Does reduced heat production during mild whole body cooling override increased heat generation by pre-shivering muscle tension?

INTRODUCTION Preshivering thermogenesis (developing muscle tension) may not be noticed in static situations due to reduced metabolism under mild body cooling. This paper describes the observations of metabolic and heart rate changes under mild body cooling. METHODS Eight men participated in the study. The subjects were dressed in shorts, socks and shoes, and were seated. The air temperature (32 °C) was after 25 minutes gradually reduced to 13°C (0.2 °C/min). Rectal, finger, central skin and air temperatures were recorded. Heart rate was measured continuously. Oxygen consumption was analysed under 5 minutes at 30th and 90th minutes. RESULTS AND DISCUSSION Metabolic rates were commonly decreasing (57((+/-)8 at 30th and 55(+/-)11 W/m2 at 90th minute) during cooling. In one subject (quickest finger cooling and increased final core temperature) the metabolic rate did not reduce. His momentary AVAs reaction indicates a more complex neural input than core cooling. In one subject onset of strong shivering raised metabolism considerably. The mean heart rate of all subjects followed the major trend in metabolic rate (80(+/-)12 at 30th and 72(+/-)12 beats/min at 90th minute). CONCLUSION Increased metabolic rate under mild cooling was not observed, eventually due to the absence of strong enough thermo regulatory responses such as increased muscle tension and superficial shivering. There is a need to repeat the study involving EMG in order to separate thermoregulatory effects from postural effects. (Less)