Different Runners, Different Muscles

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By Guest Author Tanya L. Brown

Guest Author Tanya L. Brown is a graduate student at the University of Colorado Anschutz Medical Campus, Program in Cell Biology, Stem Cells and Development


Runner _Adobe Stock _77706016


Types of Runners

There are two types of runners: sprinters and long distance. You’ve probably noticed that the builds of these runners are completely different. Sprinters have massive, thick muscles compared to the skinny (but strong!) legs of a long distance runner. These differences hold the key to why a marathoner can outrun a sprinter for hours but then get trounced in a short race.

Our muscles are made up of two types of fibers, slow twitch and fast twitch. While all people have a combination of these fibers in their muscles, they each use different sources of energy. Which type of running you do the most determines your energy needs and can change how much of each fiber type you have. You can think of these fibers like the dark and light meat from a chicken.


Long distance runners: “dark meat” slow twitch muscle fibers

Slow twitch fibers use more fat, protein and carbohydrates for energy. They also need more oxygen as they develop force slowly and contract for longer periods of time. These type of muscle fibers are darker in color because they have higher numbers blood vessels to increase the amount of blood and oxygen delivered to the muscle fibers. Similarly, chicken legs are generally dark meat because they are full of slow twitch fibers that they use for large portions of the day for standing and walking. In general, the slow twitch muscle fibers are slow to contract and slow to fatigue, making them extremely useful for endurance sports such as marathon running.


Sprinters: “light meat” fast twitch muscle fibers

Fast twitch fibers are able to generate more force; they contract faster and don’t need oxygen for fuel. Instead of oxygen, fast twitch fibers use sugars for quick bursts of activity. The light meat of chicken wings and breasts are primarily composed of fast twitch fibers that they use for bursts of energy such as short bursts of flight. Generally, fast twitch muscles are bigger than slow twitch muscles, which also helps explain body physique when looking at a sprinter compared to a long distance runner.


Your Muscles: dark or light meat?

On average, humans have about 50% slow twitch and 50% fast twitch muscle fibers. However, marathoners will be more “dark meat” with an increase in slow twitch muscles while sprinters will have more of a “light meat” composition full of fast twitch muscles. Next time you’re out for a long run or extended hike in shorts, you can be the judge of if you have “chicken legs!” Then you can smile and remember that those slow twitch “dark meat” muscles will take you farther than you realized was possible!

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