What makes one person stronger than the other? What gives another person the ability to run for miles and miles without losing steam? Experts believe muscle fiber type plays a role in athletic giftedness. Athletes with a higher proportion of a particular muscle fiber type have more potential to excel at a given sport because the predominant muscle fiber they have makes them better suited for strength or endurance.
What are muscle fibers? They’re individual muscle cells that band together to make up the skeletal muscles in your body. These individual cells or fibers are wrapped into tight bundles to form fascicles, which, in turn, group together to form a muscle.
The muscles in your body are made up of two main types of muscle fibers. Slow-twitch muscle fibers (type 1) are those that contract slowly, can’t generate a lot of force but are resistant to fatigue. They also have a rich blood supply and lots of mitochondria to supply the muscle with energy, in the form of ATP, through pathways that require oxygen. Type 1 fibers are the ones you want a lot of when you’re running a marathon.
In contrast, fast-twitch muscle fibers (type 2) contract quickly and with large amounts of force. Unfortunately, they, unlike type 1 fibers, fatigue quickly. Fast-twitch muscle fibers are sub-divided into two classes, type 2A (fast-twitch) and type 2B (super-fast twitch). As you might expect, type 2B are the ones with the highest force generation capabilities. Type 2A fibers have some aerobic and anaerobic capacity, so they’re intermediate between type 1 and type 2B.
How Muscle Fiber Type Impacts Training
When you train, muscle fibers are recruited based on the strength of the contraction required to move a certain weight. If you’re using light weights and don’t need to generate much force, you recruit mainly slow-twitch fibers to do the job. As the force you need increases, you begin to recruit fast-twitch fibers to tap into their superior ability to generate force. Muscle fibers are recruited in sequence. You always recruit slow-twitch muscle fibers first. That’s because slow-twitch fibers are the smallest in size and have the lowest threshold for firing. If more force is needed, larger, higher threshold fast-twitch muscle fibers are recruited as back up.
What is your muscle fiber ratio? Most people are born with roughly 50% slow-twitch and 50% fast-twitch muscle fibers, but guys and gals who excel at certain sports may have a disproportionate number of one type of fiber over another. For example, long-distance runners often have a higher ratio of slow-twitch to fast-twitch fibers. This puts them at an advantage when running distances over people with lots of fast-twitch fibers. Strength athletes, power athletes, and sprinters typically have a higher ratio of fast-twitch fibers. Although fiber types don’t completely determine why some people perform well at certain sports, motivation, training and other factors like body type and V02 max are important elements, it is a factor.
How do you know what fiber type ratio you have? Having the ability to jump high, lift heavy weights or fast sprint times, is one possible indicator you have a predominance of fast-twitch fibers, but the only way to really know is to get a muscle biopsy. To do this, you’d have to go to a facility and have someone stick a needle into your muscle and pull out a few muscle fibers. Then they would look at the fibers under the microscope to see what type they are.
Another less, high-tech way is to see how many reps you can do using a weight that’s 85% of your one-rep max. If you can do 5 reps, you probably have a fairly equal quantity of fast and slow-twitch fibers. If you can eke out more than 5 reps, you’re likely slow-twitch dominant. If you do less than 5, you’re predominantly a fast-twitch fiber type. Of course, this method hardly compares with a muscle biopsy but it will give you some idea of your muscle fiber ratio.
Can You Change Your Muscle Fibers Type Through Training?
If you’ve always wanted to be a strength or power athlete, you might wonder whether, with training, you can increase the proportion of fast-twitch muscle fibers you have. Yes, you can change type 2A to type 2B fibers with training, but most experts believe you can’t change type 1 fibers to type 2, although there is some evidence this COULD be possible. But as far as we know at this point, type 1 stays type 1 and type 2 stays type 2, although 2A and 2B can interconvert based upon how you train. For example, if you cut back on weight training and begin endurance training, you will likely, over time, change some of your type 2B fibers into type 2A since you don’t need those super-fast type 2B fibers to excel at endurance activities.
At the other end of the spectrum, if you make the main focus of your workouts heavy resistance training, you’ll likely convert a portion of your type 2A fibers to type 2B fibers, the type that can generate the most force. On the other hand, if you have a predominance of type 1 fibers suited for endurance activities and start a heavy resistance training program, you’ll increase the SIZE of your type 2 muscle fibers but you won’t increase their number. What happens when you train heavy is your fast-twitch muscle fibers (type 2A and 2B) grow in size and your slow-twitch (type 1) diminish in size but the number of fibers in each class stays the same.
The Bottom Line
You’re born with a certain ratio of fast-twitch to slow-twitch fibers and it seems unlikely that you can change that ratio through training, although this hasn’t been completely ruled out. Regardless of your muscle fiber ratio, you can still change the size of the muscle fibers you have through resistance training. Fiber type shouldn’t limit you. Your body is capable of adapting and becoming more proficient both from a strength and an endurance standpoint through the appropriate training.
Muscle Fiber Types and Training (Jason R. Karp, M.S.)
Journal of Applied Physiology Published 1 November 2004 Vol. 97 no. 5, 1591-1592 DOI: 10.1152/classicessays.00010.2004.
Strength and Conditioning Research. “Does resistance-training change muscle fiber type?”
Physical Therapy November 2001 vol. 81 no. 11 1810-1816.
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