Curtin University of Technology
Skip to content
Link to Curtin homepage
Curtin School of Physiotherapy

Heat Acclimatisation of Athletes

Proposition for Debate - by Bobby Bacic

Contents

Statement of the Topic

Heat Acclimatisation of Athletes

Introduction

Heat acclimatisation(HA) involves a complex of adaptations which involves decreased heart rate, rectal temperature, perceived exertion as well as increased plasma volume and sweat rate. These adaptations serve to reduce physiological strain, improve an athletes ability to exercise in a hot environment and reduce the incidence of some forms of heat illness. Furthermore acclimatisation induces a lower oxygen consumption at a given workload (Armstrong et al 1998).

Figures show that in the last 20 years more than 100 players of American football have died from excessive heat stress during practice or actual competition (McArdle et al 1996).

Team Sports

Team sports have unique qualities which make the players particularly liable to heat stress. These include the fact the work rate is intermittant, largely unpredictable and random in nature. Furthermore analysis of various team sports reveal that such games are characterised by a high degree of inter and intra personal individual variability work rate between players from the same sport. Finally, team players are less able to anticipate sweat losses than athletes involved in prolonged, continuos moderate intensity exercise .However compared with most endurance events, many team sports offer frequent opportunites to ingest adequate volumes of fluid and thus prevent exercise induced hypohydration.

Core Temperature

Core temperature is recorded normal at 37 degrees and may vary 1 degree either side. The factors responsible for heat gain are Basal metabolic rate, muscular activity, hormones, thermic effect of food, menstrual changes and environment.

Those factors that are responsible for heat loss include conduction ,convection and evaporation. If heat gain exceeds heat loss, as can happen readily during vigorous exercise in a warm environment, core temperature rises; in a cool environment on the other hand, heat loss often exceeds heat production and core temperature falls.

Sweating

Sweating is the most important mechanism to dissipate excess body heat generated by muscular activity, or gained via convection from a hot environment, sweat rate varies among indivduals markedly .They are influenced by the relative intensity of exercise, the state of fitness and acclimatisation of the athlete aswell as the environmental playing conditions. The sweat losses of athletes participating in long distance athletic events have been documented and are typically around 1 litre to 1.2 litres a hour when conditions are temperate. However some individuals can sustain sweat rates of 2 litres a hour during prolonged exercise in hot and humid conditions where there is restricted air flow.(Noakes 1998).

Shapiro et al 1998 reported that HA led to quicker sweating responses and these were already improved by previous non HA training. This helped explained why previous studies by Roberts et al 1977 were no longer valid as they reported that changes in sweat rate did not occur but there subjects had already undergone HA.

Evaporation of sweat provides the major physiological defence against overheating. Heat is continually transferred to the environment as water and is vaporised from the respiratory passages and skin surface. For each litre of water that vaporises, 580 kcal are extracted to the body and transferred to the environment.

When sweat reaches the skin a cooling effect occurs as the fluid evaporates, the cooled skin then serves to cool the blood that has shunted from the interior to the surface. Along with the heat loss through sweating, approximately 350m/l of water seeps through the skin each day and evaporates to the environment as perspiration.

As ambient temperature increases, the effectiveness of heat loss decreases by conduction, convection and radiation. When ambient temperature temperature increases exceed body temperature,heat is actually gained by these mechanisms of thermal transfer. Therefore sweat is the only measure of heat dissipation.

Heat loss in high humidity

The total amount of sweat vaporised from the skin depends on three factors:

The relative humidity is the most important factor that determines the effectiveness of evaporative heat loss. Relative humidity is defined as the ratio of water in the ambient air at a particular temperature to the total quantity of moisture that could be carried in the air as a percentage. When humidity is high the ambient vapour pressure approaches that of the moist skin and evaporation is greatly reduced. Therefore this avenue for heat loss is closed even though large quantities of sweat bead on the skin and roll off. This form of sweating represents a useless water loss that can lead to a dangerous state of hydration and overheating.

Pandolf 1998 reported that the benefits of HA appear to be better retained for dry rather than humid heat. A peripheral factor related to excessive wetting of the skin, interferes with the free flow of eccrine sweat during exercise in humid environments. Known as hidromeiosis this factor is believed to result from fatigue of sweat glands due to lack of neurotransmitters.

Atlanta

The Olympic games held in Atlanta had average temperatures of 31 degrees centigrade with a humidity averaging 69% (Maughan 1998).

Concerning HA there have been no actual studies on Olympic athletes so therefore in reality it is difficult to give definite recommendations to athletes about HA. Maughan 1998 whose study focused on the British HA program prior to Atlanta states that medal tally should not be used as a measure of success of HA programs. However Athletes may disagree with this especially as Britain scored very poorly in the medal tally. Regardless of medal counts it does mean it is difficult to judge the success of HA programs for elite athletes.

Other heat dissipating mechanisms

Circulation

With extreme heat stress, 15 to 25 percent of the cardiac output will pass through the skin. Cardiac output during sub maximal exercise in the heat results in the heart stroke volume lowering in proportion to the fluid deficit created during exercise, which causes the heart rate to be higher at all levels of submaximal exercise. (Montain and Coyle 1992)

Hormonal adjustments

Because both water and electrolytes are lost through sweating, hormonal adjustments are initiated in heat stress as the body attempts to conserve salts and fluids. The pituitary gland releases vasopressin or anti duirectic hormone (ADH), which increases water reabsorption from kidney tubucles. This causes the urine to become more concentrated during heat stress. Concurrently during exercise in hot weather, the sodium, conserving hormone aldosterone is released from the adrenal cortex. This hormone acts on the renal tubules to increase the reabsorption of sodium. Aldosterone also acts to reduce the osmolaty of sweat. Thus sodium concentration in sweat is decreased during repeated heat exposure, which further results in conserving electrolytes.

Vascular constriction and dilation

In the heat adequate cutaneous and muscle blood flow is achieved at the expense of other tissues, which can temporalily compromise their blood supply especially the splenic vascular bed and renal complications are associated with exertional heat stress.

Maintenance of blood pressure

During heavy exercise with its accompanying dehydration, relatively less blood is shunted to peripheral areas for heat dissipation. This probably reflects the body's attempt to maintain cardiac output in the face of a diminishing plasma volume caused by sweating, under these conditions circulatory regulation and muscle blood flow take precedence over temp regulation.

Even when sub maximal exercise in the heat is well tolerated, work is generally accompolished with a greater dependence on anaerobic metabolism than in cooler conditions (Young 1990).

Latic Acid

The results in early accumilation of latic acid and encroacment of glycogen reserves. An increase latic acid level is probably due to A) decrease lactate uptake by the liver because of a reduction in hepatic blood flow during exercise in the heat B) a reduced muscle circulation as a large portion of the of the cardiac output is shunted to the periphery for heat dissipation. Both of these factors contribute to the early fatigue noted during only moderate exercise in the heat. This is followed by a greater subjective sensation of effort in the heat and a reduced exercise capacity. An example of this was England's football team who "laboured" in the heat during the 1970 defence of the world cup in Mexico.

Clothing

With regards to clothing it effects the air velocity and increases humidity at the skin’s surface garments usually suppress local sweating. Dry clothing no matter how lightweight retards the change more than the same clothing soaking wet ( Mc Arldle et al 1996). Therefore the practice as seen on tennis of switching to a dry shirt makes little difference from the point of temperature regulation. However the benefits of increased comfort may outweigh the detrimental effects. Evaporative heating is also thwarted by continual drying of the skin with at towel before the sweat has had a chance to evaporate. Sweat does not cool the skin but the actual evaporation does.(McArdle et al 1996) .Therefore towelling off as seen in tennis should not be advised.

Acclimatisation

During the initial days of training in a hot environment, athletes display a reduced ability to exercise at the same intensity or duration when compared to exercise performed in a cool environment. Weakness dizziness flushed skin and other signs and symptoms of heat stress may accompany this phenomenon (Hubbard and Armstrong 1998). Following several days of exercise exposure, the heat tolerance of athletes improves. This occurs as the body adapts to the combined stress of internal metabolic heat production and high ambient temperature. The improved ability to exercise in a naturally hot environment is known as HEAT ACCLIMATISATION(HA). It is different from HEAT ACCLIMATION which produces similar responses but is accomplished in an artificially controlled environmental chamber.

A comparison of physiological responses before and after HA has shown that the following adaptations occur during when humans exercise at a controlled intensities in the range of 40 to 95 % of maximal aerobic power (VO2 max): decreased heart rate, decreased core body temperature, increased exercise tolerance time, increased plasma volume, and decreased psychological rating of perceived exertion. (Wegner 1988).

It has also been reported that an increase sweat rate, increased sweat sensitivity and decreased (i.e the sweat loss expressed per degree rise of core body temperature), and decreased sodium chloride (NaCl) losses in sweat and urine are also observed during HA. The results of these changes is an improved heat transfer from the body's core to the skin, and ultimately to the environment.

A few hours of exercise in a hot environment causes dehydration from both the intracellular and extracellular compartments. In a HA person water loss by sweating may reach a peak of 3 litres per hour during intense exercise and may average nearly 12 litres on a daily basis. Furthermore several hours of intense sweating can lead to sweat fatigue which can lead to an inability to regulate core temperature. Therefore from a sporting point of view elite marathon runners are most at risk as they may lose up to 5 litres during competition .In these athletes the fluid loss represents between 6 to 10 percent of body mass.

Therefore the only exceptional potential for evaporative cooling of ACCLIMATIZED humans can only be sustained with adequate fluid replacement. The primary aim of fluid replacement is to maintain plasma protein so that circulation and sweating can progress at optimal levels .Ingesting fluid during exercise increases blood flow to the skin for more effective cooling independent of any change in plasma volume.

Old fashioned methods witnessed in sport such as placing towels on one head as in boxing or a cold shower before an event is of only small benefit in facilitating heat transfer at the body's surface.

Ingestion of extra water or hyperhydration before exercising in a hot environment provides some protection because it delays the development of hydration, increases sweating during exercise and brings about a smaller rise in core temperature(Mack 1994). It was advised by the authors to consume 400 to 600ml of water 20 minutes before exercising in the heat. However this does not take into account the fact that many athletes do not like the sensation of competing on a full stomach.

Because sweat is hypotonic to the body's fluids ,replacing water is a much more immediate concern than replacing minerals. Research by Maughan and Shirreffs 1997 advocated the benefits of sports drinks with added minerals and glucose however their work was discovered on a Sports drink web site so therefore an element of caution exists over their findings.

Authors do agree that HA takes between 7 to 14 days as the major physiological effects take place in this time(Nadal 1988 Shepard 1985). To begin with training intensity and volume should be reduced then increased. The warm up intensity and duration should be decreased to keep core temperature from rising to high during full training. To begin with 30 to 60 minute training sessions should be adopted and 100 minutes becomes the optimal time for training(Dawson 1994). Furthermore training should initially begin early in the morning before the temperature rises to much. However this would probably vary dependent on the athletes event and time of actual competition.

Houmand et al 1990 stated that 56 minutes training at 95 percent VO2 max over 8 days is sufficient to gain HA.

This is further reinforced by Pandolf 1998 who stated that 80% of HA benfits occurred during the first 7 days.

After 10 days of HA the capacity to sweat is nearly doubled and more diluted so less sodium is loss this allows a HA athlete to train at a lower core temperature. This results in less blood needed at the skin's surface thus freeing a greater proportion of cardiac output to the working muscles.

Age

Studies that control factors such as body size and composition ,aerobic fitness level, degre of HA show little or no age related effects on thermoregulatory capacity or the ability to acclimatize .However older athletes do not recover from dehydration as effectively whih may be related to a blunted thirst drive. This could make them more prone to a chronic state of hypohydration with a less than optimal plasma volume that would effect their thermoregulatory capacity (Mack 1994).

Sex

The general feeling is that women are evenly matched to men and that both sexes can acclimatise at a similar rate. An early study by Frye and Kamon 1981 expressed that women's rate of HA was slower however their subjects were not evenly matched as the women were exercising at higher intensities relation to their aerobic capacities.

Body Fat

Excess body fat is a liability when working in a hot environment because the specific heat of fat is greater than muscles. Furthermore the insulatory property of fat retards the conduction of heat to the periphery. Finally the fat person has a smaller body surface area to mass ratio for the evaporation of sweat compared to a smaller person.

Conclusion

Therefore it could be argued that as no study has yet been conducted on elite athletes then the benefits of HA programs cannot truly be measured. Furthermore with regards to measurement an objective marker is needed either in the form of medal tally, times set or even perceived level of health. However all of the studies analysed presented with similar findings that seven days was sufficient for HA benefits to occur and this model seems to be the one most readily followed.

References

Armstrong EL and Maresh CM (1991)
The induction and decay of heat acclimatisation in trained athletes. Sports Medicine 12(5):302-312.
Armstong EL and Maresh CM (1998)
Effects of training, environment, and host factors on sweating response to exercise. International Journal of Sports Medicine Supplement (19):103-105.
Hargreaves M and Febbraio M (1998)
Limits to exercise performance in the heat. International Journal of Sports Medicine Supplement(19):115-117.
Hubbard RW and Armstrong EL(1998)
Heat acclimatisation and decline in sweating during humidity transients. International Journal of Sports Medicine Supplement (19):250-254.
McArdle WD, Katch FI and Katch VL(1996)
Exercise Physiology(4th ed.) Baltimore: Wilkins and Wilkins.
Mack GW(1994)
Body fluid balance in dehydrated healthy older men. Journal of Applied Physiology 76(12):1124-1129.
Maughan R(1998)
Heat acclimatisation and rehydration stratergy. International Journal of Sports Medicine Supplement (19):77.
Noakes TD(1998)
Fluid replacement during exercise.Exercise Sports Science Review 21(2):297-301.
Pandolf KB (1998)
Time course of heat acclimatisation and it's decay. International Journal of Sports Medicine Supplement(19):51-55.
Young AJ(1990)
Energy substrate utilization during exercise in extreme environments. Exercise and sports Science Reviews 18:65-117

Exercise Physiology Educational Resources 2000