Self-reported physical activity level does not alter whole-body total heat loss independently of aerobic fitness in young adults during exercise in the heat

Effects of sex and menstrual cycle on sweating during isometric handgrip exercise and postexercise forearm occlusion

NEW FINDINGS

What is the central question of this study? Do sex and menstrual cycle modulate sweating during isometric handgrip exercise and muscle metaboreceptor stimulation? What is the main finding and its importance? Sex modulates sweating during isometric handgrip exercise, as indicated by the lower sweat output per gland in women than in men, but not during muscle metaboreceptor stimulation. Sweat output per gland during isometric handgrip exercise and muscle metaboreceptor stimulation were lower in the mid-luteal phase than in the early follicular phase in women. Cholinergic sweat gland sensitivity might explain, in part, the individual variation of the response. Our results provide new insights regarding sex- and menstrual cycle-related modulation of the sweating response.

ABSTRACT

We investigated whether sex and menstrual cycle could modulate sweating during isometric handgrip (IH) exercise and muscle metaboreceptor stimulation. Twelve young, healthy women in the early follicular (EF) and mid-luteal (ML) phases and 14 men underwent two experimental sessions consisting of a 1.5 min IH exercise at 25 and 50% of maximal voluntary contraction (MVC) in a hot environment (35°C, relative humidity 50%) followed by 2 min forearm occlusion to stimulate muscle metaboreceptors. Sweat rates, the number of activated sweat glands and the sweat output per gland (SGO) on the forearm and chest were assessed. Pilocarpine-induced sweating was also assessed via transdermal iontophoresis to compare the responses with those of IH exercise and muscle metaboreceptor stimulation, based on correlation analysis. Sweat rates on the forearm and chest during IH exercise and muscle metaboreceptor stimulation did not differ between men and women in either menstrual cycle phase (all P ≥ 0.144). However, women in both phases showed lower SGO on the forearm and/or chest compared with men during IH exercise at 50% of MVC, with no differences in muscle metaboreceptor stimulation. Women in the ML phase had a lower forearm sweat rate during IH exercise at 50% of MVC (P = 0.015) and SGO during exercise and muscle metaboreceptor stimulation (main effect, both P ≤ 0.003) compared with those in the EF phase. Overall, sweat rate and SGO during IH exercise and muscle metaboreceptor stimulation were correlated with pilocarpine-induced responses (all P ≤ 0.064, r ≥ 0.303). We showed that sex and menstrual cycle modulate sudomotor activity during IH exercise and/or muscle metaboreceptor stimulation. Cholinergic sweat gland sensitivity might explain, in part, the individual variation of the response.

Individual characteristics associated with the magnitude of heat acclimation adaptations

Purpose

The magnitude of heat acclimation (HA) adaptations varies largely among individuals, but it remains unclear what factors influence this variability. This study compared individual characteristics related to fitness status and body dimensions of low-, medium-, and high responders to HA.

Methods

Twenty-four participants (9 female, 15 male; maximum oxygen uptake [\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{{V}}$$\end{document}V˙O_2peak,kg] 52 ± 9 mL kg⁻¹ min⁻¹) completed 10 daily controlled-hyperthermia HA sessions. Adaptations were evaluated by heat stress tests (HST; 35 min cycling 1.5 W  kg⁻¹; 33 °C, 65% relative humidity) pre- and post-HA. Low-, medium-, and high responder groups were determined based on tertiles (n = 8) of individual adaptations for resting rectal temperature (T_re), exercise-induced T_re rise (ΔT_re), whole-body sweat rate (WBSR), and heart rate (HR).

Results

Body dimensions (p > 0.3) and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{{V}}$$\end{document}V˙O_2peak,kg (p > 0.052) did not differentiate low-, medium-, and high responders for resting T_re or ΔT_re. High WBSR responders had a larger body mass and lower body surface area-to-mass ratio than low responders (83.0 ± 9.3 vs 67.5 ± 7.3 kg; 249 ± 12 vs 274 ± 15 cm² kg⁻¹, respectively; p < 0.005). Conversely, high HR responders had a smaller body mass than low responders (69.2 ± 6.8 vs 83.4 ± 9.4 kg; p = 0.02). \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{{V}}$$\end{document}V˙O_2peak,kg did not differ among levels of responsiveness for WBSR and HR (p > 0.3).

Conclusion

Individual body dimensions influenced the magnitude of sudomotor and cardiovascular adaptive responses, but did not differentiate T_re adaptations to HA. The influence of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\dot{{V}}$$\end{document}V˙O_2peak,kg on the magnitude of adaptations was limited.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00421-021-04626-3.

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

NEW FINDINGS

What is the central question of this study? Are fitness-related improvements in thermoregulatory responses during uncompensable heat stress mediated by aerobic capacity V ̇ O 2 max or is it the partial heat acclimation associated with training? What is the main finding and its importance? During uncompensable heat stress, individuals with high and low V ̇ O 2 max displayed similar sweating and core temperature responses whereas exercise training in previously untrained individuals resulted in a greater sweat rate and a smaller rise in core temperature. These observations suggest that it is training, not V ̇ O 2 max per se, that mediates thermoregulatory improvements during uncompensable heat stress.

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 if V ̇ O 2 max independently influences thermoregulatory sweating, maximum skin wettedness (ω_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. ω_max was similar between UF (0.74 ± 0.09) and F (0.78 ± 0.08, P = 0.22). However, ω_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, not V ̇ O 2 max per se, confers an increased ω_max , greater sweat rate, and smaller rise in core temperature during uncompensable heat stress in fit individuals.