Exercise physiology-Physiotherapy

1. Physiological responses to
exercise in the heat
Dr.Rachita Hada
M.P.T. Ortho

2. • Heat production is beneficial during exercise in
a cold environment because it helps maintain
normal body temperature.
• Even when exercise is performed in a cool
environment, the metabolic heat load places a
considerable burden on the mechanisms that
control body temperature.
• Heat stress- any environmental condition that
increases body temperature and jeopardizes
homeostasis.

3. Cardiovascular Function
• The burden placed on the cardiovascular system
is enhanced during exercise in the heat as
exercise increases the demands on the
cardiovascular system as well as to regulate body
temperature is added.
• During exercise in hot conditions, the circulatory
system has to continue to transport blood not
only to working muscle but also to the skin.

4. Effect
• First, cardiac output increases further by
increasing both heart rate and contractility.
• Second, blood flow is shunted away from
nonessential areas like the gut, liver, and
kidneys and to the skin.

5. Prolong exercise in hot environment
• During prolonged running on a hot day, the
aerobic exercise increases both metabolic heat
production and the demand for blood flow and
oxygen delivery to the working muscles.

6. Effect
• SNS signals sent from the POAH to the skin
arterioles cause these blood vessels to dilate
and increase blood flow.
• SNS signals also go to the heart to increase
heart rate and cause the left ventricle to pump
more forcefully to compensate the decrease in
stroke volume.

7. Cardiovascular drift,???
• Because blood volume stays constant or even
decreases (as fluid is lost in sweat), another
phase of cardiovascular adjustment occurs
simultaneously.
• SNS signals to the kidneys, liver, and intestines
cause vasoconstriction of those regional
circulations, which allows more of the available
cardiac output to reach the skin without
compromising muscle blood flow.

8. What Limits Exercise in the Heat?
• Cardiovascular system can no longer
compensate for the increasing demands of
continuing endurance exercise and efficiently
regulating the body’s heat.
• Consequently, any factor that tends to overload
the cardiovascular system or to interfere with
heat dissipation can drastically impair
performance, increase the risk of overheating, or
both.

9. Critical temperature theory
• In well-trained, acclimated athletes, regardless
of the rate at which core temperature (and thus
brain temperature) increases
• Brain will send signals to stop exercise when
some critical temperature is reached, usually
between 40 and 41 °C (104 and 105.8 °F).

10. Body Fluid Balance: Sweating
• The eccrine sweat glands are controlled by
stimulation of the POAH by sending impulses to
the millions of eccrine sweat glands distributed
over the body’s surface(skin).
• The sweat glands are tubular structures
extending through the dermis and epidermis,
opening onto the skin.

12. • Second type of sweat gland, the apocrine
gland, is clustered in particular regions of the
body including the face, axilla, and genital
regions.
• These are the glands associated with “nervous
perspiration,” and they do not contribute
significantly to heat loss by evaporation.

14. The eccrine sweat glands
• They are located over most of skin surface, with
~2 to 5 million covering the whole body.
• They are most densely distributed on the palms
of the hands, the soles of the feet, and the
forehead.
• The lowest densities are found on the forearms,
lower legs, and thighs.
• When sweating begins, there are large regional
variations in sweating rate.

15. Mechanism of sweating
• Sweat is formed in the coiled secretory portion of
the sweat gland and has an electrolyte composition
similar to that of the blood(Plasma).
• As this filtrate of plasma passes through the
uncoiled duct of the gland, sodium and chloride are
reabsorbed back into the surrounding tissues and
then into the blood.
• As a result, the final sweat that is extruded onto the
skin surface through the sweat gland pores is
hypotonic to (has less electrolytes than) plasma.

17. • During light sweating, the filtrate sweat
travels slowly enough through the duct that
there is time for reabsorption of sodium and
chloride containing very little of these
electrolytes by the time it reaches the skin.
• During heavy exercise, the filtrate moves
more quickly through the tubules, allowing less
time for reabsorption, and the sodium and
chloride content of sweat can be considerably
higher.

18. • With training and repeated heat exposure
(acclimation), more sodium is reabsorbed and
the sweat is more dilute (the sweat glands
become more sensitive to the hormone
aldosterone).
• Sweat glands do not conserve/absorb other ions
like Potassium, calcium
• Genetics is a major determinant of both
sweating rate and sweat sodium losses.

19. • While performing heavy exercise in hot conditions,
the body can lose more than 1 L of sweat per hour
per square meter of body surface.
• During intense effort on a hot and humid day (high
level of heat stress), an average-sized female athlete
(50-75 kg) might lose 1.6 to 2.0 L of sweat, or about
2.5% to 3.2% of body weight, each hour.
• A person can lose a critical amount of body water in
only a few hours of exercise in these conditions.

20. • A high rate of sweating maintained for a prolonged
time ultimately reduces blood volume.
• This limits the volume of blood returning to the
heart, increasing heart rate and eventually
decreasing cardiac output, which in turn reduces
performance potential, particularly for endurance
activities.
• In long-distance runners, sweat losses can approach
6% to 10% of body weight. Such severe dehydration
can limit subsequent sweating and make the
individual susceptible to heat-related illnesses.

21. vasopressin or arginine vasopressin
Loss of both electrolytes and water in the sweat,
Release of both aldosterone & antidiuretic
hormone (ADH), [vasopressin or arginine
vasopressin]

22. • Aldosterone is released from the adrenal
cortex.
• During acute exercise in the heat and during
repeated days of exercise in the heat, this
hormone limits sodium excretion from the
kidneys.
• More retention of water and sodium in
preparation for additional exposures to the heat
and subsequent sweat losses.

23. • Posterior pituitary gland to release ADH.
• Thus, the body attempts to compensate for loss
of electrolytes and water during periods of heat
stress and heavy sweating by reducing their loss
in urine.

24. Summary
• During exercise in the heat, the skin competes with the
active muscles for a limited cardiac output. Muscle blood
flow is well maintained unless severe dehydration
occurs.
• A series of cardiovascular adjustments shunt blood away
from nonessential regions such as the liver, gut, and
kidneys to the skin to aid in heat dissipation.
• At a given exercise intensity in the heat, cardiac output
may remain reasonably constant or decrease slightly at
higher intensities, as a gradual upward drift in heart rate
helps to componsate a lower stroke volume.
• Prolonged heavy sweating can lead to dehydration and
excessive electrolyte loss. To compensate, increased
release of aldosterone and ADH enhances sodium and
water retention.

25. Health Risks During Exercise in the
Heat
• Despite the body’s defenses against overheating;
• excessive heat production by active muscles,
• heat gained from the environment,
• conditions that prevent the dissipation of excess
body heat
• Elevation of internal body temperature
• Impairment of normal cellular functions.

26. Total physiological stress imposed on
the body in a hot environment.
• Six variables:
1. Metabolic heat production
2. Air temperature
3. Ambient water vapor pressure (humidity)
4. Air velocity
5. Radiant heat sources
6. Clothing

27. An individual exercising on a bright, sunny day
with an air temperature of 23 °C (73.4 °F)
Someone exercising in the same air temperature
but under cloud cover and with a slight breeze.
More Heat stress

28. • At temperatures above skin temperature, 32 to
33 °C (92 °F), radiation, conduction, and
convection
• Increase in the body’s heat load rather than
heat loss.

29. Measuring Heat Stress
• The heat index (involving air temperature and
relative humidity) measures how hot it feels.
• In the 1970s, wet-bulb globe temperature
(WBGT) was devised to simultaneously account
for conduction, convection, evaporation, and
radiation .
• Three different thermometer readings and
provides a single temperature reading to
estimate the cooling capacity of the surrounding
environment.

30. Twb
Tdb
Tg

31. • The dry-bulb temperature (Tdb) is the
actual air temperature one would measure with
a typical thermometer.
• A second thermometer has a bulb that is kept
moist by a wetted cotton “sock” dipped in
distilled water. As water evaporates from this
wet bulb (Twb), its temperature will be lower
than the Tdb(effect of sweat evaporating from
the skin).

32. • The difference between the wet- and dry-
bulb temperatures indicates the
environment’s capacity for cooling by
evaporation.
• In still air with 100% relative humidity, these
two bulb temperatures are the same because
evaporation is impossible.
• Lower ambient water vapor pressure and
increased air movement promote evaporation,
increasing the difference between these two bulb
temperatures.

33. • The third thermometer, placed inside a black
globe(Tg), typically shows a temperature higher
than Tdb as the globe, painted a flat black,
maximally absorbs radiant heat. Thus, its
temperature (Tg ) is a good indicator of the
environment’s radiant heat load.
• The overall atmospheric challenge to body
temperature in outdoor environments:
WBGT = 0.1 Tdb + 0.7 Twb + 0.2 Tg

34. • WBGT reflects only the environment’s impact on
heat stress and is most effectively used along
with a measure or estimate of metabolic heat
production.
• Clothing influences heat stress.
• WBGT, as an index of thermal stress, is used by
coaches, team physicians, and athletic trainers to
anticipate the health risks associated with
athletic practices and competitions in thermally
stressful environments.

35. • How to calculate wet-bulb globe temperature?:
Outdoor WBGT = 0.1 Tdb + 0.7 Twb + 0.2 Tg.
Indoor WBGT = 0.7 Twb + 0.3 Tg.

36. Heat-Related Disorders

37. Heat Cramps
• least serious
• Severe and painful cramping of large skeletal
muscles, involve primarily the muscles most
heavily used during exercise.
• Athletes will feel “locking up” which is different
from cramps in small muscles.
• Heat cramps are due to sodium losses and
dehydration due to sweating.

38. How to prevent/treat Heat cramp?
• Proper hydration- liberal salt intake with foods
and in beverages consumption during exercise.
• Treatment
– by moving the stricken
individual/athelet to a cooler location
-- Administering a saline solution,
either orally or intravenously.

39. Heat Exhaustion
• symptoms as extreme fatigue, dizziness, nausea,
vomiting, fainting, and a weak, rapid pulse.
• It is caused by the cardiovascular system’s
inability to adequately meet the body’s needs as
it becomes severely dehydrated.
• Heat exhaustion typically occurs when blood
volume decreases as a result of excessive fluid
loss from profuse sweating.

40. • A second form of heat exhaustion, from sodium
depletion, is rare in athletes.
• Therefore, heat exhaustion can be like a
syndrome of dehydration and should be treated
as such.

41. • With heat exhaustion, the thermoregulatory
mechanisms are functioning but cannot dissipate
heat quickly enough because insufficient blood
volume is available to allow adequate blood flow to
the skin.
• Although the condition often occurs during
moderate to heavy exercise in the heat, it is not
necessarily accompanied by extremely high core
temperatures.
• Some people who collapse from heat exhaustion
have core temperatures well below 39 °C (102.2 °F).
• People who are unfit or not acclimated to the heat
are more susceptible than others to heat exhaustion.

42. Treatment
• Rest in a cooler environment with their feet
elevated to facilitate return of blood to the heart.
• If the person is conscious, administration of salt
water is usually recommended.
• If the person is unconscious, medically
supervised intravenous administration of saline
solution is recommended.

43. Heatstroke
• A life threatening heat disorder that requires
immediate medical attention.
• Heatstroke is caused by failure of the body’s
thermoregulatory mechanism characterized by:
• an increase in internal body temperature to a
value exceeding 40 °C (104 °F);
• confusion, disorientation, or
unconsciousness.

44. • The final element—altered mental status—is the
key to recognizing impending heatstroke
because neural tissues in the brain are
particularly sensitive to extreme heat.
• In heatstroke, cessation of active sweating may
also occur, but sweat may remain on the skin.
• If heatstroke is left untreated, core temperature
will continue to rise, progressing to coma and
ultimately death.

45. Treatment(immersion)
• Involves cooling the body as rapidly as possible.
• In the field, this can best be accomplished by
immersing the victim—as much of the body as
possible excluding the head—in a bath of cold
water or ice water.

46. Temperate water immersion
• This is the next best option.
• Where immersion is not feasible, the combined
effect of wrapping the entire body in cold, wet
sheets and fanning vigorously may be used.

47. • Cooling methods that place ice bags on small
areas, like the armpits, neck, and groin, are not
effective in rapidly lowering core temperature
because of the small surface area covered.
• For the athlete, heatstroke is not just a problem
associated with extreme conditions. Studies have
reported rectal temperatures above 40.5 °C
(104.9 °F) in marathon runners who successfully
completed races conducted in temperate and
even in cool conditions.

48. Preventing Hyperthermia
• In threatening conditions, athletes must either
move the exercise session to a less stressful
environment (moving practice indoors) or
decrease their effort (metabolic heat production)
in order to reduce their risk of overheating.
• Athletes, coaches, and sport organizers should
all be able to recognize the symptoms of heat
illness

49. Guidelines for Practicing and
Competing Under Conditions of Heat
Stress
1. Athletic events (distance races, tennis matches,
sport team practices, etc.) should be scheduled to
avoid the hottest times of the day. As a general
rule, if the WBGT is above 28 °C (82-83 °F),
consider canceling, moving indoors, decreasing
intensity of practice, or otherwise altering the
event.
2. An adequate supply of palatable fluid must be
available. Athletes should be educated and
encouraged to prevent excessive (>2%) weight
loss—that is, to replace their sweat losses to
prevent dehydration but not overdrink to the point
where they gain weight during the event.

50. 3. Because individual sweating rates and sweat
sodium losses vary tremendously, athletes should
customize their fluid intake based on their
individual sweating rate. Sweating rate can be
estimated by measuring body weight before and
after exercise. Fluids containing electrolytes and
carbohydrates can provide benefits over water
alone.
4. Athletes should be aware of signs and symptoms of
heat illness. Cold-water immersion is the most
efficient method for cooling hyperthermic athletes
in the field.
5. Organizers of events and medical personnel should
have the right to cancel or terminate events and to
stop individual athletes who exhibit clear signs of
heat exhaustion or heatstroke.

51. Precautions
• Competition and practice should not be held
outdoors when the WBGT is more than 28 °C
(82.4 °F) (practices and events either in the early
morning or in the late evening avoids the severe
heat stress of midday).
• Fluids should be readily available, and drink
breaks should be scheduled every 15 to 30 min,
with a goal of matching fluid intake to sweat loss

52. • The more clothing worn, the less body area
exposed to the environment to allow for direct
heat loss.
• Distance athletes should wear as little clothing
as possible when heat stress is a potential
limitation to thermoregulation.
• It is also important to maintain adequate
hydration, since the body loses considerable
water through sweating.
• Drinking fluid both before and during exercise
can greatly reduce the negative effects of
exercising in the heat

53. Acclimation/Acclimatization to
Exercise in the Heat
• Repeated exercise in the heat causes a series of
relatively rapid adaptations for better
performance and more safely, in hot conditions.
• When these physiological changes occur over
short periods of time, like days to weeks, those
adaptations are termed heat acclimation.
• A similar but much more gradual set of
adaptations occurs in people who adapt to hot
conditions by living in hot environments for
months to years. This is known as
acclimatization (note that the word “climate” is
part of this latter term).

54. Effects of Heat Acclimation
• Repeated bouts of prolonged, low-intensity exercise
in the heat cause a relatively rapid improvement in
the ability to maintain cardiovascular function and
eliminate excess body heat, which reduces
physiological strain. this process, termed heat
acclimation
• It involves changes in plasma volume,
cardiovascular function, sweating, and skin blood
flow that allow for subsequent exercise bouts in the
heat to be performed with a lower core temperature
and heart rate response.

56. • After heat acclimation, more work can be done
before adverse symptoms occur or a maximal
tolerable core temperature or heart rate is
reached.
• This series of positive adaptations typically takes
a period of 9 to 14 days of exercise in the heat to
fully occur.
• Well-trained individuals need fewer exposures
than untrained individuals to fully acclimate.

57. • A critical physiological adjustment that occurs over
the first one to three days of acclimation is the
expansion of plasma volume.
• The process likely involves
(1) proteins being forced out of the circulation as
muscles contract,
(2) these same proteins then being returned to the
blood through the lymph, and
(3) fluid moving into the blood because of the oncotic
pressure exerted by the increased protein content.
• However, this change is temporary, and blood
volume usually returns to original levels within 10
days.

61. • The end-exercise heart rate and core temperature
decrease early in the acclimation process, while the
increase in sweating rate during exercise in the heat
occurs somewhat later.
• An additional adaptation is a more even distribution
of sweat over the body, with increased sweating on
the most exposed body areas such as the arms and
legs,
• At the beginning of exercise, sweating also starts
earlier in an acclimated person, which improves
heat tolerance; and the sweat that is produced
becomes more dilute, conserving sodium. (bcz
eccrine sweat glands become more sensitive)

62. Achieving Heat Acclimation
• The benefits of acclimation and the rate of acclimate,
depend on
• the environmental conditions during each
exercise session,
• the duration of exercise-heat exposure, and
• the rate of internal heat production (exercise
intensity).
• An athlete must exercise in a hot environment to attain
full acclimation that sustains exercise in the heat. Simply
sitting in a hot environment, such as a sauna or steam
room, for long periods each day will not fully or
adequately prepare the individual for physical exertion
in the heat.

63. How can the athlete maximize heat
acclimation?
• Because body temperature is elevated and sweating
occurs, athletes gain partial heat tolerance simply by
training, even in a cooler environment. Therefore,
athletes are “preacclimated” to heat and need fewer
exercise-heat exposures to fully acclimate.
• To gain maximal benefits, athletes who train in
environments cooler than those in which they will
compete must achieve heat acclimation before the
contest or event.
• Heat acclimation will improve their performance
and reduce the associated physiological stress and
risk of heat injury.