Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to curiouskidsus@theconversation.com.

Why are some people faster than others? – Jon, age 14, Macon, Georgia

Usain Bolt, the world’s fastest human, ran a 100-meter sprint at a speed of 23.35 miles per hour (37.57 kilometers per hour).

That’s mind-blowingly fast for a human. It’s about the same speed as cruising in a car through your neighborhood or in a school zone. It might not seem that fast when you’re in the car, but for a person? Few runners in the world can even come close.

There are several reasons why some people can run very fast while others tend to run more slowly. Genetics – the traits you inherit from your parents – play a role, but so do your choices and experiences.

As pediatric exercise scientists, we create and evaluate programs that help children be healthy. The exciting news is that while you have no control over your genetics, you can train to improve your speed.

One major factor that influences your ability to run fast is the structure of your body, including how your muscles work.

The human body has more than 600 muscles that work together, allowing you to move in different directions and at various speeds. These muscles are made up of groups of fibers. There are two main types: fast twitch and slow twitch.

Muscles have different mixes of these fiber types. For example, two muscles make up the calf: One is predominantly fast twitch – that’s the gastrocnemius, used for sprinting and jumping. The other is mostly slow twitch – that’s the soleus, used for walking and jogging.

Fast-twitch muscle fibers are larger and help your body move quickly and generate significant force. Sprinters tend to have an abundance of fast-twitch muscle fibers. However, this muscle fiber type also tires quickly, which limits how long you can run at top speed to relatively short distances.

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