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“Hot” Yoga: Physiological Concerns while Exercising in the Heat
Leslie S. Funk
Introduction
As “hot” Yoga classes gain in popularity, it is crucial to inform
potential students how heat affects the body while exercising. This article
explains the physiological demands placed on the body while exercising in a
heated environment. It is important to understand how the body cools itself
when heat stress is present, and how excessive heat stress can cause heat
illness. Recommendations for how to exercise in a heated environment are given,
including hydration and acclimatization guidelines to minimize the risk of heat
illness.
Thermoregulation: A Balancing Act
The body maintains its core temperature at 98.6º F (37º C) by
balancing the rate of internal heat production with heat loss to the
environment. This balancing act is technically called thermoregulation, and it
is crucial, maintaining core temperature within a narrow range to avoid
life-threatening conditions. To regulate core temperature, the cardiovascular
system, comprised of the heart, blood vessels, and blood, adapts during
exercise in the heat. Adaptations to transport heat from the body’s core
to the skin surface include increased heart rate and stroke volume (the volume
of blood ejected from the heart with each beat). These cardiovascular changes
are controlled by the central nervous system. At core temperatures greater than
103–104º F (39–40º C) the central nervous system becomes
severely impaired, rendering the body helpless in lowering core temperature.
Basically the nerve impulses are decreased, leading to an inability of the
brain to send a message to the blood vessels and sweat glands to increase heat
loss. If the core temperature reaches 106º F (41º C), medical
intervention is needed immediately or death will ensue.
Thermoregulation is challenging in a heated environment, especially
if the air temperature is greater than the skin temperature. During moderate
exercise, core temperature rises and the additional heat must be moved from the
core to the skin. Heat is removed from the body to the environment in four
ways: radiation, conduction, convection, and evaporation. The first three occur
in environments where air temperature is cooler than skin temperature or where
cooler air blows over the skin. In a “hot” Yoga class the room is
typically heated to 90–105º F (35–40º C), which exceeds
skin temperature, and is possibly above core temperature in an exercising
person. Assuming a fan is not present in the Yoga environment, three of the
four ways the body releases heat are not viable options, forcing the body to
rely on evaporation to dissipate the heat generated by exercise. Additionally,
when the room is warmer than the body, the body will actually gain heat from
the environment, increasing core temperature.
Sweat and Heat Loss
Evaporation is the body’s primary protection against overheating. Through
this process, the body transfers heat from its core by evaporating sweat from
the skin and respiratory passages. It is important to understand, however, that
sweating alone does not cool the body. To produce a cooling effect, the sweat
must be evaporated to lower the skin temperature. By blowing cooler air over
the skin, sweat vaporizes and heat is released. Humidity impairs evaporation,
which increases the rate of sweating, causing a greater loss of body water,
which in turn leads to more severe dehydration. In a Yoga class, humidity may
be high, depending on the heat source, the room ventilation, and the number of
students participating. Humidity will increase if the ventilation is poor and
there are lots of students exercising and sweating. A separate concern is the
practice of wiping sweat from the body using a towel. If the sweat is removed
from the skin, evaporation will not occur, and heat will be retained. Sweating
will subsequently increase, leading to a greater loss of body water and
increased dehydration.
Dehydration
As the sweat rate increases, body water loss increases, and the need for
replacement fluids becomes crucial. Without adequate fluid replacement during
exercise, the body’s ability to dissipate heat is compromised. Hydration
status prior to exercise is equally as important in avoiding dehydration. If
properly hydrated at the start, the effectiveness of fluid replacement while
exercising is increased. As dehydration occurs, the body experiences a decrease
in the plasma volume of the blood. As plasma volume decreases, the body’s
ability to lose heat is compromised. As little as a 2% loss of body mass from
fluid loss will impair exercise performance. This means a 150-pound person who
loses 3 pounds during a “hot” Yoga class from increased sweating will
experience increased heart rate and decreased blood volume, causing a loss of
endurance.
Heat Illnesses
There are several types of heat illness, the most common being heat exhaustion.
Heat exhaustion occurs when blood plasma volume is reduced and the heart is
unable to maintain cardiac output (heart rate and stroke volume combined). The
symptoms of heat exhaustion include a weak, rapid pulse, dizziness, headache,
general weakness, and low blood pressure when upright. Body temperature is not
severely elevated and sweating may be moderately reduced. To treat heat
exhaustion one should move to a cooler area and rehydrate by taking fluids
orally or by IV if necessary. In most cases, a 0.1% NaCl electrolyte solution
will be used to replace sodium. If blood sodium is depleted, symptoms of heat
exhaustion can include vomiting and muscle cramping, combined with progressive
weakness. Heat exhaustion can range from mild to severe.
Heat stroke is the most dangerous heat illness and is considered a
medical emergency. Heat stroke occurs when the core temperature of the body
increases to 103–104º F (39–40º C), a life-threatening
situation. In heat stroke, the body’s heat-dissipating efforts have
failed, usually due to central nervous system impairment. At this point,
sweating ceases and the skin becomes hot and dry and the core temperature
continues to increase. Internally, organ damage occurs as cell membranes are
ruptured. Individuals experiencing heat stroke will be disoriented and
confused, their mental acuity will be impaired, and they may lose
consciousness. Immediate medical intervention is required to prevent death.
Heat cramps are a less dangerous but painful form of heat illness.
By definition, heat cramps are muscle spasms that occur during or after intense
physical activity. Imbalanced body fluids and/or electrolytes are the believed
cause of heat cramps. In most cases, the cramps occur in the abdomen and
extremities.
Prevention of Dehydration
The ideal situation is the prevention of dehydration during exercise. To
achieve this goal, students must be informed of good hydration practices before
they participate in a “hot” Yoga class. The recommendations for
hydration before, during, and after exercise are as follows:
1. Daily intake of 8–10 glasses of water (8 oz. per glass) for normal
activity, not including exercise.
2. At least 2 hours prior to exercise, drink 16 oz. of water or fluid without
caffeine.
3. If possible, determine body weight while hydrated, prior to exercise.
4. During exercise, drink frequently and early, before you are actually
thirsty. Your goal is to replenish fluids at the same rate you are sweating.
The maximal rate of replacement is between 20–40 oz. per hour, based on
absorption of fluid by the stomach.
5. After exercise, evaluate change in body weight. Consume fluids to return to
body weight prior to exercise.
Since the environment in a “hot” Yoga class increases body fluid
loss, the above guidelines become even more important. Of additional concern,
caffeinated beverages such as coffee, many teas, and sodas will increase fluid
loss by increasing urinary output. Wine, beer, and other alcoholic beverages
cause fluid loss as well. Advise students who drink beverages with caffeine or
alcohol to consume equal amounts of water (in addition to the daily
recommendation of 8–10 glasses) to avoid dehydration.
Heat Acclimatization
Acclimatization is a way to train the body to tolerate a particular
environment. In the case of “hot” Yoga, it would be ideal to
acclimate to the temperature of the room to reduce risk and improve exercise
performance. Heat acclimatization involves the body adapting to heat stress by
stimulating physiological changes that will improve heat tolerance. The
physiological changes include changes in cardiovascular function and autonomic
nervous system adaptations. Ideally, the body should be exposed to heat stress
gradually, over a period of 10–14 days. The initial exposures to heat
should be without exercise, then with light exercise in the heat for 15–20
minutes. The duration and intensity of the exercise sessions should be
systematically increased, in small increments, over 10–14 days.
Summary
By increasing your understanding of how the body regulates its core
temperature, of proper hydration, and of acclimatization, you can decrease your
risk for heat illnesses. Your understanding will be applicable to exercise of
any type in the heat, including the “hot” Yoga class environment.
References
Armstrong, L. E. Performing in Extreme Environments. Champaign, Ill.:
Human Kinetics, 2000.
McArdle, W. D., F. I. Katch, and V. L. Katch. Exercise Physiology:
Energy, Nutrition and Human Performance. 4th ed. Baltimore, Md.:
Williams & Wilkins, 1996.
About the author: Leslie S. Funk holds
a B.S. in nutrition science and dietetics and was awarded her master’s
degree in exercise physiology in May 2001. She has been teaching fitness and
wellness for the past 17 years and Yoga for the past 7 years. Currently Leslie
teaches at San Jose State University and the Courtside Club in Los Gatos. She
can be contacted by email at: funkenterprises@msn.com.
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