TRAINING AND RACING IN HEAT AND HUMIDITY Michael Turnbull 8 th October 2005

INTRODUCTION Physiology in the heat Performance in the heat Heat acclimatization Adaptations Gender and age differences Dehydration Heat injuries Loss of acclimatization Summary Questions and Answers

PHYSIOLOGY IN THE HEAT Exercise in Asia poses a particular stressful challenge to the maintenance of normal body temperature and fluid homeostasis Heat and humidity reduce the body’s ability to lose heat resulting in a greater core temperature and a higher sweat rate compared to the same exercise in a moderate environment.

PERFORMANCE IN THE HEAT Triathlon performance is impaired in hot and humid environments Contributory factors include: –Hyperthermia – Impairs Central Nervous System. –Muscle blood flow changes – Muscle blood flow is reduced due to competition between the working muscles and the skin. –Metabolism – Muscle glycogen usage is increased and lactate production is elevated Leads to an increased risk of heat injury

HEAT ACCLIMATIZATION Acclimatization by repeated exercise in the heat (either strenuous interval training or continuous exercise above 50% VO2 max) promotes higher core temperatures. This results in physiological adaptations that increase heat tolerance: –Increased plasma volume –Earlier onset of sweating –Higher sweat rate –Reduced sodium chloride loss in sweat –Reduced skin blood flow

HEAT ACCLIMATIZATION After acclimatization, the increased sweat rates improve the ability to control body core temperature, leading to an improved performance capacity.

GENDER DIFFERENCES Most women appear to be less heat tolerant than men. Contributory factors include: –Lower sweat rates –Generally higher percentage of body fat However, when men and women are matched for the same acclimatization and similar body compositions – gender differences in the physiological responses to thermal stress are small.

AGE DIFFERENCES Does ageing impair your ability to thermoregulate and exercise in the heat? –Previously controversial in earlier studies More recent studies have shown that heat tolerance does not appear to be compromised by age in healthy and physically active older subjects –Physical deconditioning and a lack of heat acclimatization are more likely causes of a decline in performance

DEHYDRATION An earlier onset of sweating at a higher rate can lead to potential dehydration Sweat loss can reach up to 3 litres per hour in some athletes A 5% loss in body weight through dehydration can lead to a 30% loss in exercise capacity Even if hydration techniques are correct, it has been reported that some marathon runners can lose 8% of their body weight in one race

HYDRATION RECOMMENDATIONS Dehydration can be avoided through correct hydration techniques: –Be adequately hydrated prior to the exercise (400 – 800ml of fluid 2 – 3 hours before) –During the exercise, most athletes should consume between 150 – 300ml of fluid every 15 to 20 mins (PRACTICE!!!) –Following the exercise, replace what you have lost Drink cold drinks, they are absorbed by the body faster Fluid replacement during exercise reduces heart rate, body temperature and perceived exertion levels For triathlons, water alone is not enough Electrolytes lost through sweating need to be replaced

HEAT INJURIES Symptoms include: –Nausea, dizziness, reduction in sweat rate and general ability to think rationally To prevent overheating: –A maximum surface area should be exposed for evaporation –Frequent water breaks couple with rest periods to allow the core temperature to decline If the symptoms have already developed: –Stop exercising –Drink water or a balanced electrolytic drink –Submerge the body in cool water

LOSS OF ACCLIMATIZATION The rate of decay of heat acclimatization is rapid Reductions in heat tolerance can occur within a few days of inactivity Heat tolerance can decline significantly within 7 days of no heat exposure. A complete loss of heat tolerance can occur following 28 days of no heat exposure. Repeated exposure is required to maintain acclimatization.

PROMOTING HEAT ACCLIMATIZATION Training in sweat clothing in a cool environment can promote heat acclimatization The magnitude of acclimatization is generally less than training in a hot and humid environment BUT artificial heat training appears to be better than attempting no heat acclimatization measures. Important for those of you who travel to Europe

SUMMARY Heat and humidity have a negative effect on performance. Heat acclimatization can improve performance A side effect of acclimatization is a greater risk of dehydration Maintain hydration! When moving to a cooler climate for a short period of time, train in the heat