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 running:
video here