Thermogenic control during exercise in a cold environment.

To determine whether the voluntary contractions of exercise interfere with involuntary shivering contractions, four male subjects were each exposed to a 10 degrees C environment for 60 min of rest followed by either another rest period or 30 min of cycleergometer exercise. On different days exercise was performed at zero load, light load, and moderate load. Each experiment was performed twice, resulting in a minimum of eight experiments for each subject. Esophageal temperature (Tes), eight skin temperatures (Tsk), oxygen uptake (VO2), and the integrated electrical activity from a neck muscle (EMG) were continuously monitored. Pedaling flushed cold blood into the body core, causing Tes to fall. The rate and absolute magnitude of the decrease in Tes was proportional to the intensity of exercise. Thermoregulatory VO2 (attributable to shivering) and EMG were inversely related both to Tsk during rest, prior to any changes in Tes, and to Tes during exercise, when Tsk was constant, once shivering thresholds were surpassed. The slope of the thermoregulatory VO2-to-Tes relation was significantly suppressed by increasing exercise intensity. The slope of the EMG-to-Tes relation was similarly suppressed; since the neck muscles are not involved in the additional acitivity of exercise, we concluded that the graded inhibition of shivering during exercise was of central origin rather than from the rhythmic contractions required to sustain exercise.