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Most fitness-minded people have probably heard of fast- and slow-twitch muscle fibers. However, the distinction can be somewhat mysterious, especially in the context of understanding how it relates to our ability to do endurance versus sprinting, and our body’s ability to develop one versus the other, depending on our training.
The answer, as with so many other aspects of fitness, is complicated. Increasing one muscle fiber type over another is definitely possible through training, although there are still a lot of unknowns. Can a person with a higher level of fast-twitch muscle fiber, which is used primarily for fast, explosive movements, get better at endurance, building more slow-twitch muscle fibers in the process? And if so, to what extent?
Although we still don’t have all the answers, there is a lot we do know about our ability to change our muscle fiber type.
What are fast- and slow-twitch muscle fibers?
According to the National Academy of Sports Medicine (NASM), our muscle fiber types can be divided into three subcategories: slow-twitch Type I, fast-twitch Type IIA, and fast-twitch Type IIB, which is sometimes referred to as Type IIX.
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As NASM points out, slow-twitch muscle fibers are small and contain a lot of mitochondria. Although they don’t generate a lot of force and are slow to contract, they also don’t tire very easily, which means they get used during activities that require a lot of endurance, such as distance running, swimming, or cycling.
“This is why people can run for hours at a time—because they rely on slow-twitch muscle fibers,” said Steve Stonehouse, a certified USATF running coach and the director of education for STRIDE. Whenever you do a cardio workout, one that requires a lot of aerobic activity, you are primarily engaging your slow-twitch muscles.
Fast-twitch muscle fibers are much bigger, can generate a lot more force—but they also tire a lot more easily. Type IIB muscle fibers generate the most force but tire the quickest, while Type IIA muscle fibers are in the middle of Type I and Type IIB in terms of how much power they generate and how quickly they tire out.
For Type IIB, these are the muscle fibers that get engaged during really high-intensity—primarily anaerobic efforts, such as the 100m sprint—while Type IIA muscle fibers get engaged in activities that require a mixture of aerobic and anaerobic activity, such as running a mile or a 5K race.
“Fast-twitch A fibers have both endurance and power characteristics,” Stonehouse said. In this way, fast-twitch A fibers represent an intermediate between slow-twitch fibers and fast-twitch B fibers.
What type of training promotes one muscle fiber type over another?
Most people are born with an equal proportion of muscle fiber types, although there is a high amount of variability between individuals. This proportion can also shift depending on the type of training you do. Endurance training will help shift muscle fibers toward slow-twitch, while sprinting, along with explosive movements, such as powerlifting and strength and agility training, will help shift your muscle fibers towards a higher proportion of fast-twitch.
In the context of running, Stonehouse recommends longer, lower intensity runs for training slow-twitch muscles, while shorter, faster, or higher-intensity workouts will train your fast-twitch muscles. That said, there is a certain amount of crossover.
“Fast-twitch fibers are also recruited when the slow-twitch fibers fatigue,” Stonehouse said. “When someone runs long enough, the slow-twitch fibers begin to fatigue, causing some fast-twitch fibers to be recruited to pick up the slack so the pace can be maintained.”
As with so much of fitness, the important part is balance and moderation. If you primarily do endurance training, it would be a good idea to incorporate some strength training and sprints into your routine, while if you primarily do sprints and strength training, it would be good to add in some endurance work.
When you add in high-intensity workouts, “you will typically need more time to recover,” Stonehouse said. His recommendation is to add in no more than 1-2 high-intensity workouts a week, and to prioritize rest and recovery.
How much can you change your body’s muscle fiber composition?
Although we can train in a way that increases one type of muscle fiber over another, it’s not quite certain to what degree our muscle fibers can change from one type to another. According to a recent review in the journal Sports, elite athletes have a higher proportion of either Type I or Type IIB muscle fibers, with the composition varying depending on their sport.
However, since there haven’t been any long-term studies following elite athletes from their early days throughout the development of their careers, it’s hard to know just how much of this muscle fiber plasticity is due to their genetics or their training.
However, although we don’t know the full extent to which we can change our muscle fiber composition, the research does indicate that yes, the type of training you do can help shift it from to one type to another. You may never become the next Usain Bolt or Haile Gebrselassie, but with the right approach to training, you can change the relative proportion of muscle fiber types in your body.
