Exercise, performance and temperature control: temperature regulation during exercise and implications for sports performance and training

Thermoregulation is an important consideration not only for athletic performance but also for the safety of the athlete. This article presents a broad overview of the mechanisms by which body heat is dissipated in an individual exercising in a hot environment. Particularly emphasised are more recent views of body heat loss mechanisms and the influences of non-thermal inputs, such as effects due to changing blood volume or blood flow distribution. During exercise in a hot environment, metabolic heat produced by the exercising muscles is transported by the circulating blood to the surface of the body where it is released to the environment, either by radiation and convection or by evaporation of sweat. The primary drives for both the increased skin blood flow and increased body sweating are the thermal inputs which are sensed by receptors in the deep body core, with a lesser drive from skin receptors. These thermal signals are integrated in the hypothalamus and proper heat loss responses are effected. When exercise is prolonged, however, and body rehydration is not adequate, the total blood volume may be compromised. In addition, as the core temperature increases during exercise, larger proportions of the blood volume are distributed to the cutaneous vessels, thus effectively reducing cardiac return and central blood volume. During severe exercise, a reduction in cardiac filling may result in a fall in central venous pressure and stimulate baroreceptor vasoconstrictor reflexes. As discussed below, the outputs from these baroreceptors compete with and modify the thermal drives for both the control of the skin blood flow and control of the sweat glands. The effect of high ambient temperatures on exercise performance is most evident in prolonged submaximal exercise. Normally, maximal exercise performance is not altered by high temperatures unless the individual has an elevated deep body temperature before the start of the exercise task. However, submaximal exercise performance is often impaired by high ambient temperatures, but may be improved by programmes of physical training and heat acclimatisation. Both training and heat acclimatisation significantly modify the control systems which regulate skin blood flow and sweating. Only acclimatisation programmes, however, are effective in preventing heat stress during prolonged exercise in hot environments.     

Control of skin circulation during exercise and heat stress.

At any given environmental and mean skin temperature, exercise brings about an increase in internal body temperature and skin blood flow. At high environmental temperatures, when skin temperature is elevated, skin blood flow at any given internal temperature reaches higher levels than at cooler skin temperatures. Increased cutaneous blood flow serves to deliver metabolic heat from the core to the skin, where the heat is lost to the environment by convective, radiative, and evaporative mechanisms. However, at high levels of skin blood flow, peripheral vascular pooling and fluid losses by filtration lead to reduced central venous pressure. This lowers cardiac stroke volume, and requires a higher heart rate to maintain a given cardiac output. Mechanisms which alleviate some of the cardiovascular strain produced by exercise in the heat include the following: acutely, reflexes which arise from receptors in working muscles produce vasoconstriction in a number of central and peripheral vascular beds. Other reflexes, arising from cardiac baroreceptors, produce additional peripheral vasoconstriction when cardiac filling is impaired. In the long term, physical conditioning and heat acclimation lead to increases in sweat output during thermal stress, leading to cooler skin and core temperature during exercise, and decreasing the level of skin blood flow needed for regulation of body temperature.     

Thermoregulation during exercise in individuals with spinal cord injuries

The increased participation in wheelchair sports in conjunction with environmental challenges posed by the most recent Paralympic venues has stimulated interest into the study of thermoregulation of wheelchair users. This area is particularly pertinent for the spinal cord injured as there is a loss of vasomotor and sudomotor effectors below the level of spinal lesion. Studies within this area have examined a range of environmental conditions, exercise modes and subject populations. During exercise in cool conditions (15-25 degrees C), trained paraplegic individuals (thoracic or lumbar spinal lesions) appear to be at no greater risk of thermal injury than trained able-bodied individuals, although greater heat storage for a given metabolic rate is evident. In warm conditions (25-40 degrees C), trained subjects again demonstrate similar core temperature responses to the able-bodied for a given relative exercise load but elicit increased heat storage within the lower body and reduced whole-body sweat rates, increasing the risk of heat injury. The few studies examining a wide range of lesion levels have noted that, for paraplegic individuals where heat production is matched by available sweating capacity, excessive heat strain may be offset. Studies relating to tetraplegic subjects (cervical spinal lesions) are fewer in number but have consistently shown this population to elicit much faster rates of core and skin temperature increase and thermal imbalance in both cool and warm conditions than paraplegic individuals. These responses are due to the complete absence or severely reduced sweating capacity in tetraplegic subjects. During continuous exercise protocols, the main thermal stressor for tetraplegic subjects appears to be environmental heat gain, whereas during an intermittent-type exercise protocol it appears to be metabolic heat production. Fluid losses during exercise and heat retention during passive recovery from exercise are related to lesion level. Future research is recommended to focus on the specific role of absolute and relative metabolic rates, sweating responses, training status and more sport- and vocation-specific exercise protocols.     

Effect of prewarming in the cold season on thermoregulatory responses during exercise.

OBJECTIVE: To assess whether thermoregulation in the cold season can be affected by prewarming before exercise. METHODS: Four healthy non-athletic unacclimatised males were exercised to the same degree in summer and winter on a bicycle ergometer without prewarming (experiment 1) and after prewarming by sitting for 30 min in a room at 30 degrees C (experiment 2). During exercise, sweat production and rectal and skin temperatures were measured continuously. RESULTS: There was seasonal variation in sweating capacity and sensitivity and in heat storage during exercise without prewarming (experiment 1). After the subjects were warmed before exercise, there was no such seasonal variation in their sweat rates during exercise at 30 degrees C and 40 degrees C (experiment 2). In both cases, the sweat rate and skin temperature were dependent on the environmental temperature, and the sweat rate and core temperature were dependent on the workload. In the cold season, sweating sensitivity and evaporative cooling response could be enhanced by thermal stimulation. There was no seasonal difference in the relation between evaporative heat loss and metabolic rate in the two thermal conditions. These values did not differ significantly between winter after prewarming and summer (P > 0.05), neither did heat storage and metabolic heat production at various workloads (P > 0.05). CONCLUSIONS: There is adaptation of the thermoregulatory mechanisms during temperature acclimatisation. Body warming enhances not only the heat dissipating activity of the thermoregulatory centre but also the induction of peripheral sweat gland activity. Seasonal change of sweat rate in exercising men can be eliminated through a different type of acclimatisation by prewarming in the cold season.     

Wet-bulb globe temperature (WBGT)—its history and its limitations

Wet-bulb globe temperature (WBGT) is nowadays the most widely used index of heat stress, yet many users appear to be unaware of its history and its limitations.History of WBGT: WBGT was invented and first used during the 1950s as one element in a successful campaign to control serious outbreaks of heat illness in training camps of the United States Army and Marine Corps. Control measures based on air temperature and humidity, and applied to all trainees alike, had proved effective but had entailed excessive compliance costs in the form of lost training time. New control measures introduced in 1956 further reduced heat illness and also lost fewer training hours. Crucial innovations were (1) replacing the temperature and humidity measurements with WBGT, which additionally responds to sun and wind, (2) using epidemiologic analyses of casualty records to identify hazardous levels of WBGT and vulnerable trainees, and (3) protecting the most vulnerable trainees by suspending drill at lower levels of WBGT, and by improving their heat tolerance in special conditioning platoons. This campaign has considerable relevance to the prevention of heat illness in sport.Limitations of WBGT: WBGT's most serious limitation is that environments at a given level of the index are more stressful when the evaporation of sweat is restricted (by high humidity or low air movement) than when evaporation is free. As with all indices that integrate elements of the thermal environment, interpretation of the observed levels of WBGT requires careful evaluation of people's activity, clothing, and many other factors, all of which can introduce large errors into any predictions of adverse effects. Moreover, the accuracy of WBGT is being eroded by measurement errors associated with the omission of the globe temperature, with non-standard instrumentation, and with unsatisfactory calibration procedures. Because of the above limitations WBGT can provide only a general guide to the likelihood of adverse effects of heat. A much clearer assessment can be obtained by measuring the individual elements of the thermal environment, and using those measurements to estimate the requirement for evaporative cooling, the likelihood of achieving it, and more accurate and comprehensive indices of heat stress.     

湿热环境下运动对汗液中尿素、乳酸和电解质的影响

目的 揭示湿热环境下运动时机体的适应性,为训练提供理论依据.方法 随机抽取某大学男生30名,平均(21.3±1.1)岁.选择第二军医大学湿热环境训练实验室,环境温度控制在39℃,相对湿度80%.训练周期为7 d,训练过程按照循序渐进的原则递增运动负荷.训练过程中收集汗液冷冻保存,采用等离子体发射光谱仪分析汗液成分.训练后即刻测试肛温.结果 训练第7天汗液成分中尿素、乳酸(La)、Na+、K+、Cl-较训练第1天均下降,且差异具有统计学意义(P<0.05).除La浓度从训练的第6天下降开始趋于平稳外,其余汗液成分指标均在训练的第6天降至最低值,随后出现明显的回升趋势.结论 在本实验设计的前提下,湿热环境习服训练适应时间至少需要6 d.     

热环境对机体功能影响的机制及防护研究进展

热环境增加高温作业人员发生热损伤的风险,充分认识热环境对高温作业人员机体的影响和危害,对提高高温作业人员防暑意识、减少作业时的职业性热损伤具有重要意义.笔者主要从热环境的定义、热环境对人体主要功能系统影响的机制、热应激的影响因素及防护研究进展进行综述,为高温作业人员对热致疾病的认识及防护提供参考.     

The pathopysiology of heat stroke: an integrative view of the final common pathway

Heat stroke is defined as a condition in which body temperature is elevated to such a level that it becomes a noxious agent causing body tissue dysfunction and damage with a characteristic multi-organ clinical and pathological syndrome. Marked hyperthermia, usually above 40.5°C and associated encephalopathy, occurs after thermoregulation is subordinated to circulatory and metabolic demands and to the associated systemic inflammatory reaction. Exertional heat stroke is a function of both intrinsic and extrinsic modulators. Intrinsic modulators like genetics, fitness, acclimatization, illness, medications, and sleep quality can alter individual risk and outcomes, while extrinsic modulators like exercise intensity and duration, clothing and equipment, ambient temperature, relative humidity, and solar radiation can affect the group risk and outcomes. This review integrates the current theoretical and accepted knowledge of physiological alterations into one model that depicts a common pathway from heat stress to heat stroke.     

Personalized fluid and fuel intake for performance optimization in the heat

It is well appreciated that a loss of body water (dehydration) can impair endurance performance and that the effect is magnified by environmental heat stress. A majority of professional sports medicine and nutrition organizations recommend drinking during exercise to replace sweat losses and prevent dehydration, while also avoiding frank over-hydration. Knowledge of sweating rate, which is highest in the heat for any given metabolic rate, is therefore considered key to developing a sound drinking strategy. Exercise duration and the provision of liquid fuel interacts with required drink volumes in important ways that are infrequently discussed but are of utmost practical concern. This review details some challenges related to the optimized coupling of fluid and fuel needs during prolonged exercise in the heat and the need for personalization.     

Thermoregulation and marathon running: biological and environmental influences

The extreme physical endurance demands and varied environmental settings of marathon footraces have provided a unique opportunity to study the limits of human thermoregulation for more than a century. High post-race rectal temperatures (Tre) are commonly and consistently documented in marathon runners, yet a clear divergence of thought surrounds the cause for this observation. A close examination of the literature reveals that this phenomenon is commonly attributed to either biological (dehydration, metabolic rate, gender) or environmental factors. Marathon climatic conditions vary as much as their course topography and can change considerably from year to year and even from start to finish in the same race. The fact that climate can significantly limit temperature regulation and performance is evident from the direct relationship between heat casualties and Wet Bulb Globe Temperature (WBGT), as well as the inverse relationship between record setting race performances and ambient temperatures. However, the usual range of compensable racing environments actually appears to play more of an indirect role in predicting Tre by acting to modulate heat loss and fluid balance. The importance of fluid balance in thermoregulation is well established. Dehydration-mediated perturbations in blood volume and blood flow can compromise exercise heat loss and increase thermal strain. Although progressive dehydration reduces heat dissipation and increases Tre during exercise, the loss of plasma volume contributing to this effect is not always observed for prolonged running and may therefore complicate the predictive influence of dehydration on Tre for marathon running. Metabolic heat production consequent to muscle contraction creates an internal heat load proportional to exercise intensity. The correlation between running speed and Tre, especially over the final stages of a marathon event, is often significant but fails to reliably explain more than a fraction of the variability in post-marathon Tre. Additionally, the submaximal exercise intensities observed throughout 42 km races suggest the need for other synergistic factors or circumstances in explaining this occurrence. There is a paucity of research on women marathon runners. Some biological determinants of exercise thermoregulation, including body mass, surface area-to-mass ratio, sweat rate, and menstrual cycle phase are gender-discrete variables with the potential to alter the exercise-thermoregulatory response to different environments, fluid intake, and exercise metabolism. However, these gender differences appear to be more quantitative than qualitative for most marathon road racing environments.     

Eccrine sweat glands. Adaptations to physical training and heat acclimation

Heat dissipation, under conditions of thermal stress, is mediated primarily by evaporation of sweat. Physical training has been shown to enhance sweat production by eliciting changes in the sensitivity of eccrine glands, total sweat output and distribution of gland activity. These adaptations afford partial acclimation. Heat acclimation produces similar changes, and also results in reduced sweat thresholds. To account for these different responses it has been hypothesised that physical training induces peripheral adaptations, while acclimation produces both peripheral and central modifications. It is suggested that repeated cutaneous heat detection may be essential to the development of central sudomotor changes.     

补水补盐联合辅助降温对热应激反应的影响

 目的 探讨训练前补水补盐联合训练中辅助降温措施对热应激反应的影响。方法 参加研究男性士兵62人,兵龄1~2年,训练科目为5 km及10 km跑步,户外跑步环境热指数40~42,随机分为实验组和对照组,实验组跑前30 min饮用60 mmol/L氯化钠溶液500 ml。跑步中辅助降温:实验组跑步中每间隔400 m给予用海绵块吸取20 ℃水浇头1次及20 ℃水全身喷雾降温3 s,对照组给予跑前30 min饮纯净水500 ml,跑步中无降温措施。对比5 km跑步实验组(n=20),对照组(n=21)以及10 km跑步实验组(n=10),对照组(n=11)生理应力指数(PSI)、出汗量、钠丢失量。结果 实验组与对照组比较,PSI、出汗量差异显著,有统计学意义(P<0.05)。5 km跑步与10 km跑步PSI比较差异显著,有统计学意义(P<0.05);而最快心率比较无统计学意义(P>0.05)。结论 训练前预补水补盐加训练中辅助降温可减小热应激反应,并减小脱水量,从而有预防热射病作用。     

Physiological consequences of hypohydration: exercise performance and thermoregulation.

During exercise in the heat, sweat output often exceeds water intake, which results in a body water deficit or hypohydration. This water deficit occurs from both the intracellular and extracellular fluid compartments, and causes a hypertonic-hypovolemia of the blood. Aerobic exercise tasks are likely to be adversely affected by hypohydration; and the warmer the environment the greater the potential for performance decrements. Hypohydration causes greater heat storage and reduces one's ability to tolerate heat strain. The greater heat storage is mediated by reduced sweating rate (evaporative heat loss) and reduced skin blood flow (dry heat loss) for a given core temperature. Reductions of sweating rate and skin blood flow are most tightly coupled to blood hypertonicity and hypovolemia, respectively. In addition, hypovolemia and the displacement of blood to the skin make it difficult to maintain central venous pressure and thus an adequate cardiac output to simultaneously support metabolism and thermoregulation during exercise-heat stress.     

News briefs

ABSTRACT

BACKGROUND: Scant evidence documents the physiological and environmental stresses for football players wearing partial or full uniforms, but such information would be useful for determining the ambient temperatures and humidities associated with uncompensable heat stress during practice and games.

OBJECTIVE: This laboratory study used a physiological approach to determine critical heat balance limits (various combinations of ambient temperature and relative humidity) for subjects exercising in typical American football uniforms.

DESIGN: Eight nonheat-acclimatized men exercised at 35% Vo₂max in a programmable environmental chamber. In multiple trials, either dry-bulb temperature (T_db) was held constant and ambient water vapor pressure (P_a) was systematically increased, or P_a was held constant and T_db was systematically increased. The critical heat balance limits were determined as the environments at which body core (esophageal) temperatures were forced out of equilibrium, reflecting uncompensated heat storage imposed by those environments.

RESULTS: Critical environmental limits are presented that define the combinations of air temperature and relative humidity above which thermal balance cannot be maintained. These zones of uncompensable heat stress result in continuously rising core temperatures. Retrospective analysis reveals that documented heatstroke deaths in football players wearing full uniforms occurred at or above these critical environments.

CONCLUSION: Heat balance limits can be used in decision making and are especially relevant for preventing heat-related illness or injuries early in the football practice season. The critical limits are expanded when shorts are substituted for football pants with pads.     

热习服和热应激生物学效应差别的实验研究

目的:探讨热应激和热习服生物学效应的差别。方法:通过观察日本大耳白兔在热应激时和热习服时心肌酶谱、肝功能、肾功能、免疫力及电解质等的变化。结果:(1)热应激组动物体温较热习服组升高快、幅度大;(2)热习服组动物心肌酶谱、肝功能、电解质指标的异常程度明显高于热应激组;(3)热习服组血糖和免疫功能较热应激组降低更明显。结论:热习服动物虽然体温升高不明显,但其生理机能的异常程度明显高于热应激组,而其组织和器官的损伤程度更重。     

Thermal regulation in the heat during exercise after caffeine and ephedrine ingestion.

BACKGROUND: Ingesting a combination of caffeine and ephedrine (C+E) has been shown to raise metabolic heat production and body temperature. This side effect of C+E ingestion may be positive during a cold stress scenario, however, during heat stress it could prove to be detrimental. Thus, the purpose of this study was to clarify the effect of C+E ingestion on body temperature regulation during moderate exercise in a hot dry environment. METHODS: Ten, healthy, non heat acclimated, males exercised at 50% VO2peak in a 40 degrees C and 30% RH environment until rectal temperature reached 39.3 degrees C; heart rate (HR) remained at 95% of peak value or greater for 3 min, dizziness or nausea precluded further exercise, or 3 h had elapsed. They did this four times at weekly intervals: familiarization (Fam), control (Cont), placebo, and C+E (5 mg . kg(-1) caffeine + 1 mg . kg(-1) ephedrine) trials. The Fam and Cont treatments were done first and sequentially while the placebo and C+E treatments were balanced and double-blind. Tolerance times, mean skin temperature (Tsk), rectal temperature (Tre), Vo2, Vco2, VE, sweat rate (SR), HR, and sensation of thermal comfort were measured. RESULTS: Tolerance times (mean+/-SD in minutes) were similar for the placebo (120.0+/-28.4) and C+E (121.3+/-33.9) trials and both times were significantly longer than Cont (106.6+/-24.0) trial. C+E did not affect Tsk, initial TrC, delta Tre, SR or the sensation of thermal comfort. VO2 and VF, were significantly increased by C+E. HR was elevated by C+E compared with the other trials, but only during the initial 20 min of exercise. CONCLUSION: Although the metabolic rate was slightly increased with C+E treatment, it was sufficiently offset by increased heat loss mechanisms so that internal body temperature was not increased during moderate exercise in a hot, dry environment.     

Clothing and thermoregulation during exercise

Exercise increases heat production. During exercise in both warm and cold conditions, the major dilemma is the dissipation of the heat produced from muscular activity. The use of clothing generally represents a layer of insulation and as such imposes a barrier to heat transfer and evaporation from the skin surface. In warm environments, additional clothing increases thermal insulation causing more rapid increases in temperature during exercise and imposes a barrier to sweat evaporation. However, clothing can serve a protective function by reducing radiant heat gain and thermal stress. Recent research suggests that neither the inclusion of modest amounts of clothing nor the clothing fabric alter thermoregulation or thermal comfort during exercise in warm conditions. In the cold, most reports do not support an effect of clothing fabric on thermoregulation; however, there are reports demonstrating an effect. Clothing construction does alter thermoregulation during and following exercise in the cold, where fishnet construction offers greater heat dissipation. Future research should include conditions that more closely mimic outdoor conditions, where high work rates, large airflow and high relative humidity can significantly impact thermoregulation.     

高温、噪声、振动复合应激时人体体温调节的改变

本研究探讨了高温、噪声、振动复合应激时人体体温调节的改变。采用均匀设计，11名受试者进行了19次试验。结果表明，复合因素使受试者的平均皮肤温度（Tsk）、直肠温度（Tre）和出汗量（Qsw）显著增加。高温和噪声对Tsk、Tre和Qsw具有协同效应；振动对Tre的作用为协同效应，而对Tsk和Qsw则为拮抗效应。高温是影响体温调节的主要因素。可以看出，降低直升机座舱高温、噪声、低频振动有助于保障飞行员的健康和飞行安全。     

Responsiveness of thermal sensors to nonuniform thermal environments and exercise

BACKGROUND: We investigated the utility of finger temperature, hand heat flux, and mean skin temperature as indices of overall thermal balance during nonuniform thermal manipulations combined with exercise, with a view to identifying useful feedback sites for input into personal thermal control systems. METHODS: There were 16 subjects who performed 4 x 30 s of 120% VO2peak cycling with a 4-min recovery. During recovery, subjects either received no cooling (CON), upper-body cooling (UC), or upper-body cooling combined with leg heating (UCLH) using a multi-zone liquid conditioning garment. Heat loss during recovery was approximately equal to heat production during exercise. Skin temperature was measured on the mid-medial phalanx of the fourth finger. Heat flux was measured on the dorsum of the hand. RESULTS: Neither hand heat flux or finger temperature distinguished between the two cooling conditions during any of the recovery periods, though hand heat flux was very sensitive to the onset and cessation of exercise. Mean skin temperature was significantly different (p < 0.05) during CON (34.0 +/- 0.1 degrees C), UC (32.5 +/- 0.2 degrees C), and UCLH (33.0 +/- 0.2 degrees C). CONCLUSION: Mean skin temperature may serve as a good indicator of overall heat exchange in the body, even when exposed to nonuniform thermal environments. As hand heat flux was very sensitive to the onset and cessation of exercise, it may be useful as a supplemental thermal feedback to modulate heat exchange in microclimate thermal control systems.     

Creatine supplementation does not impair the thermoregulatory response during a bout of exercise in the heat

AIM: The aim of this work was to examine the effects of creatine (Cr) supplementation on resting body water volumes and on core temperature and sweat loss during a bout of exercise in a warm environment. METHODS: Twenty-four aerobically trained male subjects (age 22.93+/-3.01 years, height 179.52+/-7.28 cm, mass 82.06+/-14.32 kg) volunteered to participate in this study. Each subject was assessed for resting body water volumes and for body mass (BM), heart rate (HR), blood pressure (BP), and core temperature immediately before and following a 60-min bout of exercise in a warm environment. Core temperature, HR, and BP were also recorded at 10-min intervals during exercise. Subjects were then randomly assigned to either a Cr or placebo (P) group. Each subject returned following a 5-day supplementation period and was reassessed using identical testing procedures. BM was measured using a standard electronic scale. Body water volumes were assessed using a multi-frequency BIS (Xitron Technologies, San Diego, CA, USA). Core body temperature was measured using a CorTemp Disposable Temperature Sensor and a CT2000 Miniaturized Ambulatory Recorder (HTI Technologies, Inc., Palmetto, FL, USA). RESULTS: The Cr group experienced a significant increase in all body water volumes. No changes were observed in the P group. No changes in core temperature or sweat loss were observed in either group following supplementation. CONCLUSIONS: Cr loading did not impair the thermoregulatory response during a bout of exercise in the heat.