K - 31 Environmental Influences on Exercise

7/27/2019 K - 31 Environmental Influences on Exercise

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Deparment of PhysiologyFaculty of Medicine

Universitas Sumatera Utara


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During exercise in the heat, the heat loss mechanisms

compete with the active muscle for more of the limited

blood volume. Thus, neither area is adequately supplied

under extreme conditions.Though cardiac output may remain reasonably constant,

stroke volume may decline, resulting in gradual upward

drift in heart rate.


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Stroke volume increases with the acclimatization. This aids

delivery of more blood to the active muscles and skin when

necessary.

Heat acclimatization reduces the rate of muscle glycogen

use, delaying the onset of fatigue.


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Heat acclimatization requires exercise in a hot

environment, not merely exposure to heat.

The amount of heat acclimatization attaineddepends on the conditions to which you are

exposed during each session, the duration of

exposure, and your rate of internal heatproduction.


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Shivering (involuntary muscle contractions)increases metabolic heat production to helpmaintain or increase body temperature in the cold.

Nonshivering thermogenesis accomplishes thesame goal, but through stimulation of thesympathetic nervous system and by the action of

hormones such as thyroxin and the catecholamines.


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Peripheral vasoconstriction decreases the transfer of coreheat to the skin, thus decreasing heat loss to theenvironment.

Body size is an important consideration for heat loss. Bothincreased surface area and reduced subcutaneous fatfacilitate the loss of body heat to the environment. So thosewho have a small surface area-to-body mass ratio and thosewith more fat are less susceptible to hypothermia.


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Exercise triggers release of catecholamines,which increase the mobilization and use of

free fatty acid for fuel. But in the cold,vasoconstriction impairs circulation to thesubcutaneous fat tissue, so this process isattenuated.


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Wind increases heat loss by convection and conduction, so

this effect, known as wind chill, must be considered along

with air temperature during cold exposure.

Immersion in cold water tremendously increases heat loss

through conduction. Exercise generates metabolic heat to

offset some of this loss.


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When muscle is cooled, it is weakened, andfatigue occurs more rapidly.

During prolonged exercise in the cold, asenergy supplies diminish and exercise intensitydeclines, a person becomes increasinglysusceptible to hypothermia.


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Cold acclimatization results in three physiologicaladaptations: 1. increased nonshivering thermogenesis 2. a higher intermittent blood flow to hands and feet

3. improved ability to sleep in cold environments

The overall goal of these adaptations is to increaseheat production and maintain core temperature,

which will make the individual more comfortableduring cold exposure.

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K - 31 Environmental Influences on Exercise