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International Conference on Environmental Ergonomics
Sex-based differences in core temperature during repeat exercise in the heat
INTRODUCTION: When working in extreme heat, the Australian Army refer to work tables that assume body core temperatures will on average peak at 38.5°C at the end of work. We examined the sex-based differences in peak body core temperature following four work and recovery cycles in the heat.
METHOD: Fourteen males (M: 32 ± 8.21yr; 179.78 ± 4.82cm; 76.5 ± 6.4kg, 54.39 ± 8.68ml/kg/min) and thirteen females (F: 31.2 ± 7.25yr; 166 ± 6.73cm; 58.06 ± 6.09kg, 51.73 ± 7.18ml/kg/min) performed four successive bouts of treadmill walking in 32.5°C Wet Bulb Globe Temperature (WBGT) at a constant work rate, alternating between 35 minutes (~600W) and 55 minutes (~400W), each separated by 30-min seated rest at 28°C WBGT as per current work tables. Participants wore standardised military clothing including body armour and a helmet. Women were tested on days 5-8 of their menstrual cycle. Peak heart rate (HR), rectal (Tc) and 4-site mean skin temperature (Ts) were compared across exercise periods. Statistical analyses were conducted using repeated measures ANOVA.
RESULTS: Peaks in Tc were significantly different between the exercise bouts (E1-4) (M: E1 38.20 ±0.31°C, E2 38.36 ±0.34°C, E3 38.49 ±0.42°C, E4 38.32 ±0.41°C / FE1 38.05 ±0.40°C, E2 38.09 ±0.37°C, E3 38.33 ±0.38°C, E4 38.15 ±0.35°C) (P=<0.001), but not between males and females (P=0.163). Five males and four females reached 38.5°C by the conclusion of the fourth work. There was no difference in peak HR (P=0.522) or peak Ts (P=0.336) between males or females, nor between exercise bouts (HR: P=0.194) (Ts: P=0.586).
CONCLUSION: Participants on average did not meet the assumed body core temperature of 38.5°C within four work and recovery cycles. These findings indicate the current work tables can apply to both males and females across four work bouts.
Daily cold-water recovery may impair training load tolerance during short-term heat acclimation
INTRODUCTION: Currently, there is limited understanding of the effects of heat acclimation (HA) on perceptual training load (TL) or the interaction with common thermal recovery strategies. This study aimed to examine the effects of daily cold- and hot-water recovery on perceived TL during short-term HA training.
METHODS: Eight healthy, trained males undertook 5-days of cycle training for 60 min in four different conditions, using a block counter-balanced order design. Three conditions were completed in the heat (35 °C) and one in a thermoneutral environment (24 °C, CON). Each day after cycling, participants’ completed recovery 20 min seated rest (CON and HA), cold- (14 °C; HACWI) or hot-water immersion (39 °C; HAHWI). Heart rate, rectal and skin temperature, and rating of perceived exertion (RPE) were collected during training. Session RPE (sRPE) was collected after training for the determination of perceived TL. Data was analysed using Bayesian hierarchical regression, Cohens d was calculated, and for perceived TL, the probability that d >0.5 was also computed.
RESULTS: Bayesian analysis showed evidence of increased perceived TL in HACWI compared to HA on days 3–5 (d=2.26-2.69). The probability that the d >0.5 for days 3, 4 and 5 were 0.98, 0.98 and 0.96, respectively. There was evidence that the increased perceived TL coincided with a greater exercise heart rate (3–8 b·min-1 higher; d=2.34-3.00) and higher RPE on days 4 (d=2.46) and 5 (d=2.27). There was little evidence that hot-water altered perceived TL or heat adaptation.
CONCLUSION: Daily cold-water recovery increases percevied TL, interefers with heat adpatation, and impairs TL tolerance during 5-days of fixed-intensity HA, and hot-water immersion provides no additional benefit. Considerations for the effects of thermal recovery strategies on TL are required when implementing to avoid counteracting the desired HA outcomes.
Short-term heat acclimation training enhances knee extensor strength and improves cycling performance in hot conditions
INTRODUCTION: The redistribution of blood flow from splanchnic regions during exercise in the heat may compromise gastrointestinal permeability and facilitate endotoxin leakage. Subsequent inflammatory responses are suggested to cause neuromuscular fatigue. This study examined the protective neuromuscular and inflammatory effects of short-term heat acclimation (HA) on cycling performance in the heat.
METHODS: Eight recreationally-trained males completed a 5-day cycling training block (60 min∙day-1 at 50% Pmax) in hot (HA: 35±1 °C, 53±4% relative humidity (RH)) and thermoneutral (CON: 22.2±2.6 °C, 65±8% RH) conditions using a randomised cross-over design. Pre- and post-intervention TT’s were completed in the heat. Neuromuscular assessment of the knee extensors was completed pre- and immediately after the TT’s and on the first and last day of each training block. Blood samples were also collected at these same time points and analysed for endotoxins, inflammation and markers of gut damage. Data were analysed using Bayesian hierarchical regression, and Cohens d effect sizes were also calculated.
RESULTS: Statically faster TT completion times was apparent after HA compared to CON (MD=55s [11, 98], d=2.51 [0.49, 4.46]). While pre- to post-intervention improvements were observed in HA (MD=62s [18, 104], d=2.86 [0.82, 4.75]), no clear difference was seen in CON (MD=30s [-6, 67]). Interestingly, knee extensor strength increased with HA but declined in CON. Further, despite the faster post-intervention HA TT performance, no difference was found for central fatigue, circulating endotoxin levels, inflammation, or markers of gut damage between conditions.
CONCLUSION: Short-term HA training improves subsequent 20TT cycling performance in the heat by 2.9% [0.8-4.9] without an associated increase in intestinal damage or inflammation. These findings suggest that short-term HA training may be a time-efficient training method to improve neuromuscular function and cycling performance in hot conditions.
Extending work tolerance time in the heat in protective ensembles with commercially available cooling methods
INTRODUCTION: Physical roles necessitating the use of chemical, biological, radiological, or nuclear (CBRN) protective ensembles in the heat accentuate thermal and cardiovascular strain during work. As a result, work tolerance times are shortened with an increased threat of heat exhaustion if work is maintained. We, therefore, investigated commercially available cooling methods and their ability to reduce physiological strain and/or extend work tolerance time in those working in the heat dressed in a CBRN protective ensemble.
METHODS: Eight males wore a CBRN ensemble (MT94, Lion Apparel, USA; 15.3 kg) and walked for a maximum of 120 minutes at 35 °C, 50 % relative humidity. In a randomised order, participants completed the trial with no cooling and one of four cooling protocols: 1) ice-based cooling vest (IV), 2) a non-ice-based cooling vest (PCM), 3) ice slushy consumed before work, combined with IV (SLIV) and 4) a portable battery-operated water-perfused suit (WPS). Mean with 95 % confidence intervals are presented.
RESULTS: Tolerance time was extended in PCM (46 [36–56] mins, P = 0.018), SLIV (56 [46–67] mins, P < 0.001) and WPS (62 [53–70] mins, P < 0.001), compared with control (39 [30–48] mins). Tolerance time was longer in SLIV and WPS compared with both IV (48 [39–58 mins]) and PCM (P ≤ 0.011). After 20 min of work, HR was lower in SLIV (121[105–136] beats·min–1) and WPS (117[101–133] beats·min–1) compared with control (137[120–155] beats·min–1), IV (130[116–143] beats·min–1) and PCM (133[116–151] beats·min–1) (all P < 0.001).
CONCLUSION: All cooling methods utilised in the present study can reduce physiological strain, while SLIV and WPS are most likely to extend tolerance time for those working in the heat dressed in a CBRN ensemble.
Commercially available cooling systems can extend work time in gas-tight protective clothing
Ice vests extend physiological work time while wearing explosive ordnance disposal protective clothing in hot conditions
Agreement between chest and mean skin temperature: Influence of clothing ensemble and measurement device