The muscles in our body contract and relax to walk and move us through our day. Even when we are not in motion, our muscles are actively working to keep us upright and steady. Surprisingly, this constant action doesn’t fatigue us like running at top speed for 30 seconds does. What is the physiological basis for why some activities exhaust us while others we don’t even register?

Muscles are made up of three types of fibers identified by how quickly they contract: slow, fast or super-fast. Besides contraction speed differences, the fibers fuel themselves differently—either through oxygen, glucose (sugar) or both. They also range in size, amount of power they produce and how quickly they get tired. Every muscle group in the body contains all three types but the proportions of each reflect the muscle’s purpose.

• Slow-contracting fibers derive their energy mainly from oxygen. They are resistant to fatigue and can contract for long periods of time. Muscles in the back contain a large number of slow fibers, which help sustain an upright posture for extended periods.

• Super-fast fibers get their energy mostly from glucose stores in the body. These fibers are larger in diameter than slow fibers and, because of their size, can generate more powerful contractions. However, super-fast fibers exhaust quickly. Muscles in the arms have more of these fibers, enabling them to produce large amounts of tension quickly, as when lifting objects.

• Fast-contracting fibers use both oxygen and glucose for energy. The size and fatigue rate of these fibers are in between the other two.

Just as fiber makeup varies between muscle groups, it also varies between individuals and can reflect the sports a person is best suited for. Marathon runners, who run for extended periods of time, have a large number of slow fibers in their quadriceps. Sprinters, on the other hand, need quick bursts of power and have a large number of fast oxygen/glucose-using fibers in theirs.

Former world champion sprinter Colin Jackson. Credit: Guy Evans/Flickr

A recent study in Journal of Applied Physiology looked at the fiber makeup of the quadricep of former world champion sprinter Colin Jackson. The investigators found that Jackson has a high number of the super-fast glucose-using fibers, which was surprising to them because other elite sprinters studied have very few. The researchers noted that animals that sprint, such as cheetahs and horses, also have a high percentage of these super-fast fibers and suggested that sprinting ability could be partly related to the number of these fibers. Jackson’s unique muscle profile “provides a scientific basis for the high level of sprinting success he achieved during his career,” the researchers stated.

Maggie Kuo, PhD, is the former Communications and Social Media Coordinator for APS. Catch more of her writing in the Careers Section of Science Magazine.

Reviewed by Scott Trappe, PhD