Summary Article Périard, Eijsvogels and Daanen –
Exercise under heat stress: thermoregulation,
hydration, performance implications, and mitigation
strategies
Introduction
Humans have the ability to maintain a stable body core temperature with
features to commensurate with heat dissipation. These features include the
multiplication of eccrine sweat glands, an elongated body form and reduced body
hair favoring convective heat loss, as well as changing from nasal to oronasal
breathing to permit greater airflow rates with less resistance and work.
Due to extreme heat exposure, there might be challenges for humans, which are
associated with the rise in core temperature and loss of body water related to
exercising in the heat.
Although it is well established that aerobic performance is impaired when
undertaken in hot relative to cool conditions, the mechanisms mediating this
impairment remain contentious and differ based on the type of exercise being
performed (i.e., incremental, constant work rate, and self-paced).
Human thermoregulation and heat balance
- Thermoregulation constitutes one aspect of homeostasis and represents
the ability of an organism to keep its body temperature within certain
boundaries in varying environmental conditions
Body temperature
- Body core temperature is typically regulated to
~36.6°C, but may deviate considerably when
exposed to extreme conditions
- Body core temperature is the main regulated
variable in thermoregulation. Core temperature is
most commonly determined in the digestive
system (e.g., oral, esophageal, gastrointestinal,
and rectal) and the head (e.g., ear and forehead).
- Resting body core temperature is dependent on
age, sex, ethnicity, ambient temperature, dew
point, time of day, and month of year.
- Skin acts as the interface with the environment,
but unlike core temperature, skin temperature is not regulated and varies
across the body in response to the thermal environment. Mean skin
temperature can be categorized as cool (<30°C), warm (30-34.9°C), and
hot (>35°C).
Behavioral and autonomic thermoregulation
- Body temperature regulation is accomplished through the parallel
processes of behavioral and autonomic thermoregulation. Behavioral
temperature regulation operates largely through conscious behavioral
adjustments
- Thermoregulatory behavior decreases the requirement for autonomic
responses, which operate through physiological processes that are
independent of conscious voluntary behavior. These responses include the
control of vasomotor (i.e., cutaneous vasodilation) and sudomotor (i.e.,
, sweating) function in the heat, along with metabolic heat production (i.e.,
shivering) and vasomotor function (i.e., cutaneous vasoconstriction) in cold
environments.
- Thermoregulation during exercise in the heat is regulated similarly as
during rest and influenced by factors such as hydration state and ambient
conditions, as well as work rate.
Heat balance
Human heat balance refers to the equilibrium between the internal rate of
metabolic heat production and rate of heat exchange to the surrounding
environment via sensible (i.e., convection, conduction and radiation) and
insensible (i.e., evaporation) pathways
S= M −W ±C ± K ± R−E(W )
M = metabolic work C ± K ± R determines rate of dry
W = external work rate heat exchange
C = rate of convection E reflects rate of evaporative heat
K = rate of conduction loss
E = rate of evaporation
Environmental parameters
Ambient temperature
During exercise, ambient temperatures higher than skin
temperature led to sensible (i.e., dry) heat gain, whereas
lower temperatures lead to heat loss. Environments with a
high ambient temperature and low humidity favor evaporative
heat loss, since sweat and moisture from mucosa (slijmvlies)
can more easily evaporate.
Humidity
Absolute humidity refers to the amount of water vapor
present in the air
High absolute humidity compromises the capacity to evaporate sweat from the
skin because the difference in water vapor (i.e., moisture) between the skin
surface and the environment is low.
,Air velocity
At human level (i.e., 1-2m above ground), air velocity during exercise is
dependent on factors such as direction of travel, wind direction and terrain. Air
displacement across the body results in convective heat exchange, depending on
the thermal gradient between the air and the skin. The displacement of air also
aids with evaporative heat loss as it removes the layer of saturated water vapor
that may stagnate across the skin
Solar radiation
The amount of solar radiation that reaches the human body depends on the
location on earth, time of day, season, and the level of skin area exposed.
The most commonly used index in sport and exercise is the wet-bulb-globe-
temperature index (WBGT)
Task-dependent parameters
Metabolic heat production
Human metabolism is the sum of resting (~65W/m2) and exercise metabolism.
The oxidation of substrates during exercise contributes significantly to increase
body core temperature as only ~20 to 25% of metabolic energy is converted to
mechanical work, with the majority released as heat
Clothing
Clothing acts as a barrier between the skin and the environment, altering heat
exchange properties in relation to environmental conditions.
As such, the insulative properties and water vapor resistance of garments worn
during exercise in the heat should be as low as possible. The water absorption
capacity of the material should be low, as sweat trapped in a garment is not
evaporates and does not provide cooling. Furthermore, the reflective properties
of a garment are important in high radiative load scenarios (i.e., direct sunlight).
It is not so much the color of the garment that is important, but the reflective
properties of the dyes used in the garment.
Personal parameters
Body surface area
Heat generated during metabolism is lost over the surface area of the body to
prevent excessive heat storage.
A large body surface area is beneficial for evaporative heat loss, as the number
of active sweat glands is proportional to surface area.
Dry heat loss is also enhanced by having a larger body surface area when
ambient temperature is lower than skin temperature. Hence, for a given thermal
environment, heat loss potential is greater in those with a large body surface
area.
Body surface area-to-mass ratio
Individual with a high body surface are-to-mass ratio experience less heat
storage during uncompensable heat expose than those with a lower ratio, due to
the larger area for dry and evaporative heat loss relative to body mass.
For heavy females it is more difficult to release body heat than for equally heavy
males.
Sex
When standardized for body surface area, metabolic heat production during
several tasks is similar between the sexes.
, Females have a higher density of activated sweat glands during moderate
exercise but sweat rate per body surface area is higher in males during light
exercise in humid heat and similar between sexes in dry heat. Females having a
higher onset threshold for sweating and better ability to suppress sweating when
the skin is wet.
There is currently no evidence that females have an inherent disadvantage in
thermoregulation when exercising in the heat compared with males of similar age
and health status.
Age
Older individuals (>60y) have a lower resting body core temperature, attenuated
cutaneous vasodilatory capacity, less effective sweat response, and decreased
thermoreceptor sensitivity compared with younger individuals.
In children, thermoregulatory capacity has traditionally been viewed to be less
effective than in adults and thermal tolerance inferior during exercise under heat
stress due to a higher body surface area-to-mass ratio, diminished sweating
capacity, lower mechanical efficiency, and lower cardiac output.
Aerobic fitness
Regular endurance exercise leading to improved aerobic fitness (i.e., VO2max)
has been shown to enhance heat loss capacity. Aerobic training activates
cutaneous vasodilatation at a lower core temperature and increases skin blood
flow for a given core temperature. The increase in skin blood flow is largely
mediated by the expansion of blood volume and greater cardiac output that
characterize the trained state. Endurance training has also been reported to
reduce the internal temperature threshold for the onset of sweating, increase
sweat rate at a given core temperature, and increase maximal sweat rate.
Core and skin temperature measurement
- Blood temperature of the pulmonary artery is considered the gold standard
as it best represents average internal human body temperature.
- Oral temperature may underestimate core temperature, making its
measurement less reliable in dynamic conditions or when core
temperature is elevated.
- Axilla temperature typically underestimates core temperature and is less
accurate compared with measurements at other body locations
- Earwax or dirt in the ear canal, inaccurate placement, and/or the influence
of environmental conditions (heat or cold) are known to reduce the
reliability of aural canal temperature.
- Esophageal temperature rapidly responds to temperature changes, making
it a preferred method for determining core temperature.
- Rectal temperature measurement is considered an accurate method for
determining core temperature, as long as the rectal thermistor is places
>10cm beyond the anal sphincter. Rectal temperature is less sensitive to
rapid changes in core temperatures, such as observed during exercise,
compared with esophageal temperature. Rectal temperature is considered
the clinical gold standard for obtaining core body temperature in patients
suspected of exertional heat stroke.
Exertional heat illness
Exercise in the heat may increase the risk for developing exertional heat illness
(EHI) as heat production often exceeds heat dissipation capacity.
