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1.7. INCREASE IN BODY TEMPERATURE AS A FACTOR,
LIMITING MUSCLE PERFORMANCE
If the core temperature of the body rises above 40 °, the ability of a person to perform physical and mental work worsens. When performing physical exercises in relatively cool temperatures, the increase in body temperature due to physical work is not so significant as to limit the results of physical exercises.
When performing physical exercises at high temperatures, the accumulation of body heat can limit achievements, cause heat strokes, loss of consciousness, impaired coordination, and even death.
If the air is dry, a person is able to tolerate exceptionally high temperatures for a certain time - 120 ° C for 10 minutes, 200 ° C for two minutes (if the humidity was close to zero). The main difficulties arise when it comes not just to being passively in conditions of high external temperatures, but to performing physical work.
Since the metabolic efficiency of the vast majority of motor tasks is close to 20% or even less, at least 80% of the free chemical energy released during physical work is converted into heat. In untrained subjects who, when performing those motor tasks, are forced to perform a lot of mechanical work, and in addition, in whom, due to low values of aerobic performance, the anaerobic energy supply mechanism is more sharply activated, the amount of heat generated during the same physical work is even greater. At high air temperature, the main way of heat transfer is perspiration and evaporation of sweat. Each liter of sweat during evaporation corresponds to the return of 2.43 kilojoules of heat. Let's assume, for example, that an athlete weighing 70 kg with a maximum oxygen consumption of 5 liters /min performs long-term physical work. Its oxygen consumption in this case, let's assume, will be 75% of the maximum, i.e. 3.75 l/min. In this case, the energy consumption will correspond to 20.9 kj/l, and if we assume that the mechanical efficiency is 20%, then the heat output will be 3.75 x 20.9 x 0.8 = 63 kj/min, or 3.75 kilojoules per hour. To achieve thermal equilibrium in this case, it is necessary that 1.5 liters of sweat per hour evaporate from the surface of the skin.
However, it may happen that even with the release of sweat up to 2 liters / hour, temperature equilibrium will not be achieved due to the fact that a significant part of the sweat will flow down the body, without evaporation.
Consider the effect of an increase in body temperature on the achievement of running physical exercises. A preliminary stay in conditions of higher temperature can improve the achievements to some extent due to the "warm-up effect".
However, with prolonged exercise, the accumulation of heat in the body leads to a decrease in athletic performance.
Usually, for example, the time of running long distances at high
the ambient temperature is worse than the achievements in similar competitions in conditions of comfortable temperatures.
When performing physical exercises in conditions of high ambient temperature, the supply of oxygen to the working muscles decreases due to the need to increase subcutaneous blood flow. Subcutaneous blood flow in some parts of the body can increase many times. For example, the blood flow to the fingers during the transition from a cold to a warm room can increase from 0.2 to 120 ml /min per 100 ml of tissue. Thus, when the ambient temperature changes, the blood flow of the fingers can change 600 times (!).
As for the maximum oxygen consumption, here the data of different authors differ significantly.
When performing physical exercises in conditions of high external temperatures, the indicators of muscle work decrease and, moreover, to a greater extent, the longer the time of thermal exposure was. This is largely due to an increase in body temperature and the associated dehydration of the body, combined with a number of other factors, such as the redistribution of blood between working muscles and skin.
The isolated definition of the above-mentioned temperature regime indicators is summarized in the indicators of the so-called effective temperature, which is based on data on a person's subjective sensations, the active temperature as some single integral indicator makes it possible to characterize the combined effect of temperature, humidity and air velocity. To offset the effective temperature, it is necessary to identify the values on the vertical scales on the left in a straight line on the given nomogram and thus find the intersection point of this line from the same grid that corresponds to the air velocity.
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