Buzz
Many fitness enthusiasts are familiar with the concept of fast-twitch and slow-twitch muscle fibers. Yet, the difference can be a bit unclear, especially when trying to understand how it affects our endurance vs. sprinting abilities, and how our bodies can develop one type over the other based on our training.
The answer, as with many fitness-related questions, is not simple. While it’s possible to enhance one muscle fiber type at the expense of another through training, there are still many unanswered questions. For example, can someone with more fast-twitch fibers—those used for quick, powerful movements—improve their endurance and build more slow-twitch fibers in the process? And if so, how much?
Although we don’t have all the answers, there’s still a great deal we do understand about our ability to alter our muscle fiber composition.
What exactly are fast-twitch and slow-twitch muscle fibers?
The National Academy of Sports Medicine (NASM) categorizes muscle fibers into three distinct types: slow-twitch Type I, fast-twitch Type IIA, and fast-twitch Type IIB, sometimes called Type IIX.
As NASM explains, slow-twitch muscle fibers are smaller in size and rich in mitochondria. While they generate less force and contract more slowly, they are highly resistant to fatigue. These fibers are predominantly recruited during endurance-based activities, such as running, swimming, and cycling.
Steve Stonehouse, a certified USATF running coach and the director of education at STRIDE, points out that this endurance capability is why people can run for hours. During aerobic exercises, slow-twitch fibers are the primary muscles at work.
Fast-twitch muscle fibers are larger, capable of producing significantly more force, but they fatigue much faster. Type IIB fibers generate the most power but tire quickly, while Type IIA fibers strike a balance between Type I and Type IIB in terms of force production and fatigue resistance.
Type IIB muscle fibers are activated during extremely high-intensity, primarily anaerobic efforts, like the 100m sprint, while Type IIA fibers come into play in activities that combine both aerobic and anaerobic demands, such as running a mile or participating in a 5K race.
As Stonehouse explains, “Fast-twitch A fibers exhibit both endurance and power traits.” In this sense, Type IIA fibers serve as a middle ground between slow-twitch and fast-twitch B fibers.
What type of training is most effective in developing one muscle fiber type over another?
Most individuals are born with an equal distribution of muscle fiber types, though this ratio can vary significantly from person to person. It can also change based on the kind of training you engage in. Endurance exercises promote slow-twitch fibers, while activities like sprinting, powerlifting, and strength or agility training shift muscle fibers towards a greater proportion of fast-twitch fibers.
Stonehouse suggests that for training slow-twitch muscles, longer, lower-intensity runs are ideal, while for fast-twitch muscles, shorter, more intense sessions are recommended. However, there is some overlap between the two.
“Fast-twitch fibers are also activated when slow-twitch fibers begin to tire,” Stonehouse explained. “As a person continues running, the slow-twitch fibers start to fatigue, prompting the recruitment of fast-twitch fibers to help maintain the pace.”
As with many aspects of fitness, balance and moderation are key. If your focus is primarily endurance training, it’s beneficial to incorporate strength training and sprints into your routine. Conversely, if you mainly focus on sprints and strength training, adding some endurance work would be advantageous.
When you introduce high-intensity workouts, “you will generally require more recovery time,” Stonehouse noted. His advice is to limit high-intensity sessions to 1-2 times per week and emphasize rest and recovery.
How much can you alter your body’s muscle fiber composition?
While it is possible to train in ways that promote one muscle fiber type over another, it is uncertain how much muscle fibers can truly change from one type to another. A recent review in the journal Sports notes that elite athletes tend to have a higher proportion of either Type I or Type IIB fibers, with the exact composition varying by sport.
Since there haven’t been any long-term studies tracking elite athletes from the early stages of their careers, it’s difficult to determine how much of the muscle fiber plasticity seen in them is a result of genetics versus their training routines.
Although we don't yet fully understand the extent to which muscle fiber composition can change, research suggests that the type of training you engage in can indeed influence this shift from one fiber type to another. While you may never become another Usain Bolt or Haile Gebrselassie, with the right training regimen, you can alter the relative balance of muscle fiber types in your body.
