Exercise in heat

1. Exercise In Heat Dr. Usha (PT)
Assistant Professor

2. Introduction
•Cardiovascular adjustments and evaporative cooling
facilitate metabolic heat dissipation during exercise,
particularly in hot weather.

3. 1
• Fluid loss in thermoregulation (sweating)
2 • Relative state of dehydration
3
• (Excessive) more serious fluid loss
4
•Reduce plasma volume
5
• (Extreme) end result involves circulatory failure with
core temperature increasing to lethal levels
S
T
E
P
S

4. •2 cardiovascular demand in exercise
•Oxygen delivery to active muscles must increase to sustain
exercise energy metabolism
•Peripheral blood flow to the skin must increase to transport
metabolic heat from exercise for dissipation at the body’s
surface; this blood no longer remains available to active
muscles.

5. Stroke Volume In Heat
•Decrease in proportion to fluid deficit in the body created
during exercise increase heart rates at all submaximal
exercise level
•Maximal cardiac output and aerobic capacity decrease during
exercise in the heat because the compensatory increase in
heart rate does not offset the decrease in stroke volume.

6. Vascular Constriction & Dilation
EX.INHEAT
Adequate skin
and muscle blood
flow
RESULTIN
Other tissues
(renal &
splanchnic)
temporarily
compromise their
blood supply
IFOCCURSFORLONG
Liver and renal
complication
(heat stress)

7. What Happens To B.P ?
•Arterial blood pressure remains stable during exercise in the heat
because visceral vasoconstriction increases total vascular
resistance as blood redirects to areas in need.
•During near-maximal exercise with accompanying dehydration,
relatively less blood diverts to peripheral areas for heat dissipation.
•This reflects the body’s attempt to maintain cardiac output despite
sweat-induced decreases in plasma volume.
•Circulatory regulation and maintenance of muscle blood flow take
precedence over temperature regulation, often at the expense of a
spiraling core temperature and accompanying health risk.

8. Core Temperature During Exercise
•Heat generated by active muscles can increase core temperature to
fever levels that incapacitate a person if caused by external heat
stress alone.
•Within limits, increased core temperature with exercise does not
reflect heat-dissipation failure.
•Core temperature increases in proportion to exercise intensity.
•A modest core temperature increase reflects favorable internal
adjustments that create an optimal thermal environment for
physiologic and metabolic function.

9. Water Loss In The Heat
•Dehydration induced by a few
hours of intense exercise in the
heat often reaches levels that
impede heat dissipation and
severely compromise
cardiovascular function and
exercise capacity.
•It shows average water loss/hr
from sweating at various air
temperatures for a typical adult
during rest, light and moderate
physical activity.

10. Magnitude Of Exercise Fluid Loss
•For an acclimatized person, sweat loss peaks at about 3 L h1 during
intense exercise in the heat and averages nearly 12 L (26 lb) on a
daily basis.
•Intense sweating for several hours can induce sweat gland fatigue
that impairs core temperature regulation.
•Elite marathon runners frequently sweat in excess of 5 L of fluid
during competition this represents 6% to 10% of body mass. For
slower paced marathons or ultramarathons, the average fluid loss
rarely exceeds 500 mtr/hr. For more intense exercise even in a
temperate climate, soccer players lose approximately a 2 L of sweat
during a 90-minute game played at about 50F (10C).

11. •Hot, humid environments impede
the effectiveness of evaporative
cooling because of the high vapor
pressure of ambient air and
promote large fluid losses.
•illustrates a linear relationship
between sweat rate during rest
and exercise and the air’s moisture
content (expressed as wet bulb
temperature

12. Consequences of
Dehydration

13. As Dehydration Progress…
Plasma volume decreases
Peripheral blood flow and sweating rate
decrease
Thermoregulation progressively more
difficult
Increases heart rate, perception of
effort, and core temperature
Premature fatigue occurs

14. •A fluid loss equivalent to only 1% of body mass increases
rectal temperature compared with the same exercise
performed fully hydrated.
•Dehydration equivalent to 5% of body mass increases rectal
temperature and heart rate while decreasing sweating rate,
VO2max, and exercise capacity compared with the normally
hydrated condition.
•Blood plasma supplies most of the water lost through
sweating; thus, maintaining cardiac output becomes
problematic as sweat loss progresses.

15. Do You Remember!
Loss of plasma volume does the following:
1. Initiates increases in systemic vascular resistance to
maintain blood pressure
2. Reduces skin blood flow, which thwarts a major avenue for
heat dissipation.

16. Dehydration reduces circulatory and temperature-regulating
capacity to meet the metabolic and thermoregulatory
demands of exercise.
sweat-
loss
dehydr
ation
Exercise
Duration
Environment
al
Temperature
Solar
Load
Exercise
Intensity
Wind
Speed
Relative
Humidity
Clothing

17. •A 100F (37.8C)ambient air temperature increases the resting
water requirement by 50% to 60%.
•Adding physical activity and radiant heat increases the
requirement even more.
•Eight hours of strenuous outdoor physical effort at
temperatures of 96F or higher(relative humidity 20%) could
increase total fluid requirements to 15 L.
•Replacing this much fluid requires drinking water a regular
intervals throughout the day.

18. •First initiated during the 1990 to 1991
Desert War in Iraq and Kuwait and
continued currently in Iraq and
Afghanistan, the U.S. military imposed
forced water intake through a planned
drinking program before, during, and
after job tasks.
•They also outfitted each soldier with a
personal 2.5-L water pack hydration
system

19. • Marathon performance decreases
progressively as wet bulb-globe
temperature(WB-GT) increases.
• The figure shows the slowing of
marathon running performance of
men and women as the WBGT
increased from 10 to 25C (50–
77F) with performance more
negatively affected for slower
runners.

20. Fluid Loss In Winter
Environments

21. Use of Diuretic and Laxative:
•Athletes who take diuretics to rapidly lose body water and
body weight reduce their plasma volume; this negatively
affects thermoregulation and cardiovascular function.
•Diuretic drugs can also impair neuromuscular function not
noted when comparable fluid loss occurs by exercise.
•Athletes who vomit and take laxatives to lose weight not only
become dehydrated but also lose minerals.
•These practices weaken muscles and impair motor function.

22. Water Replacement and
Rehydration

23. Need…
•Maintenance of plasma volume so that circulation and sweating
progress optimally
•athlete’s hydration status and its impact on exercise performance and
safety
•Cold treatments (e.g., periodic application of cold towels to forehead
and abdomen during exercise or taking a cold shower before
exercising in a hot environment) at best provide only minimal benefits
to facilitate heat transfer at the body’s surface compared with the
same exercise without skin wetting.

24. •Adequate hydration provides the most effective defense
against heat stress by balancing water loss with water intake,
not by pouring water over the head or body.
•well-hydrated athlete always functions at a higher physiologic
and performance level than a dehydrated one.

25. Determining The Rate and
Quantity of Rehydration

26. How Much Do We Sweat?

27. What To Do Then…
•With a sweat rate of 1152 mL/h, this person needs to consume
about 1000 mL (32 oz) during each hour at a rate of 250 mL (8.5
oz) at 15-minute intervals to match total fluid loss during activity.
•Partitioning rehydration periods into 10- to 15-minute intervals
allows for the maintenance of optimal stomach volume and
properly matches fluid loss with fluid intake Provide for
unrestricted access to water during practice and competition.
•Athletes must rehydrate on a regular schedule because the thirst
mechanism imprecisely gauges water needs.

28. Use of Glycerol:
•Ingesting a concentrated mixture of glycerol (now permitted by the World Anti-
Doping Agency [WADA]) with water increases the body’s fluid volume to
produce a state of hyper-hydration.
•The typically recommended pre-exercise glycerol dose of 1 g of glycerol per kg
of body mass in 1 to 2 L of water lasts up to 6 hours.
•This glycerol solution facilitates water absorption from the intestine and causes
extracellular fluid retention, mainly in the plasma fluid compartment.
•The hyper-hydration effect of glycerol supplementation reduces overall heat
stress during exercise as reflected by increased sweating rate; this lower heart
rate and body temperature during exercise and enhances endurance
performance under heat stress and increases safety for the exercise participant.

29. Is Glycerol Enough?
•Its use may be more advantageous during high-intensity
endurance exercise. Side effects of exogenous glycerol
ingestion include nausea, dizziness, bloating, and
lightheadedness.
•This area requires further research.

30. Pre-exercise Hyper hydration
•Ingesting “extra” water (hyper hydration) before exercise in the heat offers some
protection because it delays hypo hydration, increases sweating during exercise,
and brings about a smaller increase in core temperature.
•Acute hyper hydration
(1) At least 500 mL of water before sleeping the night before exercising in the heat
(2) Another 500 mL upon awakening
(3) 400 to 600 mL (13–20 oz) of cold water about 20 minutes before exercise. This
final pre-exercise intake provides fluid and increases stomach volume to optimize
gastric emptying.
(4.)An extended regimen of pre-exercise hyper hydration (4.5 L fluid per day,
starting few days before heat exposure) also increases body water reserves and
improves temperature regulation

31. •During intense exercise in the heat, matching fluid loss with
fluid intake becomes virtually impossible because only 800 to
1000 mL of fluid empty from the stomach each hour.
•This rate of stomach emptying does not match a water loss
that may average nearly 2000 mL per hour. Under these
conditions, pre-exercise Hyper hydration would prove
beneficial.
•Adequacy of rehydration is determined by large vol. of urine
output without a strong smell in contrast to small amount of
dark yellow urine with strong odour.

32. Electrolyte Replacement
•With prolonged exercise in the heat, sweat loss can deplete the
body of 13 to 17 g of salt (2.3–3.4 g per L of sweat) daily, about
8 g more than typically consumed.
•The volume of ingested fluid after exercise must exceed by
25% to 50% of the exercise sweat loss to restore fluid balance
because the kidneys continually form urine regardless of
hydration status.
•Unless the beverage contains sufficiently high sodium content,
excess fluid intake merely increases urine output without
benefit to rehydration.

33. •Maintaining a relatively high plasma concentration of sodium by
adding sodium to ingested fluid sustains the thirst drive, promotes
retention of ingested fluids (less urine output), and more rapidly
restores lost plasma volume.(0.5 to 0.7 g of sodium per liter –last
>1 hour)
•A beverage that tastes good to the individual also contributes to
voluntary rehydration during exercise and recovery.
•With heavy sweating, increasing the intake of potassium rich foods
such as citrus fruits and bananas replaces potassium losses. A
glass of orange juice or tomato juice replaces almost all the
potassium, calcium, and magnesium excreted in 3 L of sweat.

34. Thank you