Most fitness-minded people have probably heard of fast- and slow-twitch muscle fibres. 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 fibre 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 fibre, which is used primarily for fast, explosive movements, get better at endurance, building more slow-twitch muscle fibres 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 fibre type.
What are fast- and slow-twitch muscle fibres?
According to the National Academy of Sports Medicine (NASM), our muscle fibre 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.
As NASM points out, slow-twitch muscle fibres 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 fibres,” 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 fibres are much bigger, can generate a lot more force — but they also tire a lot more easily. Type IIB muscle fibres generate the most force but tire the quickest, while Type IIA muscle fibres 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 fibres that get engaged during really high-intensity — primarily anaerobic efforts, such as the 100m sprint — while Type IIA muscle fibres 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 fibres have both endurance and power characteristics,” Stonehouse said. In this way, fast-twitch A fibres represent an intermediate between slow-twitch fibres and fast-twitch B fibres.
What type of training promotes one muscle fibre type over another?
Most people are born with an equal proportion of muscle fibre 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 fibres toward slow-twitch, while sprinting, along with explosive movements, such as powerlifting and strength and agility training, will help shift your muscle fibres 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 fibres are also recruited when the slow-twitch fibres fatigue,” Stonehouse said. “When someone runs long enough, the slow-twitch fibres begin to fatigue, causing some fast-twitch fibres 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 prioritise rest and recovery.
How much can you change your body’s muscle fibre composition?
Although we can train in a way that increases one type of muscle fibre over another, it’s not quite certain to what degree our muscle fibres 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 fibres, 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 fibre 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 fibre 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 fibre types in your body.