Jamaican Olympic Athlete Usain Bolt is the fastest human. This aptly named sprinter has reached speeds above 23 miles an hour (37 kph). Bolt has the unique muscular build shared by most of the very best sprinters. All human muscles are made of a mix of slow- and fast-twitch fibers—as well as some that are undifferentiated, and will become slow- or fast-twitch depending on how we use them most often. Slow-twitch fibers are built for efficiency and use oxygen to generate energy from sugar. They’re most effective for activities sustained over a long period of time, like distance running. Fast-twitch muscle fibers are used to generate huge amounts of force, but they don’t use oxygen and as a result can’t carry us far. Training can help shape undifferentiated fibers into either slow- or fast-twitch, but for the most part the best runners were born with an imbalance of one or the other. Elite marathoners have way more slow-twitch fibers, and sprinters like Bolt have an abundance of fast-twitch ones.
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