Soleus Adaptation to Combined Exercise and Heat Acclimation

Long-term heat acclimation training in mice: Similar metabolic and running performance adaptations despite a lower absolute intensity than training at temperate conditions

This study investigated the impact of long-term heat acclimation (HA) training on mouse thermoregulation, metabolism, and running performance in temperate (T) and hot (H) environments. Male Swiss mice were divided into 1) Sedentary (SED) mice kept in T (22 °C; SED/T), 2) Endurance Trained mice (ET, 1 h/day, 5 days/week, 8 weeks, 60 % of maximum speed) in T (ET/T), 3) SED kept in H (32 °C; SED/H), and 4) ET in H (ET/H). All groups performed incremental load tests (ILT) in both environments before (pre-ET) and after four and eight weeks of ET. In the pre-ET period, H impaired (∼30 %) performance variables (maximum speed and external work) and increased (1.3 °C) maximum abdominal body temperature compared with T. In T, after four weeks, although ET/H exercised at a lower (∼30 %) absolute intensity than ET/T, performance variables and aerobic power (peak oxygen uptake, VO2peak) were similarly increased in both ET groups compared with SED/T. After eight weeks, the external work was higher in both ET groups compared with SED/T. Only ET/T significantly increased VO2peak (∼11 %) relative to its pre-ET period. In H, only after eight weeks, both ET groups improved (∼19 %) maximum speed and reduced (∼46 %) post-ILT blood lactate concentrations compared with their respective pre-ET values. Liver glycogen content increased (34 %) in both ET groups and SED/H compared with SED/T. Thus, ET/H was performed at a lower absolute intensity but promoted similar effects to ET/T on metabolism, aerobic power, and running performance. Our findings open perspectives for applying HA training as part of a training program or orthopedic and metabolic rehabilitation programs in injured or even obese animals, reducing mechanical load with equivalent or higher physiological demand.

Acquired heat acclimation in rats subjected to physical exercise under environmental heat stress alleviates brain injury caused by exertional heat stroke

BACKGROUND

Exertional heatstroke (EHS) is an emergency with a high mortality rate, characterized by central nervous system dysfunctions. This study aims to establish a Heat acclimation/acclimatization (HA) rat model in locomotion to recapitulate the physical state of human in severe environment of high temperature and humidity, and investigate the mechanism of organism protection in HA. (2) Methods: Wistar rats were exposed to 36°C and ran 2 h/d for 21 days, acquired thermal tolerance test was conducted to assess the thermotolerance and exercise ability. Core temperature and consumption of water and food were observed. Expression of HSP70 and HSP90 of different tissues were determined by WB. Pathological structure of brain tissue was detected with HE staining. Proteomics was used to identify the differently expressed proteins in cerebral cortex of different groups. And key molecules were identified by RT-PCR and WB. (3) Results: HA rats displayed stronger thermotolerance and exercised ability on acquired thermal tolerance test. Brain water content of HA+EHS group reduced compared with EHS group. HE staining revealed slighter brain injuries of HA+EHS group than that of EHS. Proteomics focused on cell death-related pathways and key molecules Aquaporin 4 (AQP4) related to cell edema. Identification results showed HA increased AQP4, Bcl-xl, ratio of p-Akt/AKT and Bcl-xl/Bax, down-regulated Cleaved Caspase-3. (4) Conclusions: This HA model can ameliorate brain injury of EHS by reducing cerebral edema and cell apoptosis, offering experimental evidence for EHS prophylaxis.

Heat acclimation reduces the effects of whole-body hyperthermia on knee-extensor relaxation rate, but does not affect voluntary torque production

PURPOSE

This study investigated the effects of acute hyperthermia and heat acclimation (HA) on maximal and rapid voluntary torque production, and their neuromuscular determinants.

METHODS

Ten participants completed 10 days of isothermic HA (50 °C, 50% rh) and had their knee-extensor neuromuscular function assessed in normothermic and hyperthermic conditions, pre-, after 5 and after 10 days of HA. Electrically evoked twitch and octet (300 Hz) contractions were delivered at rest. Maximum voluntary torque (MVT), surface electromyography (EMG) normalised to maximal M-wave, and voluntary activation (VA) were assessed during brief maximal isometric voluntary contractions. Rate of torque development (RTD) and normalised EMG were measured during rapid voluntary contractions.

RESULTS

Acute hyperthermia reduced neural drive (EMG at MVT and during rapid voluntary contractions; P < 0.05), increased evoked torques (P < 0.05), and shortened contraction and relaxation rates (P < 0.05). HA lowered resting rectal temperature and heart rate after 10 days (P < 0.05), and increased sweating rate after 5 and 10 days (P < 0.05), no differences were observed between 5 and 10 days. The hyperthermia-induced reduction in twitch half-relaxation was attenuated after 5 and 10 days of HA, but there were no other effects on neuromuscular function either in normothermic or hyperthermic conditions.

CONCLUSION

HA-induced favourable adaptations to the heat after 5 and 10 days of exposure, but there was no measurable benefit on voluntary neuromuscular function in normothermic or hyperthermic conditions. HA did reduce the hyperthermic-induced reduction in twitch half-relaxation time, which may benefit twitch force summation and thus help preserve voluntary torque in hot environmental conditions.

Effect of heat acclimation on metabolic adaptations induced by endurance training in soleus rat muscle

Aerobic training leads to well‐known systemic metabolic and muscular alterations. Heat acclimation may also increase mitochondrial muscle mass. We studied the effects of heat acclimation combined with endurance training on metabolic adaptations of skeletal muscle. Thirty‐two rats were divided into four groups: control (C), trained (T), heat‐acclimated (H), and trained with heat acclimation (H+T) for 6 weeks. Soleus muscle metabolism was studied, notably by the in situ measurement of mitochondrial respiration with pyruvate (Pyr) or palmitoyl‐coenzyme A (PCoA), under phosphorylating conditions (V˙max) or not (V˙0). Aerobic performance increased, and retroperitoneal fat mass decreased with training, independently of heat exposure (p < 0.001 and p < 0.001, respectively). Citrate synthase and hydroxyl‐acyl‐dehydrogenase activity increased with endurance training (p < 0.001 and p < 0.01, respectively), without any effect of heat acclimation. Training induced an increase of the V˙0 and V˙max for PCoA (p < .001 and p < .01, respectively), without interference with heat acclimation. The training‐induced increase of V˙0 (p < 0.01) for pyruvate oxidation was limited when combined with heat acclimation (−23%, p < 0.01). Training and heat acclimation independently increased the V˙max for pyruvate (+60% p < 0.001 and +50% p = 0.01, respectively), without an additive effect of the combination. Heat acclimation doubled the training effect on muscle glycogen storage (p < 0.001). Heat acclimation did not improve mitochondrial adaptations induced by endurance training in the soleus muscle, possibly limiting the alteration of carbohydrate oxidation while not facilitating fatty‐acid utilization. Furthermore, the increase in glycogen storage observed after HA combined with endurance training, without the improvement of pyruvate oxidation, appears to be a hypoxic metabolic phenotype.

The effect of medium-term heat acclimation on endurance performance in a temperate environment

We investigated whether an 11-day heat acclimation programme (HA) enhanced endurance performance in a temperate environment, and the mechanisms underpinning any ergogenic effect. Twenty-four males (V̇O_2max: 56.7 ± 7.5 mL·kg^-1·min^-1) completed either: (i) HA consisting of 11 consecutive daily exercise sessions (60-90 min·day^-1; n = 16) in a hot environment (40°C, 50% RH) or; (ii) duration and exertion matched exercise in cool conditions (CON; n = 8 [11°C, 60% RH]). Before and after each programme power at lactate threshold, mechanical efficiency, VO_2max, peak power output (PPO) and work done during a 30-minute cycle trial (T30) were determined under temperate conditions (22°C, 50% RH). HA reduced resting (-0.34 ± 0.30°C) and exercising (-0.43 ± 0.30°C) rectal temperature, and increased whole-body sweating (+0.37 ± 0.31 L·hr^-1) (all P≤0.001), with no change in CON. Plasma volume increased in HA (10.1 ± 7.2%, P < 0.001) and CON (7.2 ± 6.3%, P = 0.015) with no between-groups difference, whereas exercise heart rate reduced in both groups, but to a greater extent in HA (-20 ± 11 b·min^-1) than CON (-6 ± 4 b·min^-1). VO_2max, lactate threshold and mechanical efficiency were unaffected by HA. PPO increased in both groups (+14 ± 18W), but this was not related to alterations in any of the performance or thermal variables, and T30 performance was unchanged in either group (HA: Pre = 417 ± 90 vs. Post = 427 ± 83 kJ; CON: Pre = 418 ± 63 vs. Post = 423 ± 56 kJ). In conclusion, 11-days HA induces thermophysiological adaptations, but does not alter the key determinants of endurance performance. In trained males, the effect of HA on endurance performance in temperate conditions is no greater than that elicited by exertion and duration matched exercise training in cool conditions.

Effects of passive heating intervention on muscle hypertrophy and neuromuscular function: A preliminary systematic review with meta-analysis

Passive heating has been therapeutically used to treat a range of health conditions. Further, this intervention presents as a potential exercise mimetic strategy showing acute and chronic effects on skeletal muscle adaptation and neuromuscular systems. This systematic review and meta-analysis aimed to synthesise the existing evidence on the effects of passive heating on muscle hypertrophy and neuromuscular function. Seven databases were searched (i.e., PubMed, Web of Science, Scopus, CINAHL, EMBASE, Cochrane, and SPORTDiscus) from 1937 to October 2019. Eligible studies included original papers using healthy animals or human samples (≥18 years; both sexes) that have used a control group or condition. Ten original articles were included in this review and four in the meta-analysis. The meta-analysis detected an increase in muscle mass in animal samples seven days after passive heating (I² = 65%, P < 0.01). The systematic review showed preliminary evidence that repeated passive heating exposures may promote muscle hypertrophy in animals and humans. Moreover, augmented muscle strength (involuntary and voluntary) may be observed after long-term passive heating (animals and humans) and increases in corticospinal excitability in humans after a single passive heating session. Passive heating has shown some potential benefits for skeletal muscle mass gain and muscle force improvement. Therefore, it is plausible to suggest that passive heating might be a worthwhile alternative to be recommended as an exercise mimetic for those people who lack or are unable to complete sufficient exercise.

Could Heat Therapy Be an Effective Treatment for Alzheimer's and Parkinson's Diseases? A Narrative Review

Neurodegenerative diseases involve the progressive deterioration of structures within the central nervous system responsible for motor control, cognition, and autonomic function. Alzheimer's disease and Parkinson's disease are among the most common neurodegenerative disease and have an increasing prevalence over the age of 50. Central in the pathophysiology of these neurodegenerative diseases is the loss of protein homeostasis, resulting in misfolding and aggregation of damaged proteins. An element of the protein homeostasis network that prevents the dysregulation associated with neurodegeneration is the role of molecular chaperones. Heat shock proteins (HSPs) are chaperones that regulate the aggregation and disaggregation of proteins in intracellular and extracellular spaces, and evidence supports their protective effect against protein aggregation common to neurodegenerative diseases. Consequently, upregulation of HSPs, such as HSP70, may be a target for therapeutic intervention for protection against neurodegeneration. A novel therapeutic intervention to increase the expression of HSP may be found in heat therapy and/or heat acclimation. In healthy populations, these interventions have been shown to increase HSP expression. Elevated HSP may have central therapeutic effects, preventing or reducing the toxicity of protein aggregation, and/or peripherally by enhancing neuromuscular function. Broader physiological responses to heat therapy have also been identified and include improvements in muscle function, cerebral blood flow, and markers of metabolic health. These outcomes may also have a significant benefit for people with neurodegenerative disease. While there is limited research into body warming in patient populations, regular passive heating (sauna bathing) has been associated with a reduced risk of developing neurodegenerative disease. Therefore, the emerging evidence is compelling and warrants further investigation of the potential benefits of heat acclimation and passive heat therapy for sufferers of neurodegenerative diseases.

Maximizing Cellular Adaptation to Endurance Exercise in Skeletal Muscle

The application of molecular techniques to exercise biology has provided novel insight into the complexity and breadth of intracellular signaling networks involved in response to endurance-based exercise. Here we discuss several strategies that have high uptake by athletes and, on mechanistic grounds, have the potential to promote cellular adaptation to endurance training in skeletal muscle. Such approaches are based on the underlying premise that imposing a greater metabolic load and provoking extreme perturbations in cellular homeostasis will augment acute exercise responses that, when repeated over months and years, will amplify training adaptation.

Effects of 10 days of separate heat and hypoxic exposure on heat acclimation and temperate exercise performance

Adaptations to heat and hypoxia are typically studied in isolation but are often encountered in combination. Whether the adaptive response to multiple stressors affords the same response as when examined in isolation is unclear. We examined 1) the influence of overnight moderate normobaric hypoxia on the time course and magnitude of adaptation to daily heat exposure and 2) whether heat acclimation (HA) was ergogenic and whether this was influenced by an additional hypoxic stimulus. Eight males [V̇o2max = 58.5 (8.3) ml·kg−1·min−1] undertook two 11-day HA programs (balanced-crossover design), once with overnight normobaric hypoxia (HAHyp): 8 (1) h per night for 10 nights [[Formula: see text] = 0.156; SpO2 = 91 (2)%] and once without (HACon). Days 1, 6, and 11 were exercise-heat stress tests [HST (40°C, 50% relative humidity, RH)]; days 2–5 and 7–10 were isothermal strain [target rectal temperature (Tre) ~38.5°C], exercise-heat sessions. A graded exercise test and 30-min cycle trial were undertaken pre-, post-, and 14 days after HA in temperate normoxia (22°C, 55% RH; FIO2 = 0.209). HA was evident on day 6 (e.g., reduced Tre, mean skin temperature (T̄sk), heart rate, and sweat [Na+], P < 0.05) with additional adaptations on day 11 (further reduced T̄sk and heart rate). HA increased plasma volume [+5.9 (7.3)%] and erythropoietin concentration [+1.8 (2.4) mIU/ml]; total hemoglobin mass was unchanged. Peak power output [+12 (20) W], lactate threshold [+15 (18) W] and work done [+12 (20) kJ] increased following HA. The additional hypoxic stressor did not affect these adaptations. In conclusion, a separate moderate overnight normobaric hypoxic stimulus does not affect the time course or magnitude of HA. Performance may be improved in temperate normoxia following HA, but this is unaffected by an additional hypoxic stressor.

Passive heat acclimation improves skeletal muscle contractility in humans

The aim of this study was to investigate the effect of repeated passive heat exposure (i.e., acclimation) on muscle contractility in humans. Fourteen nonheat-acclimated males completed two trials including electrically evoked twitches and voluntary contractions in thermoneutral conditions [Cool: 24°C, 40% relative humidity (RH)] and hot ambient conditions in the hyperthermic state (Hot: 44–50°C, 50% RH) on consecutive days in a counterbalanced order. Rectal temperature was ~36.5°C in Cool and was maintained at ~39°C throughout Hot. Both trials were repeated after 11 days of passive heat acclimation (1 h per day, 48–50°C, 50% RH). Heat acclimation decreased core temperature in Cool (−0.2°C, P < 0.05), increased the time required to reach 39°C in Hot (+9 min, P < 0.05) and increased sweat rate in Hot (+0.7 liter/h, P < 0.05). Moreover, passive heat acclimation improved skeletal muscle contractility as evidenced by an increase in evoked peak twitch amplitude both in Cool (20.5 ± 3.6 vs. 22.0 ± 4.0 N·m) and Hot (20.5 ± 4.7 vs. 22.0 ± 4.0 N·m) (+9%, P < 0.05). Maximal voluntary torque production was also increased both in Cool (145 ± 42 vs. 161 ± 36 N·m) and Hot (125 ± 36 vs. 145 ± 30 N·m) (+17%, P < 0.05), despite voluntary activation remaining unchanged. Furthermore, the slope of the relative torque/electromyographic linear relationship was improved postacclimation ( P < 0.05). These adjustments demonstrate that passive heat acclimation improves skeletal muscle contractile function during electrically evoked and voluntary muscle contractions of different intensities both in Cool and Hot. These results suggest that repeated heat exposure may have important implications to passively maintain or even improve muscle function in a variety of performance and clinical settings.

Heat acclimation and cross tolerance to hypoxia: Bridging the gap between cellular and systemic responses

Recent research has suggested a potential for some of the physiological and cellular responses to heat acclimation to carry over to improved tolerance of the novel stresses of another environment. This cross-tolerance is evident in heat-acclimated animals that exhibit enhanced tolerance to either hypoxic or ischemic stress, and is primarily attributed to shared cellular stress response pathways. These pathways include Hypoxia-Inducible Factor-1 (HIF-1) and Heat Shock Proteins (HSP). Whether these shared cellular stress response pathways translate to systemic cross-tolerance (improved exercise tolerance, reduced risk of environment-associated illness) has not been clearly shown, particularly in humans. This review highlights the HIF-1 and HSP pathways and their relationship with systemic acclimation responses, and further examines the potential cellular and systemic adaptations that may result in cross-tolerance between hot and hypoxic environments.

Two years of combined high-intensity physical training and heat acclimatization affect lymphocyte and serum HSP70 in purebred military working dogs

Military working dogs in hot countries undergo exercise training at high ambient temperatures for at least 9 mo annually. Physiological adaptations to these harsh conditions have been extensively studied; however, studies focusing on the underlying molecular adaptations are limited. In the current study, military working dogs were chosen as a model to examine the effects of superimposing endurance exercise on seasonal acclimatization to environmental heat stress. The lymphocyte HSP70 profile and extracellular HSP70 were studied in tandem with physiological performance in the dogs from their recruitment for the following 2 yr. Aerobic power and heat shock proteins were measured at the end of each summer, with physical performance tests (PPTs) in an acclimatized room (22°C). The study shows that together with a profound enhancement of aerobic power and physical performance, hsp72 mRNA induction immediately post-PPT and 45 min later, progressively increased throughout the study period (relative change in median lymphocyte hsp72 mRNA first PPT, 4.22 and 12.82; second PPT, 17.19 and 109.05, respectively), whereas induction of HSP72 protein was stable. These responses suggest that cellular/molecular adaptive tools for maintaining HSP72 homeostasis exist. There was also a significant rise in basal and peak median optical density extracellular HSP at the end of each exercise test (first PPT, 0.13 and 0.15; second PPT, 1.04 and 1.52, respectively). The relationship between these enhancements and improved aerobic power capacity is not yet fully understood.

Prolonged latency to CNS-O2 toxicity induced by heat acclimation in rats is associated with increased antioxidative defenses and metabolic energy preservation

We have previously shown that heat acclimation provides protection against central nervous system oxygen toxicity (CNS-OT). This was well correlated with increased levels of heat shock protein 72 (HSP72). We now examine other antioxidative defenses against CNS-OT that are correlated with heat acclimation. Two groups of male Sprague-Dawley rats were used. The heat-acclimated group (HA) was exposed for 4 wk to 32°C, and the control group (C) was maintained at 24°C. At the end of the acclimation period, rats were exposed to oxygen at 608 kPa. EEG was recorded continuously until appearance of the first electrical discharge. Brain samples were taken from each group after exposure to pressure. Levels of the antioxidant enzymes CuZnSOD, MnSOD, catalase, and glutathione peroxidase, as well as levels of HSP72, were quantified by Western blot. Comparative proteome analysis of the brains of HA and C rats was carried out using two-dimensional electrophoresis and mass spectrometry to define protein spot alterations. Levels of HSP72 and CuZnSOD were higher in HA rats. Levels of the other antioxidant enzymes were not affected significantly by heat acclimation. Differences in the levels of four protein spots identified as α-synuclein, valosin-containing protein, adenylate kinase 1 (AK1), and the mitochondrial H+-ATP synthase α subunit were found between HA and C rats. We conclude that elevation of HSP72, CuZnSOD, AK1, and the mitochondrial H+-ATP synthase α subunit and possible phosphorylation of α-synuclein--all proteins involved in oxidative stress or energy conservation--might contribute to the prolongation of latency to CNS-OT induced by heat acclimation.

Heat acclimation and exercise training interact when combined in an overriding and trade-off manner: physiologic-genomic linkage

Combined heat acclimation (AC) and exercise training (EX) enhance exercise performance in the heat while meeting thermoregulatory demands. We tested the hypothesis that different stress-specific adaptations evoked by each stressor individually trigger similar cardiac alterations, but when combined, overriding/trade-off interactions take place. We used echocardiography, isolated cardiomyocyte imaging and cDNA microarray techniques to assay in situ cardiac performance, excitation-contraction (EC) coupling features, and transcriptional programs associated with cardiac contractility. Rat groups studied were controls (sedentary 24°C); AC (sedentary, 34°C, 1 mo); normothermic EX (treadmill at 24°C, 1 mo); and heat-acclimated, exercise-trained (EXAC; treadmill at 34°C, 1 mo). Prolonged heat exposure decreased heart rate and contractile velocity and increased end ventricular diastolic diameter. Compared with controls, AC/EXAC cardiomyocytes demonstrated lower l-type Ca(2+) current (I(CaL)) amplitude, higher Ca(2+) transient (Ca(2+)T), and a greater Ca(2+)T-to-I(CaL) ratio; EX alone enhanced I(CaL) and Ca(2+)T, whereas aerobic training in general induced cardiac hypertrophy and action potential elongation in EX/EXAC animals. At the genomic level, the transcriptome profile indicated that the interaction between AC and EX yields an EXAC-specific molecular program. Genes affected by chronic heat were linked with the EC coupling cascade, whereas aerobic training upregulated genes involved with Ca(2+) turnover via an adrenergic/metabolic-driven positive inotropic response. In the EXAC cardiac phenotype, the impact of chronic heat overrides that of EX on EC coupling components and heart rate, whereas EX regulates cardiac morphometry. We suggest that concerted adjustments induced by AC and EX lead to enhanced metabolic and mechanical performance of the EXAC heart.

Molecular signals that shape the integrative responses of the heat-acclimated phenotype

The introduction of molecular biology to thermoregulation was delayed compared with its application in other research fields pertinent to human health and disease. Using principles from molecular biology, we revisited fundamental problems in integrative and environmental physiology and were able to explore new research horizons. Global genomic responses in tandem with an appropriate physiological experimental model are a good experimental design strategy that can unravel the molecular mechanisms underlying integrative thermoregulatory responses. In this way, dynamic adaptation models, with accentuated or diminished regulatory circuits, triggered by superimposition of novel stressors sharing similar protective pathways, have significant benefits. On the basis of this approach, we will discuss the molecular physiological linkage of heat acclimation alone or combined with exercise training and decipher stress-specific genes in the thermoregulatory circuits in the heart and skeletal muscles. Opposing/competing adaptive features are required for each of the above-mentioned physiological conditions. Aerobic training increases the capacity to store/use ATP. In contrast, the acclimated phenotype attempts to counteract excessive heat production. Nevertheless, both treatments augment muscle force generation. These changes are tissue-specific; in the exercise-trained rat heart, there is up-regulation of Ca2+-induced Ca2+ release mechanism genes, whereas in the skeletal muscle (soleus), the enrichment is found in genes involved in metabolism. The final issue discussed in this review is the possibility that heat shock proteins serve as consensus markers of heat stress. The role of the autonomic nervous system in their induction during heat stress and how they affect integrative body systems are described.

Responses of muscle mass, strength and gene transcripts to long-term heat stress in healthy human subjects

The present study was performed to investigate the effects of long-term heat stress on mass, strength and gene expression profile of human skeletal muscles without exercise training. Eight healthy men were subjected to 10-week application of heat stress, which was performed for the quadriceps muscles for 8 h/day and 4 days/week by using a heat- and steam-generating sheet. Maximum isometric force during knee extension of the heated leg significantly increased after heat stress (~5.8%, P < 0.05). Mean cross-sectional areas (CSAs) of vastus lateralis (VL, ~2.7%) and rectus femoris (~6.1%) muscles, as well as fiber CSA (8.3%) in VL, in the heated leg were also significantly increased (P < 0.05). Statistical analysis of microarrays (SAM) revealed that 10 weeks of heat stress increased the transcript level of 925 genes and decreased that of 1,300 genes, and gene function clustering analysis (Database for Annotation, Visualization and Integrated Discovery: DAVID) showed that these regulated transcripts stemmed from diverse functional categories. Transcript level of ubiquinol-cytochrome c reductase binding protein (UQCRB) was significantly increased by 10 weeks of heat stress (~3.0 folds). UQCRB is classified as one of the oxidative phosphorylation-associated genes, suggesting that heat stress can stimulate ATP synthesis. These results suggested that long-term application of heat stress could be effective in increasing the muscle strength associated with hypertrophy without exercise training.

Heat acclimation improves exercise performance

This study examined the impact of heat acclimation on improving exercise performance in cool and hot environments. Twelve trained cyclists performed tests of maximal aerobic power (VO2max), time-trial performance, and lactate threshold, in both cool [13°C, 30% relative humidity (RH)] and hot (38°C, 30% RH) environments before and after a 10-day heat acclimation (∼50% VO2max in 40°C) program. The hot and cool condition VO2max and lactate threshold tests were both preceded by either warm (41°C) water or thermoneutral (34°C) water immersion to induce hyperthermia (0.8-1.0°C) or sustain normothermia, respectively. Eight matched control subjects completed the same exercise tests in the same environments before and after 10 days of identical exercise in a cool (13°C) environment. Heat acclimation increased VO2max by 5% in cool (66.8 ± 2.1 vs. 70.2 ± 2.3 ml·kg(-1)·min(-1), P = 0.004) and by 8% in hot (55.1 ± 2.5 vs. 59.6 ± 2.0 ml·kg(-1)·min(-1), P = 0.007) conditions. Heat acclimation improved time-trial performance by 6% in cool (879.8 ± 48.5 vs. 934.7 ± 50.9 kJ, P = 0.005) and by 8% in hot (718.7 ± 42.3 vs. 776.2 ± 50.9 kJ, P = 0.014) conditions. Heat acclimation increased power output at lactate threshold by 5% in cool (3.88 ± 0.82 vs. 4.09 ± 0.76 W/kg, P = 0.002) and by 5% in hot (3.45 ± 0.80 vs. 3.60 ± 0.79 W/kg, P < 0.001) conditions. Heat acclimation increased plasma volume (6.5 ± 1.5%) and maximal cardiac output in cool and hot conditions (9.1 ± 3.4% and 4.5 ± 4.6%, respectively). The control group had no changes in VO2max, time-trial performance, lactate threshold, or any physiological parameters. These data demonstrate that heat acclimation improves aerobic exercise performance in temperate-cool conditions and provide the scientific basis for employing heat acclimation to augment physical training programs.