In ancient times humanity had a much stronger body hair than protection against adverse climatic conditions. The lush body hair acted as a warming air cushion for the entire organism. These days, we do not have this "warming fur" and the body has to manage differently. When the temperature drops in the winter and we get cold, the small muscles in the skin contract involuntarily. This reflex ensures that the hairs on our skin set up and - as before - an air cushion is formed. However, this reflex is no longer enough.
What happens in the body?
The body has different mechanisms to minimize heat loss. In addition to the contraction of the small skin muscles, the muscles of the arterial skin vessels contract and thus cause a lower blood flow to the skin. Instead, the blood increasingly flows into internal organs, spinal cord and brain, thus keeping the body core temperature constant. In addition, the body stops its sweat production, because sweat on the skin is known to cause evaporation. In addition, the muscles under the skin tighten, trying to produce heat. If all of these measures are unsuccessful, the body tries to rekindle by stronger muscle contraction.
How does this work?
More than a third of our body mass consists of muscles that enable us to move. In a cold ambient temperature, the muscles contract (contraction). But muscle contraction also means movement, and movement creates heat. Everything that the muscles do not need for energy themselves becomes body heat. Anyone who cycles for a few minutes or runs a jog will notice that this is the case. That is why we start to shiver when we freeze. And the more we freeze, the more we start to tremble. Our body tries to help itself in this way. A violent tremor is the best way to quickly raise your body temperature.