Exercise in the Heat and the Physiological Rationale behind Acclimatisation

INTRODUCTION Most sports have to compete in change of climate Pushes bodies ability to control its internal environment Balance between preventing hyperthermia and maintaining an adequate fuel supply to the muscles. This is an abnormally high core temperature, and has significant implications on performance and health. Two significant competitive demands

HYPOTHALAMUS Temperature is regulated by the hypothalamus Posterior hypothalamus is concerned with heat loss Controls sweating and skin blood flow responses

HEAT LOSS Radiation Convection Conduction Evaporation Hot dry environment 98% of cooling

ACUTE RESPONSES Acute responses similar to exercise however exacerbated in the heat. Increase Skin and Muscle vasodilation/ Visceral vasoconstriction Increase Core Temperature Increase Sweat Response Increase HR BP Maintenance Takes precedence over skin blood flow Increase Lactate production Changes in Substrate Metabolism Aldosterone and Vasopressin release

CARDIOVASCULAR RESPONSE Progressive decline in SV Due to Sweat losses and Decrease in PV Increase in HR to compensate Attempt to maintain CO, progressive loss in CO causes a decrease in BP Extreme Cases – BP regulation wins out over temp regulation Increase stimulus from baroreceptors Blood diverted away from the skin Maintain BP IMPLICATIONS Hyperthermia as no evaporative sweat losses

SUBSTRATE METABOLISM/ LACTATE PRODUCTION Increased RER suggests an increase in CHO usage during exercise in the heat Epinephrine levels increase with exercise in the heat Could result in Increase in Lactate production Hepatic BF decreases – less ability to oxidise lactate back to pyruvate Less chance of efflux during prolonged exercise

FLUID LOSS 2-3L per hour during exercise in the heat Hypo hydration/ Dehydration Lack of fluid intake/sweat rate hypo hydration impairs thermoregulation. Leads to… Hyperthermia Inability to use cooling mechanisms in extreme environments

PERFORMANCE IMPLICATIONS Decreases muscular endurance and max aerobic power Unsure about the effect on anaerobic performance but research suggests no impact if progressive dehydration does not occur before the event.

PERFORMANCE IMPLICATIONS Debate over theories Less substrate availability Increased lactate production Suggested critical core temperature Central regulation of skeletal muscle recruitment inhibited during exercise in the heat Tucker (2004) showed power output began to fall within the first 30% of maximal self-paced time trial in the heat. This suggested the decrease in performance was not associated with an altered temperature, heart rate or exercise perception.

STRATEGIES TO REDUCE EFFECTS Pre Cooling Ice vests Cold air Reduces physiological strain Lower Core Temperature Delays dehydration and hyperthermia Clothing Light weight/ little as possible

NUTRITIONALSTRATEGIES Hyper hydration Chronic, with acclimatization Glycerol + Water,Gastric Discomfort, urination, Increase body mass Hydration Only replace 30-70% of sweat losses Rehydration Water ingested -> dilution of plasma osmolarity-> reduced thirst Sodium drinks such as Gatorade CHO Loading Daily intakes 7-10g/kg of BM

WET BULB GLOBE TEMPERATURE (WBGT) Estimation of heat stress. WBGT accounts for the levels of humidity, radiation, wind movement and ambient temperature WBGTRisk <18Low 18-23Moderate 23-28High >28Hazardous

ACCLIMATISATION Repeated exposures to the heat results in adaptations within the body that make the athlete less susceptible to the demands imposed by exercise in the heat. Heat tolerance is improved Therefore performance in the heat is improved

STRATEGIES AND DURATION Conflicting Views Long term/ short term Recent research suggests 7-14 days minutes however depends on sport Major adaptations plateau after 14 days Most of adaptation undertaken 5-6 days Acclimation Artificial Environment Same Intensity and duration Dehydration can impair outcomes

ADAPTATIONS Improved skin blood flowDissipate heat effectively Lower Heart Rate at given exercise levels Work at a higher intensity Effective distribution of COMeet thermoregulation and metabolism demands Reduction in sweating threshold Evaporative cooling begins earlier Increased distribution of active sweat glands Maximizes evaporative cooling Increased sweat rateMaximizes Evaporative Cooling

ADAPTATIONS Reduction in loss of water and electrolytes from sweat Preserves sodium in extra cellular fluids, promotes water retention Better maintenance of Core Temperature Fatigue delay, increase thermoregulatory capacities Increasing sweating sensitivity to increasing core temp Maximizes Evaporative cooling Less reliant on CHO metabolism CHO sparing, less lactate accumulation?

Sawka et.al (2000) Core temperature does not rise as quickly Prolongs onset of dehydration and hyperthermia Thermoregulatory responses carried out as normal for longer CORE TEMP

PLASMA VOLUME Expansion of plasma volume. Large shift of blood to the peripheral areas. I.e. decrease in plasma volume. Stimulates increased renal sodium and water retention. Aldosterone and Vasopressin (ADH) released to help mediate expansion.

HEART RATE Due to increase in Plasma Volume Negates need to maintain CO Heart rate decreases rapidly in the first four days of an acclimation program and then slowly till the sixth day. The HR still increased during exercise, but at a much slower rate after acclimation. A decrease of about 22 from the first day was shown.

SWEATING Sweating rate occurs at a lower core temperature. Rate of sweating increases, more effective evaporative cooling in dry temperatures. Increased distribution of active sweat glands Decrease Sodium concentration due to Aldosterone release. Retains sodium at renal tubules Plasma osmolarity main stimulator Higher Plasma sodium concentrations allows for greater water retention in the body.

Hue et.al (2004) Sweat Rates

HOW CAN THIS HELP?? Acclimatisation allows the body to exercise for longer at a higher intensity in the heat Body adapts and delays dehydration and hyperthermia Better thermoregulation Hence, delays onset of fatiguing elements and increases performance capacity.