Heat transfer through the head, and cold-induced muscle failure

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This long-term research program focuses on human thermoregulatory, neuromuscular and metabolic responses during thermal/environmental stress, using both basic and applied approaches. 1) It is known that cold-induced decrement in physical performance correlates more with local than core cooling.  We intend to use Transcranial Magnetic Stimulation and electric stimulation to provide a detailed analysis of 'central fatigue' and test the hypothesis that hypothermia will cause central fatigue that includes decreased motor cortical excitability and increased intracortical inhibition. 2) Although much is known about substrate utilization during muscular exercise, comparative analysis between cold-induced shivering metabolism vs voluntary exercise at the same metabolic intensity is lacking. We will compare substrate utilization in iso-metabolic shivering and exercise conditions at low intensity and high intensity metabolic rates. We hypothesize that shivering will rely more on lipids vs carbohydrates for fuel compared to exercise. 3) We also intend to determine mechanisms for exaggerated core cooling effects of partial and total head submersion in cold water.  A series of studies will determine the cardiovascular and peripheral blood flow effects of short term (breath holding) and long-term (breathing) immersion of the back of the head, the face or both, in cold water.  We expect facial immersion to impose a greater effect on peripheral blood flow and core cooling. A validation study will determine if measurements of whole limb heat content can be used as an indicator of peripheral blood flow. Long term body submersions with dorsal, facial or whole head submersions will then be conducted while measuring whole limb heat content. This will allow confirmation of the blood flow effects of submersion of different parts of the head.   4) Finally, a study will test the ability to actually prevent core cooling of a cold-stressed body by comparing effects of warm water-perfusion of a vest on the torso, or of a hood on the head.