How to Change Muscle Fiber Composition (and Why You Want It)
Most fitness enthusiasts have probably heard of fast and slow twitch muscle fibers . However, the difference can be somewhat cryptic, especially in the context of understanding how it relates to our ability to perform endurance versus sprint running, and our body’s ability to develop one versus the other, depending on our training.
The answer, as with many other aspects of fitness, is complex. It is definitely possible with exercise to increase one type of muscle fiber over another, although there are still many unknowns. Can a person with higher levels of fast-twitch muscle fibers, which are used primarily for fast, explosive movements, improve endurance by building more slow-twitch muscle fibers in the process? And if so, to what extent?
While we still don’t have all the answers, there is a lot we 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, type I, fast twitch, type IIA, and fast twitch, type IIB, sometimes referred to as type IIX. …
As the NASM points out , slow-twitch muscle fibers are small and contain a lot of mitochondria. While they don’t create much strength and contract slowly, they also don’t fatigue very quickly, which means they get used to activities that require a lot of endurance like running long distances, 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, USATF Certified Running Trainer and Director of Training for STRIDE . When you do a cardio workout that requires a lot of aerobic activity, your slow-twitch muscles are used first.
Fast twitch muscle fibers are much larger and can generate much more strength, but they also tire much more easily. Type IIB muscle fibers generate the most strength but fatigue the fastest, while type IIA muscle fibers are in the middle between types I and IIB in terms of how much energy they generate and how quickly they fatigue.
For type IIB, these are muscle fibers that are recruited during really high-intensity – mostly anaerobic efforts, such as a 100m sprint – while type IIA muscle fibers are involved in activities that require a combination of aerobic and anaerobic activity, such as running … mile or 5 km run.
“Fast twitch A fibers have both endurance and power characteristics,” Stonehouse said. Thus, fast-twitch A fibers are intermediate between slow-twitch fibers and fast-twitch B fibers.
What type of training promotes the development of one type of muscle fiber over another?
Most people are born with an equal ratio of muscle fiber types , although there is a lot of variation between people. This proportion can also vary depending on the type of training. Endurance training will help shift muscle fibers towards slower contractions, while sprinting along with explosive movements such as powerlifting and strength and agility training will help shift your muscle fibers towards more rapid contractions.
In the context of running, Stonehouse recommends longer runs at lower intensity to train slow-twitch muscles, while shorter, faster, or more intense workouts will train your fast-twitch muscles. However, there are a certain amount of crossovers.
“Fast-twitch fibers are also recruited when they are fatigued,” Stonehouse said. “When someone is running long enough, the slow twitch fibers begin to fatigue, causing some of the fast twitch fibers to engage to compensate for the slack so that the pace can be maintained.”
As with most fitness, the important part is balance and moderation. If you mainly train for endurance it would be nice to include some strength training and sprints in your routine, and if you mainly do sprints and strength training it would be a good idea to add some endurance work.
When you add high-intensity workouts, “you usually need more recovery time,” Stonehouse said. He recommends adding no more than 1-2 high-intensity workouts per week and prioritizing rest and recovery.
How much can you change the composition of the muscle fibers in your body?
While we can train in such a way that one type of muscle fiber increases over another, it is not entirely clear to what extent our muscle fibers can transition from one type to another. According to a recent review in Sports magazine, elite athletes have a higher proportion of type I or type IIB muscle fibers, the composition of which varies by sport.
However, since there have not been any long-term studies of elite athletes since the early days of their careers, it is difficult to say how much the plasticity of these muscle fibers is due to their genetics or training.
However, while we don’t fully know to what extent we can change the composition of our muscle fibers, research shows that yes, the type of workout you do can help you switch from one type to another. You may never be the next Usain Bolt or Haile Gebrselassi, but with the right training, you can change the relative proportion of muscle fiber types in your body.