Effect of short‐term heat acclimation with permissive dehydration on thermoregulation and temperate exercise performance

We examined the effect of short‐term heat acclimation with permissive dehydration (STHADe) on heat acclimation (HA) and cycling performance in a temperate environment. Ten trained male cyclists [mean (SD) maximal oxygen uptake: 63.3(4.0) mL/kg/min; peak power output (PPO): 385(40) W; training: 10 (3) h/week] underwent a STHADe program consisting of 5 days of exercise (maximum 90 min/day) in a hot environment (40 °C, 50% RH) to elicit isothermic heat strain [rectal temperature 38.64(0.27) °C]. Participants abstained from fluids during, and 30 min after, HA sessions. Pre‐ and post‐STHADe HA was evaluated during euhydrated fixed‐intensity exercise (60 min) in hot conditions; the effect of STHADe on thermoregulation was also examined under temperate conditions (20 min fixed‐intensity exercise; 22 °C, 60% RH). Temperate cycling performance was assessed by a graded exercise test (GXT) and 20‐km time trial (TT). STHADe reduced thermal and cardiovascular strain in hot and temperate environments. Lactate threshold [Δ = 16 (17) W] and GXT PPO [Δ = 6 (7) W] were improved following STHADe (P < 0.05), but TT performance was not affected (P > 0.05), although there was a trend for a higher mean power (P = 0.06). In conclusion, STHADE can reduce thermal and cardiovascular strain under hot and temperate conditions and there is some evidence of ergogenic potential for temperate exercise, but longer HA regimens may be necessary for this to meaningfully influence performance.     

Effects of hydration status during heat acclimation on plasma volume and performance

The impact of hydration status was investigated during a 5‐day heat acclimation (HA) training protocol vs mild/cool control conditions on plasma volume (PV) and performance (20 km time‐trial [TT]). Sub‐elite athletes were allocated to one of two heat training groups (90 min/day): (a) dehydrated to ~2% body weight (BW) loss in heat (35°C; DEH; n = 14); (b) euhydrated heat (35°C; EUH; n = 10), where training was isothermally clamped to 38.5°C core temperature (T_c). A euhydrated mild control group (22°C; CON; n = 9) was later added, with training clamped to the same relative heart rate (~75% HR_max) as elicited during DEH and EUH; thus all groups experienced the same internal training stress (%HR_max). Five‐day total thermal load was 30% greater (P < 0.001) in DEH and EUH vs CON. There were significant differences in the average percentage of maximal work rate (%W_max) across all groups (DEH: 24 ± 6%; EUH: 34 ± 9%; CON: 48 ± 8%W_max) during training required to elicit the same %HR_max (77 ± 4% HR_max). There were no significant differences pre‐to post‐HA between groups for PV (DEH: +1.7 ± 10.1%; EUH: +4.8 ± 10.2%; CON: +5.2 ± 4.0%), but there was a significant pooled group PV increase, as well as a 97% likely pooled improvement in TT performance (DEH: ?1.8 ± 2.8%; EUH: ?1.9 ± 2.1%, CON; ?1.8 ± 2.8%; P = 0.136). Due to a lack of between‐group differences for PV and TT, but pooled group increases in PV and 97% likely group increase in TT performance, over 5 days of intense training at the same average relative cardiac load suggests that overall training stress may also impact significant adaptations beyond heat and hydration stress.     

Effects of acclimation on cognitive performance in soldiers during exertional heat stress

This study investigates the effects of exertional heat stress and acclimation status on physiological and cognitive performance. Forty male soldiers performed an exertional heat stress test (EHST) either in a cool (20 degrees C, 16 degrees C wet bulb globe temperature), or in a hot environment (40 degrees C, 29 degrees C wet bulb globe temperature), unacclimatized, or after 10 days of passive or active acclimation. Mean skin and tympanic (Tty) temperatures and heart rates (HR) measured physiological strain. A cognitive test (the computerized Cambridge Neuropsychological Test Automated Batteries attention battery) is administered before and immediately after EHST. EHST in hot conditions induced physiological heat stress (increase in Tty and HR), which caused mild deficits in attention in U group (decreased number of correct responses, and prolonged movement time). Acclimated (passive and active) soldiers suffered no detrimental effects of exertional heat stress, despite almost the same degree of heat strain, measured by Tty and HR.     

Effect of heat acclimation on sweat minerals

PURPOSE: This study examined the impact of 10 d of exercise-heat acclimation on sweat mineral concentrations. METHODS: Eight male subjects walked on a treadmill at 3.5 mph, 4% grade for 100 continuous minutes or until rectal temperature reached 39.5 degrees C on 10 consecutive days in an environmental chamber set at 45 degrees C, 20% relative humidity. Arm sweat samples were collected during the first 30 min of exercise-heat stress on days 1 and 10 using a polyethylene arm glove. RESULTS: Final core temperature and HR values were significantly lower (P < 0.05) on day 10 versus day 1. Whole-body sweating rates increased by approximately 6% (P = 0.12). Sweat sodium concentration on day 10 (36.22 +/- 7.22 mM) was significantly lower than day 1 (54.49 +/- 16.18 mM) (P < 0.05). Sweat mineral concentrations of calcium (approximately 29%), copper (approximately 50%), and magnesium (approximately 43%) were also significantly lower on day 10 versus day 1 of heat acclimation (P < 0.05). A trend for lower sweat iron (approximately 75%; P = 0.07) and zinc (approximately 23%; P = 0.10) concentrations were observed from day 1 to day 10. The estimated hourly sweat mineral losses (arm concentration x whole-body sweat rate) were reduced for calcium (approximately 27%), copper (approximately 46%), and magnesium (approximately 42%) (P < 0.05), but not iron (75%) or zinc (approximately 16%) (P > 0.05), from day 1 to day 10. CONCLUSION: Exercise-heat acclimation conserves arm sweat mineral concentrations and possibly whole-body sweat losses of calcium, copper, and magnesium, and may reduce sweat iron and zinc concentrations.     

Effects of spinal cord lesion level upon thermoregulation during exercise in the heat

PURPOSE: This study examined the effects of the level of spinal cord injury upon the thermoregulatory responses of wheelchair athletes during prolonged wheelchair exercise in warm conditions. METHODS: Eight tetraplegic (TP), 10 high-level paraplegic (HP), and 10 low-level paraplegic (LP) athletes exercised for 60 min at 60% [OV0312]O(2peak) in a warm environment (31.5 +/- 1.7 degrees C, 42.9 +/- 8.0% relative humidity). Skin temperature and aural temperature were monitored. RESULTS: Aural temperature increased gradually during the exercise period by 1.1 +/- 0.3 and 1.4 +/- 0.5 degrees C for the HP and LP groups, with a more marked increase observed for the TP group (2.1 +/- 0.5 degrees C; P < 0.05). Upper-arm skin temperature was higher for the TP when compared with the HP and LP between 30 and 60 min (P < 0.05). Back skin temperature was higher for the TP when compared with the HP and LP between 45 and 60 min (P < 0.05). No differences were noted between groups for the thigh or calf skin temperatures. During recovery, skin temperature remained elevated for the TP group when compared with the HP and LP groups (P < 0.05). Heat storage was greatest for the TP athletes at the end of exercise and remained elevated throughout recovery (P < 0.05). CONCLUSIONS: All athletes completed the exercise task even though the gradual increase in aural temperature observed for the HP and LP groups suggests a degree of thermal imbalance. However, this was much less than observed for TP athletes, who demonstrated a much greater imbalance in temperature regulation. Increasing the exercise or environmental strain may result in the thermoregulatory responses of athletes with a spinal cord injury being compromised.     

Effect of daily versus intermittent exposure on heat acclimation

In order to compare the effectiveness of a daily to an intermittent acclimation protocol, 14 competitive rowers (mean +/- SD VO2peak = 48 +/- 7 ml x kg x min(-1)) were randomly assigned to either a consecutive (10 consecutive days) or intermittent acclimation group (10 sessions over 3 weeks). For every heat exposure, subjects in each group exercised for 30 min at 70% VO2peak in an environmental chamber set at 38 degrees C and 70% relative humidity. Acclimation state was monitored by measuring heart rate (HR), rectal and skin temperature (Tre and Tsk), ratings of perceived exertion (RPE) and whole body sweat rate (SR) during each heat exposure. Final exercise Tre decreased significantly by 0.6 +/- 0.7 degrees C with intermittent heat exposure but the decrease was significantly larger (p < 0.05) with consecutive day heat exposure (1.0 +/- 0.1 degrees C). Final exercise HR also decreased significantly by 13 +/- 12 bpm (p < 0.05) in the consecutive group, and non-significantly by 5 +/- 13 bpm in the intermittent group. RPE decreased with daily (5 +/- 1, p < 0.05) but did not significantly decrease with intermittent heat exposure (1 +/- 3). Similarly, Tsk significantly decreased with consecutive (0.4 +/- 0.2 degrees C, p < 0.05) but not intermittent exposure (0.2 +/- 0.3 degrees C) and SR did not change in either group. Minimal adaptation occurs with intermittent heat exposure and it appears that daily heat exposure is the most effective acclimation strategy.     

Sexual variations in thermoregulation during heat stress

Four male and three female physically fit, but untrained subjects performed a treadmill walking task in neutral (25 degrees C), warm (32 degrees C) and hot (40 degrees C) environments. The treadmill grade for each subject was based on 50% VO2 max as determined in a neutral environment. Environmental exposures were 2 h in duration divided into 40 min of rest, work, and recovery. No distinct sexual differences in rectal and skin temperature responses were observed in the three environments. The male subjects had higher heart rates and greater evaporative weight losses during exercise in all environments. The female subjects experienced less severe increases in metabolic requirements during work in the warm and hot environments than the male subjects. The greater percentage of increase in heart rates relative to changes in the metabolic cost of work in the females suggested a greater cardiovascular component of thermal regulation in the female than in the male subjects. The results of this study suggest that physically fit females are capable of working in the heat about as well as males when work load is relative to individual maximal aerobic capacity.     

Effects of precooling on thermoregulation during subsequent exercise

PURPOSE: The purpose of this study was to examine the effect of a decreased body core temperature before a simulated portion of a triathlon (swim,15 min; bike, 45 min) and examine whether precooling could attenuate thermal strain and increase subjective exercise tolerance in a warm environment (26.6 degrees C/60% relative humidity (rh)). METHODS: Six endurance trained triathletes (28+/-2 yr, 8.2+/-1.7% body fat) completed two randomly assigned trials 1 wk apart. The precooling trial (PC) involved lowering body core temperature (-0.5 degrees C rectal temperature, Tre) in water before swimming. The control trial (CON) was identical except no precooling was performed. Water temperature and environmental conditions were maintained at 25.6 degrees C and 26.6 degrees C/60% rh, respectively, throughout all testing. RESULTS: Mean time to precool was 31+/-8 min and average time to reach baseline Tre during cycling was 9+/-7 min. Oxygen uptake (VO2), HR, skin temperature (Tsk), Tre, RPE, and thermal sensation (TS) were recorded following the swim segment and throughout cycling. No significant differences in mean body (Tb) or Tsk were noted between PC and CON, but a significant difference (P < 0.05) in Tre between treatments was noted through the early phases of cycling. No significant differences were reported in HR, VO2, RPE, TS, or sweat rate (SR) between treatments. Body heat storage (S) was negative following swimming in both PC (-92+/-6 W x m2) and CON (-66+/-9 W x m2). A greater S occurred in PC (109+/-6 W x m2) vs CON (79+/-4 W x m2) during cycling (P < 0.05). CONCLUSIONS: Precooling attenuated the rise in Tre, but this effect was transient. Therefore, precooling is not recommended before a triathlon under similar environmental conditions.     

Influence of aerobic fitness on thermoregulation during exercise in the heat

It is believed that the adaptations induced by aerobic training would prevent athletes from overheating while retaining body electrolytes during exercise in the heat. Data from our laboratory suggest that aerobic training is not enough to induce these adaptations. This review considers the factors that can lead to hyperthermia in trained and untrained populations when exercising in the heat.     

Optimising the acquisition and retention of heat acclimation

Heat acclimation (HA) often starts in a moderately hot environment to prevent thermal overload and stops immediately prior to athletic activities. The aims of this study were (1) to establish whether acclimation to a moderately hot climate is sufficient to provide full acclimation for extreme heat and (2) to investigate the physiological responses to heat stress during the HA decay period. 15 male subjects exercised for 9 consecutive days at 26° C Wet Bulb Globe Temperature (WBGT) and 3 days at 32° C WBGT on a cycle ergometer for up to 2?h per day and repeated the exercise 3, 7 and 18 days later in 26° C WBGT. Rectal temperature (T (re)) and heart rate (HR) were measured during 60?min of steady state exercise (～45% of maximum oxygen uptake). During days 1-9, end-exercise T (re) was reduced from 38.7±0.1 to a plateau of 38.2±0.1° C (p<0.05), HR was reduced from 156±10 to 131±11?bpm (p<0.05). No changes in HR and T (re) occurred during the 3 days in the very hot environment. However, T (re) during rest and exercise were significantly lower by 0.4-0.5° C after HA compared with day 9, suggesting that heat acclimation did not decay but resulted in further favourable adaptations.     

The influence of exercise intensity on heat acclimation in trained subjects

Low-intensity exercise (less than or equal to 50% VO2max) has been demonstrated to produce heat acclimation (HA) in trained subjects. The purpose of this study was to determine whether shorter-duration, moderate-intensity exercise would also result in HA. Nine trained runners performed two 9-d exercise heat-stress protocols. Each protocol consisted of a 90-min heat tolerance test on days 1 (HTT1) and 9 (HTT2). On days 2-8 the subjects exercised at 50% VO2max for 60 min.d-1 (T50) or at 75% VO2max for 30-35 min.d-1 (T75). Final HTT2 heart rate and rectal temperature (Tr) were significantly (P less than 0.001) reduced, as compared to HTT1, with no differences between T50 and T75. Both protocols resulted in significant (P less than 0.05) reductions in HTT2 pre-exercise Tr and total exercising caloric expenditure, both of which are known to contribute to HA. No changes in resting plasma volume, osmolality, protein, post-HTT aldosterone, and exercising sweat rate were observed. These results demonstrate that equal levels of HA were obtained with T50 and T75, which suggests that moderate-intensity, short-duration exercise in the heat can produce HA in trained subjects.     

Effect of acute plasma volume expansion on thermoregulation and exercise performance in the heat

PURPOSE: We investigated the effects of acute plasma volume expansion on exercise performance in the heat. METHODS: Six moderately trained men cycled for 40 min at 64 +/- 2% peak pulmonary oxygen uptake (VO2peak) followed by an individual performance time trial, where subjects completed a set amount of work (267 +/- 15 kJ) in as little time as possible. Exercise trials were performed at 35 degrees C with a relative humidity of 40%. Subjects performed two exercise trials: one after 13.1 +/- 1% acute plasma volume expansion (PVE), which was achieved by the intravenous infusion of 8 mL x kg(-1) body weight of Hemaccel (35 g x L(-1) polygeline, 145 mmol x L(-1) Na+, and 145 mmol x L(-1) Cl-) and the other without prior treatment (CON). RESULTS: Core temperature, skin blood flow, and heart rate progressively increased (P < 0.05) during exercise, but no differences were observed between trials. Plasma glucose and lactate were similar at rest and during exercise, as was VO2 during exercise. Exercise performance was not influenced by plasma volume expansion (CON 17.5 +/- 0.4 min and PVE 17.1 +/- 0.2 min). CONCLUSION: These data suggest that, in moderately trained men, plasma volume expansion alone does not enhance thermoregulatory function and exercise performance during moderate intensity exercise in the heat.     

Deconditioning-induced exercise responses as influenced by heat acclimation.

Five young men were tested on a bicycle ergometer before (Test 1) and after (Test 2) 8 d of heat acclimation (exercise at 50% of Vo2max at 39.8 degrees C DB, 30.0 degrees C WB) and after 8 h of water immersion (Test 3). A control group of five subjects underwent a similar procedure in a temperate environment of 23.8 degrees C. Heat acclimation resulted in the usual decreases in exercise heart rate (30 beats/min) and rectal temperature (0.6 degrees C) and an increase in sweat rate (19%). The control group showed effects of moderate training by decreases in exercise heart rate (11 beats/min), rectal temperature (0.3 degrees C), and sweat rate (24%). Water immersion resulted in substantial diuresis in both groups, despite 1800 ml of water consumed by each subject. In the acclimation group, exercise responses in Test 2 were better than in Test 1, with little improvement shown by the control group. The acclimation group maintained exercise responses in Test 3 as in Test 1, with more adverse responses shown by the control group. The results show that heat acclimation provides an effective method to prevent the adverse effects of water-immersion deconditioning on exercise tolerance.     

The effects of heat acclimation on plasma volume and plasma protein of females

Four females were studied during a 10-day heat acclimation regimen consisting of 2 h per day of stationary cycle riding (means = 46% VO2 max) in a hot environment (40 degrees C, 50% RH) to determine the effects of heat acclimation on changes in plasma volume and vascular protein both at rest and during exercise. Changes in plasma volume were calculated from hematocrit and hemoglobin measurements. Plasma volume decreased during each 2-h session on baseline days through heat day 6 (-4.9% to -1.2%). On heat days 8 and 10, there was an expansion of plasma volume during the session (1.4% and 0.8%). This hemodilution was significantly different (P less than 0.05) from the hemoconcentration on baseline through heat day 4. The gain of protein during the 2-h session was not significantly different on any of the heat days from that on baseline days. With acclimation of females to work in the heat, there is an increased ability to maintain the fluid of the vascular compartment during a 2-h exercise session in the heat.     

Effect of heat exposure on thermoregulation and hockey-specific response time in field hockey goalkeepers

This study examined the thermoregulatory responses in field hockey goalkeepers during games (Part A), and assessed the effect of heat stress on hockey-specific response time (Part B). In Part A, core temperature (T_c), skin temperature (T_sk), body mass, fluid consumption and heart rate (HR) responses of six goalkeepers during two premier level club games in the Western Australian (winter) hockey season were recorded. Part B assessed the same measures, plus a response time test on four goalkeepers playing a simulated game inside a climate chamber in cool (COOL: 20 °C, 40% RH) and hot (HOT: 35 °C, 40% RH) conditions. In Part A, the mean (±SD) T_c and T_sk measured in games was 38.49 ± 0.20 °C and 34.99 ± 0.99 °C, with increases from baseline of 1.34 ± 0.19 °C and 1.08 ± 0.30 °C, respectively. Most of the increase in T_c resulted from the pre-game warm-up. In Part B, T_c and T_sk only increased significantly (p < 0.05) from baseline in the HOT condition (0.62 ± 0.18 °C and 1.61 ± 0.82 °C, respectively). Response time was significantly slower (0.87 ± 0.14 s, p < 0.01) after heat exposure, compared to COOL (0.75 ± 0.15 s), but the number of correct responses was not affected. For optimal performance, careful attention should be given to strategies to limit T_c increases in field hockey goalkeepers during matches.     

热适应与热应激对大鼠肝细胞HSP70表达的影响

目的 :研究不同时段的热适应及再经受热应激时大鼠肝细胞HSP70的表达变化及临床意义。方法 :采用人工热气候室 ,建立热适应 (34℃± 1℃、相对湿度 6 0 % )和热应激 (4 1℃± 1℃、相对湿度 80 % )动物模型。SD大鼠 16 0只 ,分为对照组、热适应组、热适应后热应激组、单纯热应激组 ,每组又分为 2、7、14、2 8d四个时段。取肝组织作免疫组化SP法染色 ,应用图像分析系统分析组织HSP70含量的变化。结果 :在热适应的四个时段 ,肝细胞的HSP70表达强度呈如下趋势 :热适应后热应激组 >热适应组 >对照组 ;单纯热应激组 >对照组 (P <0 .0 1或 0 .0 5 ) ;适应时段对HSP70表达强度变化无明显影响 ;2d时热适应组和热适应后热应激组的HSP70阳性枯否氏细胞数显著高于对照组 (P <0 .0 5 ) ,单纯热应激组与对照组无差别。结论 :热应激与热适应都可作为独立因素使肝细胞中HSP70表达增强 ,热适应加热应激则使HSP70表达更进一步加强 ;短时热适应使枯否氏细胞HSP70阳性表达明显增加