MODERATING EFFECTS OF THE CLIMATIC ENVIRONMENT
[Reproduced from Rushall, B. S., & Pyke, F. S. (1990). Training for sports and fitness (pp. 126-135). Melbourne, Australia: Macmillan Educational.]
The Olympic Games have drawn attention to a number of environmental influences on sports performance. During the time of the Summer Olympics it is usually hot and/or humid. On the other hand, the Winter Olympics invariably call for protection against the cold. The 1968 Mexico City Games, sited at 2,350 meters above sea-level, presented the situation of lowered barometric pressure and reduced air density. World records were set in the men's 100, 200, and 400 meter races and the long jump. Distance races were appreciably slower than in previous Games. In Los Angeles in 1984, concern was expressed for athletes possibly experiencing high levels of both heat and air pollution. While some athletes certainly suffered as a result of the climate, British middle-distance runner, Steve Ovett, and Swiss woman marathoner, Gabriela Anderson-Schiess, being the most obvious examples, the weather in Los Angeles during the Games was generally comfortable.
It is the purpose of this section to describe the physiological responses to a number of environmental conditions and to offer considerations that could be given during the performance of sporting activities.
During exercise the body produces a great deal of heat. In extreme circumstances this can elevate its core temperature from 37° C to beyond 40° C. When the surrounding air is cool heat can be lost from the body by the process of radiation (transfer of heat by electromagnetic waves), convection (by air movement), conduction (by contact), and evaporation (by sweating). As the surrounding temperature increases it becomes more and more difficult to lose heat by radiation, convection, and conduction. Hence, the predominant source of heat loss in warm to hot conditions is from the evaporation of sweat on the skin surface.
Sweat losses exceeding 6 liters have been recorded in marathon runners. These deficits constitute a body weight reduction of 8-10 percent and a body water loss of 13-14 percent (Costill, 1979). Team-game players performing in warm to hot conditions can sweat at a rate of 2 liters per hour. During a game this can amount to a loss in body weight of 5 percent and a reduction in body water of more than 10 percent (Pyke & Hahn, 1981). Losses in body weight of 2 percent have been shown to result in reductions in endurance performance as well as increase heart rate by 5 bpm.
The requirement for copious sweating places a heavy load on the circulation to provide blood flow to both the muscles to maintain work rate and to the skin for cooling. As the body progressively dehydrates the circulation is further compromised and heat storage exceeds heat removal. The resultant strain is indicated by increased heart rate, sweat rate, and core and skin temperatures. Collapse can occur if work is continued.
There are a number of factors that must be considered before individuals are exposed to work in hot conditions.
Other than air temperature, both humidity and radiant heat should be assessed before athletes engage in hard training or competition in hot weather conditions. The most commonly used heat index in sport is the WBGT index which includes measurements of air temperature (dry bulb), humidity (wet bulb), and radiant temperature. These temperatures can be easily measured with a whirling hygrometer and a black bulb thermometer placed in a black sphere.
When this climatic index exceeds 25° C and the work rate is reasonably high, coaches should be aware of the potential negative effects on athletes. When it exceeds 28° C the coach should abandon vigorous activities for poorly conditioned and unacclimatized individuals and be wary of signs of heat intolerance in others. In hotter months, training should be scheduled in the early morning or evening rather than at noon or mid-afternoon.
The impact that a hot, humid climate has on the physiological responses of a runner was well exemplified during performances in Darwin, Australia. Throughout a 30-minute run in cool conditions at a speed of 230 meters per minute, a man increased his rectal temperature from 37.7 to 39.3° C and incurred a weight loss of 750 grams. This contrasted with an increase in rectal temperature from 37.2 to 40.6° C, accompanied by a weight loss of 1,000 grams, when the run was repeated in the hot, humid conditions of Darwin. The skin temperature rose to nearly 38° C in the heat whereas in the cool it fell to 31° C. The reduced temperature gradient between the body core and skin experienced in the hot conditions meant that a large blood flow was required to transport heat from the core of the body to the periphery. This resulted in heart rates measured during the last 15 minutes of the run in the heat (190-200 bpm) being much higher than those measured in the cool (152-154 bpm). Hot, humid climates reduce endurance capacity in long-duration events.
Characteristics of the Individual
There are certain individuals who have a low tolerance to heat and need careful supervision by coaches. Those with heavier builds possess a lower ratio between skin surface area and body mass than those with more linear builds. This is a disadvantage for heat removal. High levels of body fat also encourage heat storage. Fat tissue has a lower specific heat than lean tissue and therefore, absorbs heat more readily. Individuals with a high level of endurance fitness tolerate hot conditions much better than those who are unfit. The average male has a higher level of endurance fitness than the average female and endurance fitness decreases substantially with age. This explains why males usually are more heat tolerant than females and younger adults more heat tolerant than older ones. However, when males and females and older and younger adults of equivalent levels of aerobic fitness are compared, these differences in heat tolerance disappear. One group that requires special attention in the heat is pre-pubertal children. They have poorly developed sweating mechanisms and overheat rapidly. Coaches should monitor them carefully for signs of heat intolerance during practice sessions. Risks should not be taken with them in hot, humid conditions.
It has been shown that physical training in cool conditions improves tolerance to hot conditions. However, full adaptation to heat can only be achieved by actually working in hot conditions. The adjustment is very rapid and is achievable in about 7 to 10 days if regular daily exercise for 90 minutes is undertaken. Heat acclimatization expands the blood volume, which supports an increased capacity and precision of sweating. At a given relative workload a fit, acclimatized person commences sweating sooner, sweats more evenly over the skin surface and thereby loses less salt. An acclimatized person performs in a heat tolerance test with greater circulatory stability (lower heart rate) and lower core and skin temperatures than someone who is not acclimatized. However, the acclimatization process is retarded by dehydration. For optimal adaptation to occur, fluid balance should be maintained during the recovery periods between daily bouts of work in the heat.
It might also be noted that pre-pubertal children acclimatize more slowly than do adults. Elevations in the sweating response take longer in children despite their perceiving that they are adjusting. This makes it particularly hazardous to rely on the subjective response of children as to their reaction to hot conditions. Recovery breaks for cooling and fluid replacement should be regularly scheduled to counteract young athletes' inabilities to accurately discern fluid replacement needs.
The procedure of adding extra layers of clothing (tracksuit, windcheater, and head covering) while training during the winter months has been tested as a means of promoting heat acclimatization. Despite producing elevated thermoregulatory responses during each training session, the practice has been only partially successful in improving heat tolerance of well-conditioned team-game players (i.e., field hockey). If this procedure is used, particular care needs to be taken to ensure that players do not overheat during training, since heat will produce levels of fatigue that substantially erode the capacity to perform substantial volumes of skill trials (Dawson & Pyke, 1988).
During exercise in hot conditions, it is recommended that participants wear light-colored clothing made from open-weave natural fibers (e.g., cotton, wool). As much of the skin as possible should be exposed to the air to maximize the evaporation of sweat. Clothing made from synthetic fibers, such as nylon and polyesters, offers more resistance to heat removal and, in time, becomes uncomfortable.
When fluid losses exceed 2 percent of body weight prior to exercising, significant endurance performance deterioration occurs. It is wise to drink (hydrate) before exercising so that no dehydration occurs. However, during some high energy sporting contests, despite experiencing sweat losses of 4-6 kg, it is neither necessary nor advisable to attempt to entirely replace the amount of fluid lost. The body actually produces water during exercise. Most athletes only drink enough fluid to recover between 40 and 50 percent of the sweat lost. Partial fluid replacement has been shown to reduce the risk of overheating. During a series of 2-hour runs, marathoners who ingested 100mL of fluid every 5 minutes for the first 100 minutes maintained a lower rectal temperature than those who abstained. This occurred despite only absorbing 1,660 ml of fluid while losing 4,000 ml of sweat during the run (Costill, Kammer, & Fisher, 1970). The sensation of thirst lags behind the state of negative water balance, and should not be used as the signal to drink. Drink breaks must be regularly scheduled and made compulsory during training and competitions.
Since the body loses more water than electrolytes during exercise, the body fluids become concentrated. Hence there is a greater need to replace water than electrolytes during periods of heavy sweating. The answers to questions concerning the frequency, quantity, and qualities of replacement fluids depend, to some extent, on the individual concerned, the intensity of effort, and the environmental conditions. The major concern is to replace water. Flavored drinks, commercial preparations, and other solutions are not necessarily the best forms of fluid replacement.
On hot days, fluid should be consumed before, during, and after training. This maintains the stability of circulation that is so important for endurance efforts. Water is the primary requirement and, in most circumstances, is the ideal replacement fluid. Fluids with high sugar and electrolyte concentrations empty slowly from the stomach for absorption into the blood via the small intestine. That slow emptying will actually delay the replacement of needed water. It is only when excessive sweat losses occur on successive days that small amounts of salt and sugar may be necessary in a replacement fluid. On cooler days, when fluid losses are less, a higher concentration of carbohydrate in the fluid assists in maintaining the blood glucose level. Whether the amount of carbohydrate ingested is large or small is not a critical factor in `feeding' during events or training. It has been shown that more frequent feeds maintain more stable blood glucose levels. Therefore, if carbohydrate supplementation occurs during exercise, the frequency of feeding should be considered to be of the utmost importance.
In sports such as wrestling, body-building, weight-lifting, and rowing, where weight limits have to be achieved to perform in competition categories, the loss of weight at the right time is important. Such weight loss is best achieved through gradual dietary accomplishments. Attempts to `crash diet' a short time before a contest can have debilitating effects on athletes by causing disruptions to internal well-being, feelings of distress, and reduced performance states through anti-carboloading. The even more harmful procedure of trying to lose `water weight' through taking diuretics or dehydrating should also be avoided. The maximum safe value to lose, as has been pointed out above, is 2 percent of body weight. Values that exceed that will reduce the efficiency of the body's physiology, cause the circulatory system to work harder for a stated amount of work, and will reduce endurance performance. More often than not, unsound weight loss programs cause performances to decrease. Their value and benefit to the athlete should be seriously questioned.
The following are sensible fluid replacement guidelines for exercise:
In cold climates the athlete continually tries to prevent heat loss and a fall in the core body temperature. A cooled state is referred to as `hypothermia' or `exposure'. In a fatigued person its symptoms are poor control of movement, disorientation, and poor judgment and reasoning. The two ways to cope with this problem are to produce more heat or reduce the amount being lost.
Increased Heat Production
Extra heat can be produced either by shivering or by exercising. Shivering raises the resting metabolism about fourfold but in the process interferes with the expression of skill. Nadel, Holmer, Bergh, Astrand, and Stolzijk (1974) studied breaststroke swimming in water temperatures of 18, 26, and 33° C and attributed the extra oxygen cost of performing in the cold water to the shivering response. Depending on the endurance fitness level of the individual, metabolism can be elevated twelve- or fifteenfold during intensive exercise. Fitness is necessary to maintain a high work rate and heat production during endurance sports. If a marathoner slows down towards the end of an event held on a cold day it is possible that heat loss will exceed heat production and that hyperthermic problems will arise. This is a particular threat in endurance winter sports (e.g., biathlon, cross-country skiing). Fatigue is the nemesis of the endurance athlete competing in cold conditions.
Decreased Heat Loss
There are several physical avenues for heat loss which must be considered if an athlete is to remain warm.
Radiation is the physical action whereby heat is radiated from the body to nearby cooler objects. Curling the body into a tuck and reducing the exposed surface area can minimize heat lost. Such a response is common when resting in cold conditions. Limiting the blood flow through the skin also can reduce heat loss by radiation. This is the first line of defense against cold and is managed by reflex constriction of the blood vessels supplying the skin. This mechanism is capable of improving the insulative capacity of the skin sixfold. Cooling the skin in this way reduces the temperature gradient between it and the environment and effectively reduces heat loss. However, this means of heat conservation results in the fingers and toes, with their large surface area to mass ratio, becoming particularly cold and losing their speed and dexterity. This is a problem in target and touch sports such as fishing, shooting, and golf. In extreme conditions, frostbite injuries can be sustained. Acclimatization to cold conditions promotes some improvements in local blood flow and enhances the capabilities of the extremities to perform with skill and precision.
The shutdown of blood flow to the skin of the head is much less than that in the hands and feet. If the head is exposed to the cold, substantial heat loss can occur. This has resulted in strong recommendations to wear headgear during sports played in the cold and to wear life jackets to prevent immersion of the head during aquatic rescues.
Another means of conserving heat by reducing radiation is to increase the insulative properties of the shell of the body by depositing fat under the skin. This has been observed in successful Channel swimmers (Pugh & Edholm, 1955).
Thin pre-pubertal children with a high surface area:mass ratio are particularly susceptible to cooling while swimming in cold water. Central body temperatures below 35° C have been commonly observed in children after swimming in 20° C water temperatures (Keatinge & Sloan, 1972). This is of some concern to swimming coaches who rely on the child's perception of cold to provide necessary protection. A lean and ambitious young athlete could easily become hypothermic while training enthusiastically in cool conditions (particularly when swimming) and should be watched carefully.
Convection occurs when heat is transferred from the body to free air. As cold air comes into contact with the body it is warmed, becomes less dense through expansion, and rises. The role of clothing is to trap warmed air close to the skin and develop a microclimate that is comfortable and heat retaining. Forced air convection occurs when the body is either fanned by or creates its own breeze in the process of movement. In external circumstances the `wind chill factor' is such that a temperature of -1° C in still air effectively becomes -18° C if a 40 km/h wind is blowing or if a skier or cyclist is moving at that speed. Windproof overgarments should be worn to avoid excessive heat loss in such conditions.
Conduction is the means by which heat is lost by direct contact with other surfaces that are cooler than the skin. Handling of ice axes, metal pitons, and ski poles with bare hands should be avoided since the temperature gradient between those pieces of equipment and the skin is usually very severe. Gloves and insulated boots are used to reduce the amount of conducted heat loss. The conductivity of water is 25 times greater than that of air. Much more heat is lost in water than in air at the same temperature. In one sense, the increased conductivity of water allows swimmers to perform greater volumes of work than runners since they are not inhibited by the build-up of heat.
Evaporation is the means by which heat is lost through sweating. Becoming inactive immediately after heavy sweating can invite rapid cooling and a dramatic fall in body temperature. This can occur on the bench after an intensive period of play in a team game or perhaps as a result of an enforced rest during an endurance event. It is important to have warm, dry clothing available to arrest the decrease in body temperature in such situations. The hiker or skier should try to avoid the situation arising where layers of clothing close to the skin become saturated with sweat. This destroys the insulatory value of the clothing and accelerates heat removal. Rain has the same effect. Clothing should be suited to the energy requirements of the sport, remembering that less insulation is needed as heat production increases. A doubling of the work rate from 3 to 6 Mets performed in 5° C air temperatures requires only one-third of the original insulation (Burton & Edholm, 1969). This is why it is appropriate to have layers of clothing in vigorous winter sports. The appropriate number of layers can be removed to maintain the proper level of heat loss while maintaining dry clothing. Clothing which permits insulation to be added or subtracted in accordance with the intensity of exercise is the most useful. Jackets that open down the front are more convenient than pullovers. Hoods that can be drawn back are ideal during intermittent activity. Drawstrings that allow clothes to be tightened or loosened at the collar, waist, and arm and leg cuffs conveniently vary the insulative value of garments.
It is important not to overprotect the hands and feet against the cold as the body will perceive itself to be very warm and not invoke the physiological temperature regulation processes that prevent a fall in core body temperature. It is better to insulate the trunk rather than the extremities. Three units for the torso, two units for the limbs, and one unit for the hands and feet has been recommended by Kaufman (1982) as the appropriate proportions of clothing distribution.
Should an athlete not follow these recommendations and develop hypothermia during a sporting contest it is critical to immediately start the rewarming process. After providing shelter, wet clothes should be removed and replaced with dry, warm ones. The individual should be warmed gradually under blankets or in a sleeping bag, administered warm, sugared drinks and kept awake until normal body temperature has been restored.
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