Chapter 48
Exercise in the Heat

with Timothy Noakes

*This Chapter is online in its entirety. Click on the links below to browse through the whole chapter.*

Mechanisms of heat gain and loss
Clinical Perspective
Heatstroke - a temperature above 41ºC
Exercise-associated collapse
Cramps

Cramps

Heat cramps were first described among coal miners in 1923, eventually becoming known as 'miner's', 'fireman's' 'stoker's', 'cane-cutter's' or simply 'heat' cramps. The popular belief that cramps are caused by severe dehydration and large sodium chloride losses that develop during hot conditions has no scientific basis [3]. After a lifetime studying sodium balance in persons exercising in desert heat, Epstein and Sohar [12] concluded that salt deficiency heat cramps had never been proven to exist and illustrated 'christening by conjecture'. Cramps can occur at rest, or during or after exercise undertaken in any environmental conditions; they are specific neither to exercise, nor to exercise in the heat. The more modern hypothesis proposes that cramps probably result from alterations in spinal neural reflex activity activated by fatigue in susceptible individuals (Chapter 2) [13]. The term 'heat cramps' should be abandoned as it clouds understanding of the possible neural nature of this connection. 

Management of cramps

Maintaining the muscle to length is one effective therapy. Icing and physiotherapy of the affected muscle may also help. The Boston Marathon medical team treats muscle cramps with intravenous normal saline and intravenous magnesium therapy is used in the Hawaiian Ironman Triathlon but clinical trials of either treatment have yet to be published. 

Fluid overload - hyponatremia

An important differential diagnosis in athletes who seek medical attention at an event, particularly an ultramarathon, undertaken in the heat, is hyponatremia. Athletes who become unconscious during or after ultradistance running or triathlon races and whose rectal temperatures are not elevated should be considered to have symptomatic hyponatremia until measurement of serum sodium concentration refutes the diagnosis. We emphasize that dehydration does not cause unconsciousness until it is associated with renal failure with uremia or hepatic failure [3]. To achieve such a weight loss, a 50 kg athlete would require 10 hours of high-intensity exercise at a sweat rate of 1 liter/hr without any fluid replacement. 

Athletes with symptomatic hyponatremia and serum sodium concentrations below 129 mmol/L are overhydrated by between 2 and 6 L [14, 15]. The physician should be alerted to this diagnosis in a patient with altered level of consciousness and very dilute urine. If the patient is conscious, he or she may complain of feeling bloated or 'swollen'. A helpful clinical sign is that rings, race identification bracelets and watchstraps feel and are noticeably tighter. The race bracelet is a particularly useful indicator, as it is usually loose fitting before a race. 

Management of hyponatremia

Under no circumstances should fluid be given to unconscious or semiconscious athletes with hyponatremia. All unconscious hyponatremic patients in a series [3]recovered spontaneously, without treatment other than fluid restriction and the occasional use of diuretics. Providing fluid to patients who are unconscious because of cerebral edema delays recovery and may produce a fatal result, as appears to have happened in isolated cases in recent years [16, 17]. 

It is essential that physicians caring for athletes with hyponatremia are aware of the correct management of this condition. The current management includes: 

• Bladder catheterization to establish that urine (i) is dilute, indicating a state of fluid overload and (ii) is being passed at an ever-increasing rate during recovery. Spontaneous recovery will occur if adequate amounts of urine (>500 ml/hr) are passed

• No fluids by mouth. Salt tablets and sodium containing foods can be given

• High sodium (3%) solutions can be given intravenously provided they are infused slowly (50 ml/hr). 

Other causes of exercise-related collapse in hot weather

Heatstroke and exercise associated collapse are the most likely causes of distress or collapse while exercising in hot weather. It is, however, important to consider other possible causes of distress or collapse that may also occur in these conditions. A list of possible causes of collapse and the likely circumstances surrounding that collapse is shown in Table 48.3. This highlights the importance of determining the rectal temperature as the first step in the assessment of the collapsed athlete in hot weather. A rectal temperature of greater than 40 degrees C (104-106 degrees F) indicates heat illness is the most likely cause of collapse. A rectal temperature of less than 40 degrees C (104 degrees F) should encourage the clinician to consider other causes of collapse.

Table 48.3 Other causes of collapse

It is important to remember that athletes suffering from hyperthermia and hypothermia may present in the same event. The faster runners with their increased heat production may present with hyperthermia, while slower runners, particularly those who have stopped to a walk, may present later in the day with hypothermia. The possibility of hypothermia is increased if a cold wind is present or if the temperature drops over the duration of the event. Cold-related illness is discussed in the next chapter.

Heat acclimatization

Athletes are able to cope much better with hot or humid conditions if they are acclimatized [18, 19]. The human body adjusts to exposure to hot conditions by increasing blood volume and venous tone and, particularly, by alterations to the sweating mechanism. The main ways in which the sweating mechanism is affected are by:

• earlier onset of sweating

•  increased amount of sweating

•  increased dilution of the sweat

These changes result in increased heat loss for a given set of environmental conditions and a smaller rise in body temperature.

There is considerable dispute regarding the ideal length of time required for heat acclimatization, although a minimum of two weeks is probably required when coming from a cool climate to a hot or humid climate [20]. One problem that reduces the effectiveness of heat acclimatization is that in the week or two prior to a major event, the athlete is often tapering (i.e. reducing the amount of training). While there is some effect on heat acclimatization in the rested state, it may be necessary to perform relatively intensive exercise to maximize acclimatization. Therefore, exposure to the warmer environment should occur for a minimum of two weeks.

A number of other factors affect acclimatization. If the athlete wishes to compete in a hot and humid environment, it is necessary to acclimatize for both heat and humidity. Training in a hot, dry environment provides only partial acclimatization for a hot, humid environment. Another factor affecting heat acclimatization is the presence or absence of air-conditioning. To maximize acclimatization, the athlete should be exposed to the environmental conditions 24 hours a day. If the only exposure to the hot conditions is during training and the athlete then returns to an air-conditioned environment, the effectiveness of acclimatization is reduced. Therefore, it is recommended athletes spend a minimum of two weeks acclimatizing at the site of competition or in an environment very similar to that anticipated for competition. Some intense training should be performed during this period and air-conditioning should be restricted to night-time for sleeping

Although it is possible to assist the acclimatization process by exercising in a heat chamber for 3 hours per day prior to departure, it is only partially effective and should be used as an adjunct rather than as a replacement for full acclimatization. Wearing impermeable clothing while exercising may also make a small contribution to acclimatization.

Guidelines for the prevention of heat illness

Most cases of heat illness could be prevented if the following guidelines are followed.

1. Perform adequate conditioning. The athlete must have trained appropriately and be conditioned for the planned activity.

2. Undergo acclimatization if competing in unaccustomed heat or humidity

3. Avoid adverse conditions. Event organizers should ensure that high intensity or endurance events should not take place in adverse conditions of heat or humidity. If events are to occur in hot climates, they should take place in the early morning before conditions deteriorate.

4. Alter training times. Unless trying to acclimatize, the athlete should avoid exercise at the hottest time of the day.

5. Wear appropriate clothing. In hot conditions, the athletes should wear a minimal amount of loose-fitting, light-colored clothing. An open weave or mesh top is ideal. Many athletes choose to remove their top during training in hot conditions. This has the advantage of allowing better heat loss from sweating but is counterbalanced by an increased heat gain from the environment.

6.  Drink plenty of fluids before the event. The athlete should ensure that he or she is adequately hydrated in the 24 hours prior to the event. A good method of confirming this is to ensure that urine output is clear and of good volume. Fluids should be drunk right up until the commencement of exercise. It is recommended that 500 mL of fluid be drunk in the half hour prior to exercise in the heat.

7. Drink fluid during exercise. The athlete should drink at regular intervals during exercise. Ideally, 150-250 mL should be consumed every 15 minutes in hot conditions. This should occur whether the athlete is training or in competition. It is important that the athlete masters the technique of drinking while exercising. This should be practised during training. For exercise up to 1 hour in duration, plain water is the most appropriate form of fluid. For exercise lasting longer than 1 hour, a dilute glucose and electrolyte solution should be used (Chapter 33).

8. Ensure athletes and officials are well educated. It is important that event organizers, coaches and athletes understand the importance of adequate hydration, the danger of water intoxication, and the need to avoid excessive environmental conditions.

9. Provide proficient medical support. A well-equipped, well-trained medical team should be present at all endurance events occurring in hot or humid conditions. The guidelines for the medical coverage of an endurance event are discussed in Chapter 54.
   

RECOMMENDED READING

Armstrong LE, Epstein Y, Greenleaf JE, et al. American College of Sports Medicine position stand. Heat and cold illnesses during distance running. Med Sci Sports Exerc 1996;28:i-x.

Blows WT. Crowd physiology: the 'penguin effect'. Accid Emerg Nurs 1998;6:126-9.

Gambrell RC. Hyperthermia and heat-related illnesses. In: Fields KB, Fricker PA, ed. Medical problems in athletes. Malden, Massachusetts: Blackwell Science, 1997: 279-284. 

Holtzhausen LM, Noakes TD. Collapsed ultraendurance athlete: proposed mechanisms and an approach to management. Clin J Sport Med 1997;7:292-301.

Khosla R, Guntupalli KK. Heat-related illnesses. Crit Care Clin 1999;15:251-63.

Milne C, Shaw M, Steinweg J. Medical issues relating to the Sydney Olympic Games. Sports Med 1999;28:287-98.

Noakes TD. Hyperthermia, hypothermia and problems of hydration. In: Shephard RJ, ed. 2000: 

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