Exercise in the Heat and the Physiological Rationale behind Acclimatisation

1. Exercise in the Heat and the Physiological Rationale behindAcclimatisation http://laurensfitness.com

2. 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

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

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

5. 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

6. 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

7. 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

8. 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

9. 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.

10. 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.

11. 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

12. 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

13. WET BULB GLOBE TEMPERATURE(WBGT) • Estimation of heat stress. • WBGT accounts for the levels of humidity, radiation, wind movement and ambient temperature

14. 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

15. STRATEGIES AND DURATION • Conflicting Views • Long term/ short term • Recent research suggests 7-14 days • 60-90 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

16. ADAPTATIONS

17. ADAPTATIONS

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

19. 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.

20. 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.

21. 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.

22. Hue et.al (2004) Sweat Rates

23. 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.