Exercise in the Heat and the Physiological Rationale behind

Acclimatisation

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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

WBGT Risk

<18 Low

18-23 Moderate

23-28 High

>28 Hazardous

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 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

ADAPTATIONSImproved skin blood flow Dissipate heat effectively

Lower Heart Rate at given exercise levels

Work at a higher intensity

Effective distribution of CO Meet thermoregulation and metabolism demands

Reduction in sweating threshold

Evaporative cooling begins earlier

Increased distribution of active sweat glands

Maximizes evaporative cooling

Increased sweat rate Maximizes 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.