5.4. Modulation of shivering by the other forms of thermogenesis
The amplitudes of shivering EMGs measured during cold exposure were found to be dependent on the co-existence of postprandial thermogenesis (IV) or exercise thermogenesis (V).
5.4.1. Postprandial thermogenesis (IV)
After 29–35 h of fasting, the metabolic rate in the thermoneutral zone decreased 39%. As the ambient temperature decreased, so the difference in the metabolic rate between the control and the fasting group declined, the difference at 12°C being less than half of that observed at thermoneutrality. Furthermore, the body temperature decreased approximately 2°C due to fasting.
Fasted chicks increased their shivering thermogenesis to compensate
for the decrease in postprandial thermogenesis. The amplitude of
the shivering EMG increased in the pectoralis but the change was
not significant in the m. gastrocnemius.
The 
O2-to-EMG ratio decreased due to
fasting, also indicating compensation for the decreased postprandial
heat production with shivering.
5.4.2. Exercise thermogenesis (V)
After commencement of the bipedal exercise, the amplitude
of shivering in the m. pectoralis decreased
significantly within 20 s. Exercise suppressed shivering completely at
the ambient temperature of 25°C, and partially with a quantity
of 20V at 15°C and 0°C. This indicates that exercise
substitutes for shivering just as the increase in the O2-to-EMG ratio during exercise
does. The magnitude of change in the O2-to-EMG ratio indicates that substitution
is largest at 25°C and smallest at 0°C. The physical
strain of exercise, measured as oxygen consumption, was dependent
on the ambient temperature. Between 15°C and 0°C
it increased from 72.3 to 143.7 ml·min-1·kg-1. Due to shivering suppression
and increased forced convection, hypothermia developed the faster
the colder the ambient temperature was: at a rate of 1.4, 3.3 and
5.3 °C·h-1 at
ambient temperatures of 25, 15, and 0°C, respectively.