Hi Zach, good question. When you are running fast
you are using all the muscles in your body to
some extent. The muscles in your legs are
probably working the hardest, but your arms are
also pumping, your feet are moving, your abdominal
muscles are keeping you stable, and your diaphragm
is contracting as you breathe. Within each
muscle there are different types of fibers:
"fast-twitch" and "slow-twitch".
Fast-twitch fibers are the first to
start working, but quickly run out of energy. If
you continue running for several minutes,
slow-twitch fibers kick in. So when you run
fast for a short distance, you are using
fast-twitch fibers in all of your muscles.
Here is a link to a video that shows the names of
the muscles in your legs that work when you are
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