Week 14 - Lecture 2 - Heat Stress and Cold Stress PDF

KINE 442 – Exercise Physiology Week 14 Lecture 2 – Heat stress Overview of Heat Stress Hyperthermia: an increase in deep internal body temperature above normal Heat stress challenges body to dissipate excess heat from working muscles during exercise 75% of energy from...

KINE 442 – Exercise Physiology Week 14 Lecture 2 – Heat stress Overview of Heat Stress Hyperthermia: an increase in deep internal body temperature above normal Heat stress challenges body to dissipate excess heat from working muscles during exercise 75% of energy from food is released as heat Thermoregulation: ability of body to maintain constant internal temperature Human body’s ability to maintain constant body temperature makes us homeotherms (unlike amphibians, fish, reptiles) able to perform in extreme environments body works to maintain temperature Elevated heat & humidity increase this challenge by blocking heat dissipation from body Normal core temperature is 37 degrees Celsius; serious heat illness can occur at 41 degrees have a small fever-body tolerates small During exercise you Copyright © 2016 Wolters Kluwer All Rights Reserved change during exercise  Thermoregulation Receptors in periphery & CNS detect temperature changes Periphery: receptors in & under skin, in peritoneal (abdominal) cavity CNS receptors in: Brain stem Spinal cord Loading… Hypothalamus: acts like thermostat Copyright © 2016 Wolters Kluwer All Rights Reserved PO/AH negative feedback regulation of thermoregulation generae a Copyright © 2016 Wolters Kluwer All Rights Reserved Hypothalamic control of body temperature Loading… Copyright © 2016 Wolters Kluwer All Rights Reserved  KINE 442 – Exercise Physiology Week 14 Lecture 2.2 – Factors Influencing Heat Stress  External factors Factors determining thermoregulatory stress Ambient temperature Relative humidity: % of water vapor held in air humid > - can't cool body when Wind speed Heat Stress Index: an index combining temperature and humidity to determine the amount of physiological stress that will be experienced by the body under such conditions Copyright © 2016 Wolters Kluwer All Rights Reserved  Mechanisms of Heat Loss Convection: air blows over surface of skin (or running water as with swimming) Conduction: physical contact between 2 surfaces, such as sitting in a cold water bath to treat heat stroke Radiation: molecules in motion emit electromagnetic waves such as the sun; Worse at altitude (e.g. risk of sunburn when skiing) Only 4% of heat loss from radiation in hot environments (versus 67% at normal temperatures). At very hot environments, heat can be gained by body instead of lost. Evaporation: sweat on skin vaporizes, taking heat with it. Risk of dehydration. - > evaporative cooling Copyright © 2016 Wolters Kluwer All Rights Reserved Comparison of environmental and core body temperatures Copyright © 2016 Wolters Kluwer All Rights Reserved Methods of heat loss and gain Copyright © 2016 Wolters Kluwer All Rights Reserved  Circulatory and Metabolic Responses to Heat Stress Heat increases heart rate & cardiac output Redirects circulatory flow to periphery to: Dissipate heat Cool blood Reddens skin Flushes complexion Loading… Copyright © 2016 Wolters Kluwer All Rights Reserved  Influence of Body Composition and Physical Fitness Level Body composition Larger body mass = greater heat production Ratio of body volume (generate heat): body surface area (dissipate heat) less favorable in some NFL players Body fat insulates, makes heat loss more difficult Fitness level Hot environments require redistribution of blood from core and muscle to periphery to dissipate heat Greater fitness = greater cardiac output Greater cardiac output = improved ability to redistribute blood from muscle and dissipate heat Copyright © 2016 Wolters Kluwer All Rights Reserved  KINE 442 – Exercise Physiology Week 14 Lecture 2.3 – Consequences of Heat Stress  Heat Cramps Muscle cramps that occur when one is exposed to heat Result from: Dehydration Electrolyte imbalances, specifically whole body sodium deficit potassium + Neuromuscular fatigue Triggered by intense exercise Characterized by painful, involuntary muscle contractions Copyright © 2016 Wolters Kluwer All Rights Reserved Heat Cramps Copyright © 2016 Wolters Kluwer All Rights Reserved  Syncope Fainting or “passing out” Often occurs when sitting or standing or after an activity in heat, dizziness or lightheadedness may occur beforehand Caused by: Excessive peripheral dilation Pooling of blood in legs, reducing venous return Dehydration Reduction in cardiac output Brain ischemia More common when individuals have not been acclimated or acclimatized Copyright © 2016 Wolters Kluwer All Rights Reserved  Heat Exhaustion Caused by: Heavy sweating Dehydration Sodium loss Energy depletion Typically occurs in hot & humid environments Confused with exertional heat stroke Copyright © 2016 Wolters Kluwer All Rights Reserved  Heat Exhaustion: Signs and Symptoms Persistent muscle cramps Weakness, pallor Fainting, dizziness Headache Hyperventilation Nausea, diarrhea Loss of appetite Decreased urine output Core body temperature between 36 and 40 degrees Celsium (97 to 104 Fahrenheit) When diagnosis between heat exhaustion and heat stroke is unclear, treat for heat stroke with immediate cooling Copyright © 2016 Wolters Kluwer All Rights Reserved Exertional Heat Stroke (EHS) Medical emergency: if not treated quickly, death can result Caused by failure to dissipate heat after intense exercise Thermoregulatory center is overwhelmed, leading to stroke Core temperature is elevated to above 40°C or 104°F; death can occur above 43°C or 109.4°F Most accurate measure of temperature is a rectal thermometer Cellular damage to organs & tissues (including hypothalamus) Treatment: rapid cooling of body Copyright © 2016 Wolters Kluwer All Rights Reserved Complications of Exertional Heat Stroke Lactic acidosis Excessive potassium in blood Acute renal failure Rhabdomyolysis (destruction of muscle tissue releasing proteins into blood) Bleeding disorders Other medical conditions Death Copyright © 2016 Wolters Kluwer All Rights Reserved Signs and Symptoms of Exertional Heat Stroke Tachycardia (rapid heart rate) Hypotension ↓ BP Sweating (although skin may be dry on collapse) Hyperventilation Altered mental state Diarrhea Seizures Coma Treatment: immediate, rapid cooling preferably in ice water bath; monitor temperature with rectal thermometer Copyright © 2016 Wolters Kluwer All Rights Reserved  KINE 442 – Exercise Physiology Week 14 Lecture 2.4 – Treatment of Heat Stress, Performance, and Prevention Treatment of heat stroke Copyright © 2016 Wolters Kluwer All Rights Reserved Tubs used in treatment of heat stroke Copyright © 2016 Wolters Kluwer All Rights Reserved  Factors Affecting Heat Illness Fitness level Greater fitness = less susceptibility to heat illness Carefully consider fitness before engaging in activity in heat Cardiovascular fitness should be primary focus of conditioning Age Cardiovascular function declines with age Decline in cardiac output with age reduces tolerance to heat Heat acclimatization is possible Copyright © 2016 Wolters Kluwer All Rights Reserved  Endurance Performance in the Heat As heat increases, performance declines Measures to ensure optimal performance: Prior acclimatization or acclimation Proper hydration Physical conditioning Wet-Bulb Globe Temperature (WBGT): a composite temperature used to estimate the effect of temperature, humidity, and solar radiation on humans Performance decrements are predicted by wet bulb globe temperature; when WBGT is used in conjunction with NCAA guidelines, prevents heat illness Copyright © 2016 Wolters Kluwer All Rights Reserved  Anaerobic & Strength Performance in the Heat Depends on duration of event & duration of heat exposure Limited exposure in shorter activities may not affect performance at all (100-m track) Longer events are more likely to affect performance (1500-m track) and predispose to heat illness Hypohydration does reduce strength, power, and muscular endurance Improved by: Cooling methods Hydration Limiting of exposure to heat Copyright © 2016 Wolters Kluwer All Rights Reserved Prevention Strategies Acclimation: physiological adaptation to an artificial environment Acclimatization: physiological adaptation to a natural environment Copyright © 2016 Wolters Kluwer All Rights Reserved  Time course of adaptations Cardiovascular system: 1-5 days (plasma volume, reduced heart rate at workload, improved blood flow) Temperature regulation: 5-8 days (sweat rate, sweat at lower temperatures, sweat gland adaptations) Conservation of sodium chloride: 3-9 days (losses from urine and sweat decrease) Loading… All adaptations: up to 14 days (increased heat loss, decreased core & skin temp, decrease VO2 at workload, improved exercise economy) Adaptations lost after 2.5 to 5 weeks out of heat Copyright © 2016 Wolters Kluwer All Rights Reserved Hydration Sweating cools body but dehydrates it Many athletes chronically hypohydrated, which increases susceptibility to heat illness Hydration before exercise is key to responding to heat Mild hypohydration in women causes: Decreased concentration and mood Increased perception of exertion Headaches In men: Decreased vigilance Decreased memory Increased tension/anxiety and fatigue Copyright © 2016 Wolters Kluwer All Rights Reserved  KINE 442 – Exercise Physiology Week 14 Lecture 2.5 – Cold Stress  Physiological Thermoregulation in the Cold Cold receptors in body: Monitor change & rate of decrease in temperature Signal many different actions to occur Are fewer in number than heat receptors Are found in the skin, abdominal viscera, & spinal cord Copyright © 2016 Wolters Kluwer All Rights Reserved  Body’s defense against cold Vasoconstriction of blood vessels in skin Decrease sweating Release thyroid hormones to increase metabolism and heat production Shivering of skeletal muscle Epinephrine: increase metabolism; norepinephrine: increase vasoconstriction Hair stands up to increase insulation (less effective in humans) Decreased blood flow to skeletal muscle (unless physical activity increases) Curling up to preserve core heat, seeking shelter, finding heat sources, eating, putting on additional clothing Copyright © 2016 Wolters Kluwer All Rights Reserved Mechanisms related to physiological responses to cold stress Copyright © 2016 Wolters Kluwer All Rights Reserved Isotherm: line connecting points of equal temperature Copyright © 2016 Wolters Kluwer All Rights Reserved Common cold environments in sport Copyright © 2016 Wolters Kluwer All Rights Reserved  KINE 442 – Exercise Physiology Week 14 Lecture 2.6 - Hypothermia  Hypothermia Hypothermia: decrease in body’s temperature to a point that normal physiological function is impaired Stage 1: body temp 1 to 2°C below normal Loss in ability to perform complex motor tasks Breathing becomes rapid & shallow Stage 2: body temp 2 to 4°C below normal Neuromuscular function is affected Stage 3: body temp below 32°C Body systems shut down, organs fail, brain dies Copyright © 2016 Wolters Kluwer All Rights Reserved Decline in physiological status by core body temperature Copyright © 2016 Wolters Kluwer All Rights Reserved Performance Responses to Cold Reductions in neuromuscular activity Reduction in nerve conduction velocity, or rate that neural impulses travel to muscle fibers reduces force production, particularly fast-twitch fibers Prior severe cold injuy may cause long-term decrements Reduction in force production (except possibly eccentric actions, but only with mild cooling) Diminished power output Decreased heart rate at given workload Decreased time to peak power Copyright © 2016 Wolters Kluwer All Rights Reserved Cardiovascular and Endurance Performance Submaximal and maximal oxygen consumption not affected by acute exposure to cold unless core body temp decreases (indicating first stages of hypothermia) Exercise-induced bronchoconstriction (EIB) in those with asthma (bronchoconstriction: reduction in diameter of lung bronchioles) With proper attire, cold weather has minimal impact on performance Marathon speeds slow with increasing temperature from 5 to 25 degrees Celsius; hyperthermia more of a concern than hypothermia Note: pre-cooling can be used as a performance-enhancing strategy for submaximal endurance exercise in hot environments, such as U.S. marathon runners in Olympics Copyright © 2016 Wolters Kluwer All Rights Reserved  Acclimatization/Acclimation Behavioral changes Warm-up layering clothing & dressing properly Psychological adjustments: ability to endure cold Physiological adaptations Vasodilation in response to high altitude cold exposure Higher basal metabolic rates (Eskimos) Copyright © 2016 Wolters Kluwer All Rights Reserved Ice Bowl: Green Bay Packers vs Dallas Cowboys, -26.1 C Copyright © 2016 Wolters Kluwer All Rights Reserved