tenets of general adaptation syndrome (Selye 1950), which has been incorporated into exercise training guidelines (ACSM 2009). However, there has been no study to evaluate the detailed time course of change in thermoneutral V̇ O2max after HA. Therefore, it is possible that previous studies have not: (1) monitored the inter-individual time course of responses of V̇ O2max to HA across successive days and (2) provided adequate time for adaptation to the combined thermal and exercise training stimuli. Based on this reasoning, we investigated the effects of a 10-day HA programme on the time course of changes in thermoneutral V̇ O2max, during and up to 10 days postHA against a control training group. We hypothesised that increases in V̇ O2max would occur in the HA group in the days following the intervention but presumed variability between individuals in the time course of this response in the postHA 10-day period. 13 European Journal of Applied Physiology (2019) 119:2391–2399 Methods Participants Twenty-two healthy, trained amateur male cyclists provided written informed consent to take part in this study. Twelve of the participants (age 23 ± 3 years, stature 1.77 ± 0.61 m, body mass 73.7 ± 4.8 kg, V̇ O2max 60.8 ± 6.1 ml kg−1 min−1) were randomly allocated to a HA group, while ten were allocated to a control group (age 25 ± 3 years, stature 1.78 ± 0.46 m, body mass 74.1 ± 5.6 kg, V̇ O2max 59.8 ± 6.7 ml kg−1 min−1). All of the (...truncated)