Even the limbs that are involved in exercise can alter the rate of heat loss and influence rectal temperature. Figure for example, illustrates the interactive effects that the type of exercise and water temperature had on ten test subjects who wore only a nylon swimsuit and were immersed to the first thoracic vertebrae. This graph demonstrates that leg exercise was more effective than either arm or combined arm-leg exercise in preventing the drop of rectal temperature, at three different water temperatures, and C;, and F. Physiologically, figureis interpreted to mean that arm exercise results in greater convective and conductive heat loss than leg exercise in cold water. This may result from either less fat insulation on the arms, greater blood flow per unit limb volume in arms than in legs, or a smaller cross-sectional distance from core to surface in arms than in legs less tissue to resist heat transfer. When survival is an issue, as it is when pilots are forced to land in cold ocean water, this figure suggests that leg movements should be emphasized and arm movements minimized.
Metabolic fuels also have important implications in survival situations. Muscle glycogen, the carbohydrate storage molecule consisting
Change in rectal temperature after min of water immersion. Exercise was conducted with different limbs and in three water temperatures of a chain of glucose units, is an important source of energy for human shivering thermogenesis and, therefore, maintenance of body temperature. It is logical, then, that a low muscle glycogen concentration is associated with accelerated body cooling during cold- water immersion. And, during prolonged shivering of many hours, it is possible that muscle glycogen depletion could occur, reducing the rate of heat production. In muscle with normal glycogen levels, the duration of immersion required to affect shivering via glycogen depletion is longer than
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In terms of athletic contests, it seems prudent to limit endurance events so that the duration of water immersions do not exceed the predicted safe zone for a given water temperature see figure Further, athletes and laborers should be observed closely at the beginning of a contest or work shift, when water temperatures are uncomfortable or painful. Plunging into very cold water can produce ventricular arrhythmias as well as gasping and hyperventilation; these latter two responses could result in water aspiration and drowning.
Mental performance also may suffer as a result of immersion hypothermia. For example, a lack of mental acuity and task focus obviously will affect both work and athletic performance negatively. The medical literature indicates that the signs and symptoms of mild hypothermia core body temperature of-C, -F include confusion, apathy, withdrawal, and slurred speech. Mild immersion hypothermia adversely affects complex cognitive tasks, whereas simple ones remain unaffected. Specifically, central nervous system cooling is suspected to be the critical factor, although the discomfort of cold-water exposure may cause psychological or emotional distress as well. Complete details regarding the clinical issues surrounding hypothermia appear later in this chapter.
During prolonged underwater labor, recreational diving, and endurance swimming events, it is customary to wear a wet suit for thermal insulation. However, at least two studies have shown that wearing a wet suit in moderately cold water does not prevent a decline of body temperature. For example,h of water immersion at C, F, performed by a wet-suited breath-hold diver, caused rectal temperatures to fall fromto .Cto .F. Similarly, fin swimmers and underwater scooter divers, who propelled themselves through -C -F seawater forh, experienced a continuous decline of rectal temperature, despite wearing a full wet suit. This decrease ranged fromto .C F, with the lowest Tre measurement being .C .F, suggesting that clinical hypothermia when symptoms are evident, internal body temperature below C, F may be routine during exposures lasting more than in moderately cold water. This may be due, in part, to water leaking into dry suits that reduces insulation by as much as .