Nutritional Needs in Hot Environments Applications for Military Personnel in Field Operations (1993) / Chapter Skim
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3. Physiological Responses to Excercise in Heat
3. Physiological Responses to Excercise in Heat
Pages 55-74
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From page 55... ...
Muscular exercise increases metabolism by 5 to 15 times the resting rate to provide energy for skeletal muscle contraction. Depending on the type of exercise, 70 to 100 percent of the metabolism is released as heat and needs to be dissipated in order to maintain body heat balance.
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From page 56... ...
CORE TEMPERATURE RESPONSES TO EXERCISE During muscular exercise, core temperature initially increases rapidly and subsequently increases at a reduced rate until heat loss equals heat production, and essentially steady-state values are achieved. At the initiation of exercise, the metabolic rate increases immediately; however, the thermoregulatory effecter responses for heat dissipation respond more slowly.
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From page 57... ...
Lind (1963) showed that the magnitude of core temperature elevation during exercise is independent of the environment only within a certain range of conditions or a "prescriptive zone." Figure 3-2 presents a subject's steady-state core temperature responses during exercise performed at three metabolic intensities in a broad range of environmental conditions.
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From page 58... ...
, removes most of the intersubject variability for the core temperature elevation during exercise. METABOLISM Metabolic Rate The effects of acute heat stress on a person's ability to achieve maximal aerobic metabolic rates during exercise have been thoroughly studied.
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From page 59... ...
result in a decreased effective central blood volume and thus reduce venous return and cardiac output. As skin blood flow can reach 7 liters per minute 5.0 c E cc 4.0 3 o m o IS 3.0 IS PRE-ACCLIMATION _ POST-ACCLIMATION MODERATE r=0.76 : _ _ ~ / .~/ · / ~ .~^,~/ / · ~ HOT 20 // , 1 1 1 · J · 2.0 3.0 4.0 5.0 MAXIMAL AEROBIC POWER (me mint )
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From page 61... ...
The increase in anaerobic metabolic rate exceeded the increase of total metabolic rate during exercise at the elevated ambient temperatures. Therefore, if only the aerobic metabolic rate had been quantified, Dimri et al.
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From page 62... ...
Likewise, other investigators report that plasma lactate levels are greater during submaximal exercise in a hot as compared to a comfortable environment (Dill et al., 1930/1931; Dimri et al., 1980; Fink et al., 1975; Nadel 1983; Robinson et al., 1941~. Interestingly, the oxygen uptake response to submaximal exercise does appear to be affected by heat acclimatization (Sawka et al., 1983~.
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From page 64... ...
In addition, serum glucose concentration increased, and serum triglyceride concentration decreased during exercise in the heat, compared to the opposite responses during exercise in the cold. During exercise in the heat, the increased muscle glycogen utilization was attributed to an increased anaerobic glycolysis resulting from local muscle hypoxia, caused by a reduced muscle blood flow.
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From page 65... ...
for the reduction in lactate accumulation during exercise associated with heat acclimatization remains unidentified. Evaporative Heat Loss Figure 3-1 illustrates that when ambient temperature increases, there is a greater dependence on insensible (evaporative)
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From page 66... ...
12.0 10.0 8.0 6.0 4.0 2.0 o [] PRE-EXERCISE ~ POST-EXERCISE 1 ~ _ o O COOL HOT BEFORE ACCLIMATION _ COOL HOT AFTER ACCLIMATION FIGURE 3-5 Effects of heat acclimatization on pre- and postexercise muscle lactate concentration (mean + standard error)
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From page 67... ...
Figure 3-6 (Sawka and Pandolf, 1990) provides an approximation of hourly sweating rates and, therefore, water requirements for runners based on metabolic rate data from several laboratories.
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From page 68... ...
SKIN BLOOD FLOW AND CIRCULATORY RESPONSES Blood flow from the deep body tissues to the skin transfers heat by convection. When core and skin temperatures are low enough that sweating does not occur, raising skin blood flow brings skin temperature nearer to blood temperature, and lowering skin blood flow brings skin temperature nearer to ambient temperature.
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From page 69... ...
, percentage change in plasma volume (PV) , and forearm blood flow.
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From page 70... ...
In hot environments, the core-to-skin temperature gradient is less than in cool environments, so that skin blood flow must be relatively high to achieve sufficient heat transfer to maintain thermal balance (Rowell, 1986; Sawka and Wenger, 19881. This high skin blood flow causes pooling of blood in the compliant skin veins, especially below heart level.
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From page 71... ...
· Heat stress increases the total metabolic rate and anaerobic participation during submaximal exercise, and these increases are somewhat abated by heat acclimatization. · Exercise-heat stress reduces hepatic blood flow and increases hepatic glucose release.
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From page 72... ...
Roberts, and C.B. Wenger 1979 Circulatory regulation during exercise in different ambient temperatures.
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From page 73... ...
Conn, and F Kusumi 1966 Reductions in cardiac output, central blood volume and stroke volume with thermal stress in normal men during exercise.
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From page 74... ...
Pp. 153-197 in Human Performance Physiology and Environmental Medicine at Terrestrial Extremes, K
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