Exercise and Environment

1
Exercise and Environment

• Brooks Chapter 23

2
Introduction

• People have a remarkable ability to live and work
  in very hot and very cold environments.
• Humans tolerate these hot and cold climates due
  to a well developed ability to regulate core body
  temperature.
• Humans regulate core temperature through
  behavioral and physiological means.

3

• Poikilotherms body temperature varies with the
  environment (lizards, insects).
• Homeotherms maintain constant body core
  temperature (humans, birds, bears).
• In homeotherms, various physiological processes
  depend on normal body temperature to function
  properly.
• at temps above 41oC, the interior of many cells
  begin to deteriorate.
• at temps below 34oC, cellular metabolism slows
  greatly, leading to unconsciousness and cardiac
  arrhythmias.

4
Normal Body Temperature

• Normal resting core temperature in humans is
  36.5oC to 37.5oC.
• During exercise body temp can exceed 40oC.
• There is considerable variation throughout the
  body. The core is relatively constant while skin
  temp is influenced by the environment.

5
Core Temperature

• Usually defined as the temperature of the
  hypothalamus.
• The hypothalamus is the temperature regulatory
  center of the body.
• Oral temp is influenced by breathing cool air.
• Rectal temp is a commonly used in research.
• Tympanic temp is close to the hypothalamus but it
  is influenced by head skin temp.
• Esophagus and stomach temp has also been used to
  measure core temp.

6
Hypothalamus

• Works like a thermostat.
• Controls rates of heat gain and heat loss.
• Uses feedback from peripheral heat and cold
  receptors.
• Thermal receptors in the hypothalamus itself.
• Initiates the rate of heat production when body
  temp falls, and ? the rate of heat dissipation
  when temp rises.

7

• The assessment of mean body temp (MBT) takes into
  consideration skin and core temp.
• MBT (0.33 x SkinTemp) (0.67 x RectalTemp)
• Thermal gradients are temp differences from one
  point to another that lead to the movement of
  heat.
• Thermal gradients determine the rate and
  direction of heat transfer.

8
Heat Transfer

• The temperature of an object is a measure of the
  kinetic activity of its molecules. If an object
  is hot, its molecules move rapidly.
• The temperature of a body is directly
  proportional to the amount of heat it stores.
• Core temperature is regulated by controlling the
  rate of heat gain and heat loss.

9
Energy Balance Equation

• S M Cv Cd R - E
• where
• S heat storage (kcal/min or watts)
• M metabolic heat production
• Cv convective heat loss or gain
• Cd conductive heat loss or gain
• R radiant heat loss or gain
• E evaporative heat loss

10
Body Temperature, Environment and Exercise
Intensity

• Core temp rises in direct proportion to relative
  exercise intensity when ambient temp is between
  4oC and 30oC.
• Skin temp is a function of environmental temp,
  metabolic rate, clothing, and hydration state.
• Clothing and air movement across the skin affect
  skin temp by influencing the capacity for heat
  loss.

11
Heat Production

• Heat is a by-product of all biochemical
  reactions, as these reactions are not 100
  efficient.
• Metabolism is the bodys source of internal heat
  production.
• At BMR the average body generates approximately
  100kcal of heat per hour.
• During exercise heat production can be elevated
  10 - 15 times resting values.

12
Shivering

• Shivering is an involuntary contraction of
  muscle.
• The main mechanism for ? heat production during
  negative heat balance is shivering.
• Shivering can ? the bodys heat production 5x.
• Preshivering tone can increase heat production by
  50 to 100.
• Shivering is an effective way to ? body temp as
  all of the expended energy appears as heat (no
  work).

13
Nonshivering thermogenesis

• ? thyroxin secretion from the thyroid gland and
  catecholamine secretion from the adrenals ?
  metabolic rate.
• Thyroxin increases the metabolic rate of all
  cells in the body.
• Catecholamines (norep) cause the release of fatty
  acids, which will ? metabolic heat production.

14
Q10 Effect

• ? body temp can be dangerous because metabolic
  rate ? with rising temperature.
• By increasing the temp by 10oC you will double
  the rate of enzymatic reaction.
• At high temperatures (41.5oC) the hypothalamus
  loses its ability to cool itself.

15
Heat Balance

• Heat Gain Heat Loss
• Metabolic heat prod Radiation
• -exercise Conduction
• -shivering Convection
• -thyroxine Evaporation
• -symp stim
• -Q10 effect
• Radiation
• Conduction
• Convection

16
Heat Loss

• When skin temp is greater than ambient temp, the
  body losses heat by radiation, conduction,
  convection and evaporation

17
Radiation

• The body loses and gains heat by radiation to its
  surroundings by transmitting electromagnetic
  waves.
• Radiation accounts for 60 of the body's total
  heat loss at room temperature while at rest.
• The sun is the greatest source of radiant energy.
• Human skin absorbs 97 of the radient energy that
  strikes it.

18
Conduction

• Conduction of heat occurs between 2 surfaces in
  contact with each other.
• Only 3 of the body's heat loss at room
  temperature occurs through conduction.
• The rate depends on the temperature gradient
  between the skin and a surrounding surface, as
  well as the conductive property of the surface

19
Convection

• The conduction of heat to or from air or water is
  called convection.
• Convection accounts for 12 of heat loss at
  normal room temp.
• Convective heat flow depends on the extent of the
  body surface area exposed to the surrounding
  medium, the rate of flow of the medium, and the
  difference in temp between the skin the medium.
• Heat loss by convection is greater in water
  (25x) than air.
• Heat loss by convection is greater in the wind as
  warm air is replaced with cool air (windchill).

20
Evaporation

• At rest about 25 of heat loss is due to
  evaporation.
• The body loses .58 kcal of heat for each gram of
  water that evaporates (latent heat of
  vaporization).
• Evaporation is the bodys only cooling mechanism
  at high temps.
• Sweating is critical during exercise to keep core
  temperature at a safe level.
• Sweat is only effective for cooling if it
  evaporates.

21
Evaporation

• There is insensible water loss (600 ml/day)
  through vent and diffusion through the skin.
• Sweat rates are near 0 when skin temp is low.
• Max sweat rates 1.5 L/hr sedentary unacclim
• 4 L/hr acclimatized, 5L/hr acclim athlete
• Eccrine sweat glands tubular structures that
  cover most of the body (clear odorless sweat).
• Apocrine glands under arms, genital areas
• Sweat contains NaCl, urea, KCl, lactic acid.

22
Temperature Regulation

• The hypothalamus acts as a thermostat and keeps
  the bodys core temp within a normal range.
• Thermoreceptors are located in different regions
  of the body and transmit nerve impulses to the
  spinal cord and then up to the hypothalamus.
• The greatest density of thermoreceptors are in
  the skin and hypothalamus, but there are also
  some in the blood vessels and abdominal cavity.
• There are more cold than warm receptors in the
  skin.
• When core temp goes above or below its set-point,
  the hypothalamus initiates processes to ? heat
  production or heat loss.

23
Temperature Regulation

• The anterior hypothalamus stimulates sweat
  glands.
• Normally sweating begins at precisely 37oC.
• The set-point can change temporarily in response
  to dehydration, starvation, or fever.
• When cold receptors in the skin and hypothalamus
  are stimulated, various processes will ? heat
  production.
• The hunting reflex maintains BF to the hands and
  feet.
• Vasoconstriction can ? the effective core
  insulation.
• Stimulation of the shivering center causes
  shivering.
• The post hypo initiates release of norep and
  thyroxin.

24
Behaviour

• Human behaviour is an important component in
  temperature regulation.
• When the anterior hypothalamus is overheated a
  person will have the sensation of being hot.
• If a person feels warm they can drink water,
  remove some clothing, move to a cool area.
• A cold person can put on more clothes, go
  indoors, turn up the heat.

25
Exercise in the Cold

• Cold exposure can limit physical performance.
• Exposed skin will lose sensation and is
  susceptible to frostbite.
• Movement is restricted by heavy clothing.
• Shivering may decrease muscle efficiency.

26
Clothing

• The insulation value of clothing must be balanced
  with the ? metabolic heat production of exercise.
• Clothes ? the bodys insulation by trapping warm
  air next to the skin. This reduces heat loss by
  conduction and convection.
• The clo unit is a measure of the thermal
  insulation provided by clothing.
• Optimal cold weather clothing allows for evap of
  sweat while providing insulation.
• Wear clothes in layers so that it may be removed
  when metabolic and environmental conditions
  dictate.

27
Cardiopulmonary Responses to the Cold

• Maximal O2 consumption is unaffected in the cold.
• Submaximal VO2 increases in the cold due to
  greater heat loss through conduction and
  convection.
• The ? skin and muscle blood flow during exercise
  means a greater potential for heat loss in the
  cold.
• Shivering can persist during exercise and ? VO2 .
  
• Nonshivering thermogenesis may also ? VO2 .

28

• Swimming in cold water can significantly ?
  exercise capacity.
• Body fat is an important factor in determining
  heat loss in cold water.
• It is unclear if increased body fat in distance
  swimmers is a physiological or psychological
  adaptation.
• During exercise the peripheral blood vessels
  vasodilate during physical activity, which
  effectively ? the bodys insulation to the cold.
• Exercise followed by cold exposure raises post
  exercise thresholds for vasoconstriction and
  shivering.

29
Ventilation

• Ventilation ? in the cold, particularly when cold
  exposure is sudden.
• Abrupt exposure causes a gasping reflex, which is
  accompanied by hyperventilation, tachycardia,
  peripheral vasoconstriction and ? BP.
• ? ventilation will ? blood CO2 ? vasoconstrict
  brain blood vessels ? confusion and
  unconsciousness ? drowning.

30
Heart

• Cold exposure causes peripheral vasoconstriction,
  which ? central blood volume.
• BP ? due to an ? afterload and ? preload. This
  may ? HR and ? SV.
• The incidence of arrhythmias ? in the cold.
• Cold ? secretion of epi which can ? the incidence
  of fatal arrhythmias.
• Ventricular fibrilation is the leading cause of
  death in people with hypothermia.

31
Muscle Strength

• Muscle strength and peak power output ? as muscle
  temperature ?.
• Enzymes for muscle contractions are not as
  efficient at lower temperatures.
• Other factors impairing muscle performance in the
  cold include ? muscle viscosity and ? ATP
  metabolism and muscle contractile velocity.

32
Metabolic Changes

• Cold exposure ? the use of CHO as a substrate.
• Muscle glycogen ? faster during light exercise in
  the cold.
• Prolonged cold exposure ? hypoglycemia, which ?
  shivering ? ? core temp.
• Fat metabolism is ? due to a ? in blood flow to
  subcutaneous fat cells.
• Cold exposure ? protein degradation.

33
Acclimatization to Cold

• Acclimatization physiological compensation to
  environmental stress occurring over a period of
  time.
• Habituation is the lessening of the sensation
  associated with a particular environmental
  stressor.
• Cold acclimatized people maintain heat production
  with less shivering through ? in thyroxine and ?
  sensitivity to norepi.
• Chronic exposure to cold may ? skinfold
  thickness.
• Physical conditioning results in an ? body temp
  during sleep tests in the cold.

34
Hypothermia

• The hypothalamus ceases to control body
  temperature at very low core temps (lt 30oC).
• Hypothermia ? central blood volume,
  ? peripheral resistance, ? blood viscosity,
  ? HR, more susceptible to arrhythmia.
• Hypothermia occurs when heat loss is greater than
  heat production (cold water, high wind chill,
  lack of clothing etc.).
• Frostbite occurs when ice crystals form within
  tissues. Risk ? when temp is below -6oC)

35
Rewarming from Hypothermia

• Proper management of patients with hypothermia is
  crucial due to high mortality rates.
• Afterdrop is the continued fall of deep body temp
  during active external rewarming from
  hypothermia.
• Afterdrop is thought to endanger the heart by
  further cooling from cold blood returning from
  the periphery.
• Vasodilation and the way heat moves through
  tissue causes afterdrop during rewarming .
• Afterdrop depends on the method of rewarming.
• Central rewarming with heated air (45oC) will
  heat the core and gradually heat the peripheral
  tissues.
• Passive rewarming is quick, easy and gradual.

36
Exercise in the Heat

• Exercise in the heat may produce a heat gain.
• Core temp ? proportionally with ? in exercise
  intensity.
• High environmental temp adds to the metabolic
  heat stress of exercise.

37
Cardiovascular Effects

• During intense exercise in the heat the combined
  circulatory demands of muscle and skin can impair
  O2 transport.
• Plasma volume ? during exercise, due to ? BP
  which ? the rate of plasma filtration from the
  vascular space.
• The ? in plasma volume is compounded by fluid
  loss through sweating.
• The ? in blood volume will ? preload and ? HR.

38
Cardiovascular Effects

• During submax exercise HR is ?.
• During maximal exercise skin vessels
  vasoconstrict and help maintain cardiac output.
• Circulatory regulation takes precedence over
  temperature regulation.
• VO2max is not impaired in the heat, unless the
  subject started with a thermal imbalance.

39
Sweating Response

• Sweating is the primary means of heat dissipation
  during exercise.
• During exercise sweat rate is more related to
  exercise intensity than environmental temp.
• The sweating rate differs in various parts of the
  body during intense exercise.
• The onset of sweating occurs at a lower core
  temperature in bright light.
• Pre-cooling the body prior to endurance exercise
  may improve performance.

40
Acclimatization to Heat

• Physiological Change Effect
• ? sweating threshold prevents early ? in Tcore
• ? sweat rate ? evaporative heat loss
• ? plasma volume ? HR, ? VO2max
• better control of SV and BP
• ? skin blood flow ? heat loss by radiation and
  convection
• These occur after 10 to 14 days of heat exposure.

41
Acclimatization to Heat

• 2/3 to 3/4 of acclimatization occur in 4-6 days.
• Acclimatization to heat must be accompanied by
  exercise for optimal results.
• Athletes acclimatize quicker than sedentary
  individuals.
• ? sweat concentrations of Na and Cl take 10 days.
• ? in sweat rates are not fully evident until 14
  days.

42
Thermal Distress

• Dehydration loss of body fluid.
• Moderate levels of dehydration (2) will impair
  CV and temperature regulation and ? performance.
• A fluid loss of 5 of body weight will cause
  irritability, fatigue and discomfort. This level
  of dehydration is common in football and distance
  running.
• Dehydration levels gt 7 are extremely dangerous.
  At this level salivating and swallowing are
  difficult.
• Dehydration levels gt 10 there are coordination
  problems and spasticity.
• At 15 the person experiences delirium and
  shriveled skin. Above 20 death will occur.

43
Dehydration Strategies

• Aggressive fluid replacement during ex in the
  heat.
• Regular fluid breaks.
• Drink fluids in proportion to sweat loss.
• Drink cold fluids (8 - 13oC) with moderate
  amounts of carbohydrate (7) and electrolytes.
• Physical fitness helps prevent dehydration (?
  BV).

44

• Heat Cramps involuntary cramping and spasm in
  muscles used during exercise.
• Originally thought to be due to electrolyte
  imbalance, may be due a spinal neural mechanism.
• Heat Exhaustion hypotension and weakness caused
  by an inability of the circulation to compensate
  for acute plasma volume loss and vasodilation.
• It is characterized by rapid weak pulse,
  hypotension, faintness and profuse sweating.
• Occurs at core temperature below 39.5 oC.
• Treat individual with fluids and move to a cool
  location.

45

• Heat Syncope a person faints due to hypotension.
  Occurs when blood pools in the legs following
  exercise.
• Heat Stroke occurs when there is a failure of
  the temperature regulatory function of the
  hypothalamus, and represents a medical emergency.
• Results in an explosive ? in core temperature (gt
  41oC) and is characterized by hot dry skin,
  confusion and convulsions.
• The patient should be treated in a hospital with
  a spray of tepid water and a cool air stream.
• Outside of the hospital the patient can be cooled
  with a fan and ice packs on the neck, axillae,
  and groin.

46
Preventing Thermal Stress

• Ensure athletes are in good physical condition.
• Ensure athlete does not become overheated before
  exercising in the heat.
• Athletes should not train above normal
  intensities.
• Schedule training during cooler times in the day.
• Cancel training if the temperature is too high.
• Implement regular fluid breaks.

47
Preventing Thermal Stress

• Supply cold fluids (8 - 13oC) with moderate
  amounts of carbohydrate (7) and electrolytes.
• Drink 600ml of fluid 2 hours before, and 400ml 15
  minutes prior to exercise.
• Fluid replacement should be encouraged during the
  early stages of a training session.
• Weigh athletes before each practice to ensure
  they are not dehydrated at the start of training.
• Do not use salt tablets.