Why do we shake in the cold?

There are a few reasons why we shake when we're cold. For one, our muscles tend to tense up when we're cold, and shaking is one way to release that tension. Plus, shaking helps us generate heat. When our muscles contract and relax rapidly, it causes friction, and that friction produces heat. That's why we sometimes see people shivering uncontrollably in the cold - their muscles are trying to generate as much heat as possible to warm their bodies up.
So why do our muscles tense up in the first place when we're cold? It's a reflex known as the "cold-shock response." When our skin temperature drops suddenly, our body responds by increasing our heart rate and constricting our blood vessels. This helps to preserve our core body temperature. But it also has the side effect of making our muscles tense up.
The cold-shock response is an evolutionary throwback. In our ancestors, it would have helped them survive a sudden dip in temperature, like falling into cold water. But nowadays, most of us are not in danger of hypothermia when we're out in the cold. So the cold-shock response isn't really necessary. In fact, it can be harmful, because the muscle tension can lead to injuries like sprains and strains.

There are ways to avoid the cold-shock response. One is to gradually acclimate yourself to colder temperatures. If you know you're going to be in the cold for an extended period of time, start by spending a few minutes in the cold, and then gradually increase the amount of time you spend in the cold over the course of a few days. This will give your body time to adjust and the cold-shock response should be less severe.

Another way to avoid the cold-shock response is to keep your muscles warm. This is why people often wear warm clothing or wrap themselves in blankets when they're cold. By keeping your muscles warm, you can prevent them from tensing up in the first place.

If you do find yourself in the cold and you start shaking, don't panic. It's your body's natural response to the cold, and it's not harmful. Just try to relax and let your muscles do their job. And if you can, slowly acclimate yourself to the cold so that the cold-shock response isn't so severe.