A retrospective analysis to determine if exercise training‐induced thermoregulatory adaptations are mediated by increased fitness or heat acclimation

Abstract

It remains unclear whether aerobic fitness, as defined by the maximum rate of oxygen consumption (V O 2 max), independently improves heat dissipation in uncompensable environments, or whether the thermoregulatory adaptations associated with heat acclimation are due to repeated bouts of exercise-induced heat stress during regular aerobic training. The present analysis sought to determine ifV O 2 max independently influences thermoregulatory sweating, maximum skin wettedness (max) and the change in rectal temperature (ΔT re) during 60 min of exercise in an uncompensable environment (37.0 ± 0.8 • C, 4.0 ± 0.2 kPa, 64 ± 3% relative humidity) at a fixed rate of heat production per unit mass (6 W kg −1). Retrospective analyses were performed on 22 participants (3 groups), aerobically unfit (UF; n = 7;V O 2 max : 41.7 ± 9.4 ml kg −1 min −1), aerobically fit (F; n = 7;V O 2 max : 55.6 ± 4.3 ml kg −1 min −1 ; P < 0.01) and aerobically unfit (n = 8) individuals, before (pre;V O 2 max : 45.8 ± 11.6 ml kg −1 min −1) and after (post; V O 2 max : 52.0 ± 11.1 ml kg −1 min −1 ; P < 0.001) an 8-week training intervention. max was similar between UF (0.74 ± 0.09) and F (0.78 ± 0.08, P = 0.22). However, max was greater post-(0.84 ± 0.08) compared to pre-(0.72 ± 0.06, P = 0.02) training. During exercise, mean local sweat rate (forearm and upper-back) was greater post-(1.24 ± 0.20 mg cm −2 min −1) compared to pre-(1.04 ± 0.25 mg cm −2 min −1 , P < 0.01) training, but similar between UF (0.94 ± 0.31 mg cm −2 min −1 , P = 0.90) and F (1.02 ± 0.30 mg cm −2 min −1). The ΔT re at 60 min of exercise was greater pre-(1.13 ± 0.16 • C, P < 0.01) compared to post-(0.96 ± 0.14 • C) training, but similar between UF (0.85 ± 0.29 • C, P = 0.22) and F (0.95 ± 0.22 • C). Taken together, aerobic training, notV O 2 max per se, confers an increased max , greater sweat rate, and smaller rise in core temperature during uncompensable heat stress in fit individuals.

Individual Responses to Heat Stress: Implications for Hyperthermia and Physical Work Capacity

“…Most recently, 8 weeks exercise training ( V̇ O 2 max increase from 46 to 52 ml/kg/min) increased local sweat rate, and thus skin wettedness from 72 to 85% surface area ( Ravanelli et al, 2018 ). The increased sweating function in the above studies may not be related to aerobic fitness per se , but more due to frequent and persistent rises in T c due to the training itself, evoking a mild heat adaptation ( Ravanelli et al, 2020 ).…”

Short-Term Repeated-Sprint Training in Hot and Cool Conditions Similarly Benefits Performance in Team-Sport Athletes

“…However, given the relatively small differences noted between conditions, it is debatable whether the adaptations noted in the heat had an appreciable physiological impact, despite improving thermal perception ( Figure 4 ). The similar adaptive response observed between groups highlights the partial heat acclimation phenotype exhibited by athletes regularly training in cool conditions ( Avellini et al, 1982 ; Armstrong and Pandolf, 1988 ), which could partly explain the lack of a greater difference in thermoregulatory improvements ( Ravanelli et al, 2020 ), with this trained cohort requiring a greater thermal stimulus for adaptation. Although the current results indicate the emergence of thermoregulatory and perceptual adaptations in the HOT group, there appears to be potential for an extended repeated-sprint training regimen to more fully induce heat acclimation in trained individuals.…”

“…However, given the relatively small differences noted between conditions, it is debatable whether the adaptations noted in the heat had an appreciable physiological impact, despite improving thermal perception (Figure 4). The similar adaptive response observed between groups highlights the partial heat acclimation phenotype exhibited by athletes regularly training in cool conditions (Avellini et al, 1982;Armstrong and Pandolf, 1988), which could partly explain the lack of a greater difference in thermoregulatory improvements (Ravanelli et al, 2020), with FIGURE 4 | Rating of perceived exertion and thermal sensation during a RST including a 22 min submaximal warm-up and four sets of repeated sprints performed in 40°C and 40% RH prior to and following a repeated-sprint training intervention undertaken in COOL (20°C and 40% RH) or HOT (40°C and 40% RH) conditions. Data are mean with 95% CI.…”

Effect of aerobic fitness on the relation between age and whole‐body heat exchange during exercise‐heat stress: a retrospective analysis

“…The specific mechanism(s) driving those improvements are not fully understood. However, it is presumed they represent thermoregulatory adaptations (akin to heat acclimation) associated with performing aerobic activity that causes sustained elevations in body temperature (Buono & Sjoholm, 1988; Gisolfi & Robinson, 1969), rather than a direct consequence of increases in (Avellini, Shapiro, Fortney, Wenger, & Pandolf, 1982; Ravanelli, Gagnon, Imbeault, & Jay, 2020; Stapleton, Gagnon, & Kenny, 2010) or aerobic activity per se (Amano, Koga, Inoue, Nishiyasu, & Kondo, 2013; Lamarche, Notley, Poirier, & Kenny, 2019). It follows that individuals who frequently engage in exercise in hot environments may too develop adaptations that attenuate the age‐related decline in thermoregulatory function, albeit the specific ‘dose’ required to induce those adaptations requires further exploration.…”