ECSS Paris 2023: OP-PN25
INTRODUCTION: Heat acclimation (HA) has been shown to induce plasma volume expansion, which may stimulate compensatory erythropoiesis and increase hemoglobin mass (Hbmass) [1,2]. However, findings are inconsistent, and most studies have focused exclusively on men, despite possible sex-specific differences in thermoregulatory physiology [3,4]. Additionally, these hematological adaptations may modulate blood viscosity, but this has not been explored with heat acclimation in humans. The aim of this study was to examine the effects of a short-term (10-day) active heat acclimation, compared to equivalent thermoneutral training, on hematological and blood rheological parameters in active women. METHODS: Twenty-eight healthy, active women were randomized into a 10-day HA (n = 15) or thermoneutral (CON; n = 13) training group. Participants cycled for 1 h per day at 40% maximal aerobic power in either 35°C (HA) or 20°C (CON), with 50% relative humidity. Hbmass, along with blood and plasma volumes, were measured in duplicate at baseline and after the intervention using the carbon monoxide rebreathing method. Venous blood samples were also collected for viscosity analysis at varying shear rates. Data were analyzed using a 2x2 (time*group) mixed-effects ANOVA, with statistical significance set at p < 0.05. Data are presented as Mean ± SD. RESULTS: Blood volume increased similarly in both groups (HA, 4.53 ± 0.67 L vs. 4.91 ± 0.84 L; CON, 4.53 ± 0.81 L vs. 4.66 ± 0.44 L; interaction p = 0.197; time p = 0.012), underpinned by plasma volume expansion (HA, 2.73 ± 0.38 L vs. 2.96 ± 0.59 L; CON, 2.67 ± 0.52 L vs. 2.87 ± 0.24 L; interaction p = 0.853; time p = 0.011). Hbmass remained unchanged in both groups (HA, 9.1 ± 1.1 g/kg vs. 9.3 ± 0.9 g/kg; CON, 9.4 ± 0.7 g/kg vs. 9.4 ± 0.8 g/kg; interaction p = 0.112; time p = 0.147). Native blood viscosity decreased in both groups across increasing shear rates (time p = 0.063, 0.008, 0.002, 0.006, 0.019 for 11.25/s, 22.50/s, 45.00/s, 90.00/s, 225.00/s, respectively). This reduction appeared more pronounced in CON, though a significant time*group interaction was only observed at a shear rate of 90.00/s (p = 0.040). CONCLUSION: The addition of a heat stimulus (35°C) to ten consecutive daily 1-h moderate-intensity cycling sessions did not elicit additional hematological benefits compared to equivalent thermoneutral (20°C) training. In particular, both groups exhibited plasma volume expansion with no change in Hbmass. Blood viscosity may be marginally preserved by HA at higher shear rates, though notable decreases were observed in both groups. Overall, these data suggest that short-term HA is insufficient to enhance the hematological response to daily exercise training in healthy, active women, compared to an equivalent thermoneutral training intervention. 1. Dunn et al. (2007) Am J Med Sci, 334, 65-71. 2. Oberholzer et al. (2019) Front Physiol, 10, 1379. 3. Kelly et al. (2023) Sports Med, 53, 1395-421. 4. Anderson et al. (1995) Eur J Appl Physiol, 71, 95-101.
Read CV Benjamin NarangECSS Paris 2023: OP-PN25
INTRODUCTION: Increased sports participation and global warming are posing a greater heat-related risk to individuals. Sensors assessing physiological parameters (e.g. heart rate (HR) and core temperature (Tc)) offer insights into the cardiovascular and thermal demands of individuals exercising in the heat. However, few studies have characterised both cardiovascular and thermal responses in field-based events, with only one study characterising skin temperature (Tsk) during a mass participation event, albeit in elite populations rather than in recreational populations who may be more poorly informed [1, 2]. Therefore, we utilised a real-time monitoring system to profile HR, Tc and Tsk during mass participation distance running in a warm-humid environment and investigated potential modulators of Tc in a tropical and recreational population. METHODS: 162 recreational tropical natives participated in a 21km (n=84) or 10km (n=78) run. Real-time continuous monitoring of HR (n=115) and Tc (n=102) was conducted via a multi-user dashboard. Continuous Tsk (n=34) was assessed using iButtons at four sites. Splits and finishing times were extracted from official results. Multiple linear regression (MLR) analyses were performed across various parameters and peak Tc. RESULTS: Participants achieved peak HR, Tc and Tsk as high as 209bpm, 40.8ºC, and 35.4ºC during 21km and 204bpm, 41.1ºC and 35.3ºC during 10km, respectively. Mean HR (21km: 163±10 (139-188) bpm, 10km: 163±11 (133-180) bpm; p=0.516) and Tsk (21km: 32.9±0.9 (31.6-34.9) ºC, 10km: 33.0±0.8 (30.9-34.6) ºC; p=0.735) were similar between races. Mean Tc was higher in the 21km (38.7±0.5 (37.9-39.8) ºC) than the 10km (38.5±0.4 (37.7-39.9) ºC; p<0.05). Tsk exhibited an inverted-U profile in the 21km but plateaued in the 10km. MLR analysis revealed that starting Tc (16%), mean HR (11%), body surface area (11%), and average speed (7%) significantly contributed to peak Tc. CONCLUSION: This study is the first to integrate Tsk with Tc and HR to provide continuous time-profiles of the cardiovascular and thermal demands of recreational tropical runners, adding to literature that typically focuses on elite populations. The inverted-U Tsk profile in the 21km contradicts the plateau seen in laboratory-based findings and may be explained by air velocity fluctuations and/or differences in pacing in longer distance runs. Real-time monitoring and incorporation of modulators of peak Tc can be used to inform targeted interventions to optimise safety of heat-exposed exercising individuals. References 1. Aylwin, P., et al., Thermoregulatory responses during road races in hot-humid conditions at the 2019 Athletics World Championships. J Appl Physiol (1985), 2023. 134(5): p. 1300-1311. 2. Racinais, S., et al., Association between thermal responses, medical events, performance, heat acclimation and health status in male and female elite athletes during the 2019 Doha World Athletics Championships. Br J Sports Med, 2022. 56(8): p. 439-445.
Read CV Sharifah Badriyah AlhadadECSS Paris 2023: OP-PN25
INTRODUCTION: Global population ageing, the rising prevalence of non-communicable diseases (e.g. obesity, type 2 diabetes) and sedentary behaviour suggest that humans are becoming less resilient to hotter climates. Personal cooling strategies, such as electric fans or self-dousing, have been shown to attenuate the adverse effects of environmental heat stress in a range of conditions. However, these acute strategies fail to provide individualised long-term benefits, such as those afforded by regular physical activity and high aerobic fitness (V̇O2peak). Therefore, this study aimed to determine the relationship between aerobic fitness and thermoregulatory and heart rate responses to a simulated heat wave across the human adult lifespan (18 – 80 y). METHODS: Sixty-one (27 females) participants (median [IQR]: age 44 y [30, 66], body mass 71.9 kg [63.2, 80.6], V̇O2peak 42 ml∙kg-1∙min-1 [31, 49]) completed an incremental treadmill test, dual x-ray absorptiometry scan, and a six-hour heat wave simulation (43°C, 25% relative humidity). Rectal temperature (Tre) and heart rate (HR) were measured throughout the heat wave simulation. Whole-body sweat rate (WBSR) and physiological strain index (PSI) were calculated at the end of the exposure. Age and either V̇O2peak or body fat (%) were used as predictor variables in Bayesian regression models. Data are presented as posterior means, [95% credible intervals] and probability of direction (%). RESULTS: Mean ΔTre during the heat wave simulation was 0.89°C [0.83, 0.95]. An increase in age from 35 to 65 y led to a greater ΔTre of 0.36°C [0.23, 0.49; >99%], whereas a 20 ml∙kg-1∙min-1 increase in V̇O2peak had little effect (-0.00°C [-0.14, 0.13; 55%]). Mean WBSR was 142 g·m-2·h-1 [134, 150] and increasing age from 35 to 65 y lowered WBSR by 13 g·m-2·h-1 [-26, 0; 97%], whilst a 20 ml∙kg-1∙min-1 increase in V̇O2peak increased WBSR by 21 g·m-2·h-1 [6, 35; >99%]. On average, the percentage of maximum HR (%HRmax) during the last hour of the heat wave simulation was 48% [46, 49]. Increasing age from 35 to 65 y led to a predicted change in %HRmax of -1% [-4, 2; 73%], whereas increasing V̇O2peak by 20 ml∙kg-1∙min-1 changed %HRmax by -5% [-8, -2; >99%]. Mean PSI was low (2.2 [2.0, 2.5]). Increasing age from 35 to 65 y increased the PSI by 0.6 [0.2, 1.0; >99%], whilst a 20 ml∙kg-1∙min-1 increase in V̇O2peak had no effect (-0.0 [-0.5, 0.4; 59%]). When body fat percentage was included in the models instead of V̇O2peak, a 15% increase in body fat reduced WBSR (18 g·m-2·h-1 [5, 31; >99%]) and increased %HRmax (5% [2, 7; >99%]) but had no effect on PSI or ΔTre. CONCLUSION: Ageing increases thermal and physiological strain during a six-hour heat wave simulation in 43°C and 25% relative humidity. A greater V̇O2peak attenuates cardiovascular strain (i.e. %HRmax), likely facilitated by a greater WBSR, but has little effect on ΔTre or PSI.
Read CV Harry BrownECSS Paris 2023: OP-PN25