Muscle Fiber Types: How They Impact Exercise Performance

Muscle fiber types play an active role when you work out. It’s the contraction of these fibers that enable your muscles to move and do work. Not all muscle fibers are the same. Some “specialize” in generating power while others are more suited to endurance exercise.

Most people have a roughly equal quantity of muscle fibers that specialize in power and fibers more suited to endurance exercise, although there are some who have a markedly higher ratio of one type to another. People with a greater ratio of one over the other may have a natural advantage when it comes to playing certain types of sports. In other words, sprinters have more muscle fibers that are suited to generating short bursts of activity while long-distance runners have a higher ratio of muscle fibers geared towards endurance exercise.

Differences in muscle fiber types partially explain why some people are gifted at one of these activities or the other, although this is only a single factor that determines exercise performance.

Muscle Fiber Types: Type 1 Muscle Fibers – Slow and Steady

Muscle fibers that are geared towards endurance are called type 1 fibers. You’ll also commonly see them referred to as slow-twitch fibers. If you were able to visualize type 1 fibers, you’d see they’re red in color. Their red coloration is explained by the large number of capillaries that feed into them and an abundance of myoglobin, a protein that stores oxygen and provides it to working muscles when they need it. Just as importantly, type 1 fibers have a large number of energy-producing organelles called mitochondria.

Mitochondria play a very important role in exercise. They’re the organelles inside cells that convert glucose and fatty acids into ATP. It’s this ATP that fuels muscle movement – but mitochondria need oxygen to keep producing ATP. Because type 1 fibers have lots of mitochondria, myoglobin and a rich blood supply to deliver oxygen to them, they’re well suited for sustained periods of exercise at a pace that’s relatively comfortable like a long run. The biggest benefit these fibers offer is resistance to fatigue. They allow you to keep exercising at a sub-maximal, steady pace for prolonged periods of time.

 Muscle Fiber Types: Type 2 Muscle Fibers – Give Me Some Power, Please

Type 2 muscle fibers look different from type 1 fibers and they have different strengths and weaknesses. If you looked at them, you could easily tell them apart because type 2 muscle fibers are white in color. That’s because they lack the rich capillary supply and myoglobin that type 1 fibers do.

Because type 2 fibers don’t have the same rich supply of oxygen, they’re not ideal for sustained periods of exercise since they fatigue easily. Their strength lies in power work. These fibers have the power to contract quickly and with great force, giving them the name fast-twitch fibers. It’s these fibers that are predominantly activated when you have to generate force quickly, when you’re lifting a heavy weight, jumping, throwing or sprinting or playing sports where there are lots of quick, powerful movements. These fibers use anaerobic metabolism to produce energy, a process that doesn’t require oxygen. Unfortunately, these fibers also “peter out” just as fast. That’s because the anaerobic pathway they use generates lots of lactic acid, hydrogen ions and other metabolites that build-up and contribute to muscle exhaustion.

How is all of this coordinated when you work out? When you begin to exercise, type 1 fibers are called into play first but if you’re doing the exercise at a moderate intensity or less type 1 fibers take over and will continue to be the predominant fiber type used as long as you don’t exceed a certain exercise intensity.

 Muscle Fiber Types: Types of Fast-Twitch Muscle Fibers

There are really two types of type 2 muscle fibers called type 2a and type 2b. Type 2a fibers have some of the characteristics of type 1 fibers because they have a greater ability to produce ATP aerobically than type 2b but less than type 1 fibers do. They’re a sort of intermediary fiber with some of the strengths of type 1 fibers and some of type 2. It’s possible for type 2a fibers to convert to type 2b fibers but less evidence that type 1 fibers can convert to type 2 muscle fibers even with training.

There is also a third type of fast-twitch fiber called 2m that can contract very quickly. We activate these fibers when we do super-fast movements like blink our eyes.

Muscle Fiber Types: What Determines How Much of Each Muscle Fiber Type You Have?

The majority of people have roughly equal quantities of fast-twitch and slow-twitch fibers although there are individuals who have significantly more of one than the other. This is primarily determined by genetics, but if you don’t train these fibers through exercise you’ll never be a great sprinter even if you have a predominance of fast-twitch fibers.

Even if you have a predominance of one fiber type and train hard, it doesn’t necessarily mean you’ll be a great athlete in a sport that takes advantage of that fiber type since other factors like V02 max for endurance exercise, neurological factors, biomechanical factors, and motivation come into play too. What makes a great athlete is a complex blend of genetics, motivation, and training.

What you can say is that successful long-distance runners and cross-country skiers are likely to have a predominance of type 1 of slow-twitch fibers while sprinters and powerlifters tend to have a higher ratio of fast-twitch to slow-twitch fibers. Some elite endurance runners have as high as 92% slow-twitch fibers. That certainly gives them an advantage when they run long distances.

The Bottom Line?

If you have a predominance of one muscle fiber type over another, you may have some advantage when it comes to performing endurance exercise or power sports, depending upon the type of fiber you have more of, but that alone won’t determine how successful you are at a sport. You may not be able to control your genetics or your fiber type but you can still focus your efforts to be the best you can be when you play sports and when you work out. That’s really what counts.

 

References:

PLoS Biol. 2004 October; 2(10): e348.

Exercise Physiology. Fifth Edition. McArdle, Katch, and Katch (2001)

Physical Therapy November 2001 vol. 81 no. 11 1810-1816.

 

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