What is a warm-up?

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A warm-up is light movement before your main workout that heats up your body and gets it ready for the exercise you are about to do.

What is a warm-up?
The 'let me loosen up first' before a workout
At the gym you often see people walk on a treadmill for a few minutes or do a few squats with an empty bar before their main workout. That is a warm-up. This post explains what a warm-up is, what changes in the body, and how it is split into parts.
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Exercise that heats up and prepares the body
A warm-up is light movement done before a main workout or competition to heat up the body and get it ready. It is also called 'preparatory exercise' or simply 'loosening up'.
Why 'warm'?
Moving sends blood to muscles and warms them
Once you start moving lightly, more blood flows to the muscles, and muscle and core temperature rise. Most of what a warm-up does is explained by this rise in temperature.
What changes in a warm muscle
Less stiff, faster signals
A warmer muscle is less stiff, and the nerve signals the brain sends to tell the muscle to push travel faster. That is why short, hard efforts got 2-5% better for every 1 degree rise in muscle temperature.
Step one: warm the whole body
Light walking or cycling raises body heat
A warm-up is often split into two steps. The first is the 'general warm-up': light exercise such as walking or an indoor bike to warm the whole body. Exercise guides give about 5-15 minutes of easy movement as an example.
Step two: a practice run
Do the coming exercise with a light weight first
The second step is the 'specific warm-up': doing the exercise you are about to do with a light weight first. For example, a few squats with a light weight before heavy squats. It wakes up the muscles that the move will use and lets the body rehearse the movement.
With a warm-up
About 8 out of 10 results got better
When studies comparing performance with and without a warm-up were pooled, the warm-up side did better in about 8 out of 10 results examined. Almost no results showed a well-done warm-up making performance worse.
How hard?
Only enough to stay fresh
Warming up also uses energy stored in the muscles. So when the warm-up was too hard or left no rest before the task, short, hard efforts actually got worse. The goal of a warm-up is to heat the body while staying fresh.
Common misconception
Is stretching alone a warm-up?
Many people treat stretching before exercise as their warm-up. But the core of a warm-up is moving to raise body heat, while stretching is a different exercise that lengthens muscles. The US National Library of Medicine's health guide also says not to stretch cold muscles, and to stretch after warming up or after exercise.
Summary
Light movement that heats up the body
A warm-up is light movement before the main workout that heats up the body and prepares it for the coming exercise. It is not the same as stretching; its core is raising body heat. Related terms: stretching, cool-down.