Contrasting with such situations, the premise directing the research interests of this laboratory is the possibility that emergency survival situations can arise when military personnel cannot be adequately protected from the cold by available clothing and/or shelter. The Canadian Forces recently experienced just such an event when a C130 aircraft crashed north of the magnetic North Pole in October 1991 (de Groot, 1993). Weather complicated rescue attempts until 32 hours after the crash. During this time the survivors had to deal with temperatures ranging from -20° to -60°C (-4° to -76°F) considering the windchill factor. In such situations, hypothermia is delayed in direct proportion to the capacity for, and intensity of, metabolic heat production (i.e., shivering).
By measuring the electrical activity of many muscle groups simultaneously during cold-induced shivering, it is now known that several large muscle groups are recruited and contract at relatively low intensities that are less than 20 percent of their maximum force-generating capabilities (Bell et al., 1992). Because so many muscle groups are involved in shivering, the sum total of their contractile activities can result in a four- or fivefold increase in metabolic rate and in heat production. Much of the attention of Jacobs and coworkers has been directed toward the substrates that are used by skeletal muscle to increase heat production during shivering. Until about a decade ago there was very little empirically based information available in this regard for human subjects. Therefore, some fundamental experiments were carried out in an attempt to fill this knowledge gap. (The reader is referred to Jacobs et al.  for a more detailed review of thermoregulatory thermogenesis during cold stress.)
For example, Vallerand et al. (1988) administered a clinical glucose tolerance test to subjects who were sitting in either cold air or at a comfortable temperature for 2 hours. These data were the first to show in humans that glucose is eliminated more rapidly from the circulation during cold exposure, presumably to provide more available substrate to fuel the increase in metabolic rate. It is also noteworthy that this more rapid uptake of glucose during cold exposure occurs with lower insulin levels in the cold compared to warm temperatures.
Subsequently attempts were made to quantify the rates of substrate oxidation of fat, carbohydrate, and protein in humans during cold exposure with indirect calorimetric techniques. As one might presume, the increase in metabolic rate during shivering is caused by increases in oxidation of both fat and carbohydrate, but the relative increase in the rate of substrate oxidation caused by shivering is greatest for carbohydrates (Vallerand and Jacobs, 1989). In resting subjects exposed to either cold air or cold water, carbohydrates and fat contribute approximately equally to heat production (Martineau and Jacobs, 1989a, b; Vallerand and Jacobs, 1989). From a strategic point of view, this finding seems unfortunate because the body's availability of carbohydrates is quite limited compared to the abundant fat and protein stores. Aware of the well-established positive relationship between muscle glycogen concentration and endurance exercise performance of skeletal muscle, this research group