When you picture different athletes — marathon runners, gymnasts and Olympic weightlifters, for example — you are likely to categorise them instinctively by their height, size and build. But the differences in athletic expertise extends beyond those that meet the eye.

Physiological differences in muscle fibre distribution — fast-twitch and slow-twitch muscle fibres, to be exact—play a role in exercise performance, and may make one person more inclined to perform well in one sport, compared to another.

While your genetics play a role in muscle fibre distribution, how you train and adapt muscle fibres is an interesting part of exercise science. So regardless of your inherent genetics, here’s what you need to know about training fast-twitch and slow-twitch muscle fibres based on your favourite sport.

What are fast-twitch and slow-twitch muscle fibres?
In the broadest of strokes, fast-twitch fibres refer to those responsible for faster, more powerful movements (think sprints and plyometrics). And conversely, slow-twitch fibres refer to those responsible for slower, sustained movements and exercises (think long runs and walks). Everyone has both types of fibres and both are critical, not just for exercise, but for daily life and the different movements we all perform throughout the day.

This high-level reminder is important, because often when discussing muscle fibre types, the discussion revolves around sports. And if you’re a sprinter, having more fast-twitch fibres is “good” and having more slow-twitch fibres is “bad,” with the reverse being true if you’re a marathon runner. But considering most people spend their days walking around the office, playing soccer with their kids, or sitting at a desk, the concept of “good or bad” muscle fibres is moot, considering both play an important role in everyday life.

Here’s how they differ, in more detail:

All about slow-twitch muscle fibres
“Slow-twitch muscle fibres contain high levels of mitochondria and myoglobin, an oxygen-binding protein that can release oxygen for energy production during exercise,” Scott explains. “Because slow-twitch muscle fibres store high levels of oxygen, they are able to participate in slow force generation contractions for long periods of time without fatigue. These properties make them useful to endurance athletes, such as marathon runners, who compete for extended periods of time.”

Think for a second about the difference between cardio exercise (running longer distances, cycling, swimming, or cross-country skiing) and performing exercises that require power or speed (sprinting, heavy strength training, high-jumping, or throwing the javelin). Maintaining activity during a cardio workout requires your oxygen intake to keep up with your muscle’s energy requirements. You get into a rhythm where, even if you’re breathing at a heavier rate, you’re breathing in a controlled way that you’re able to maintain. It’s this steady flow of oxygen that’s providing the fuel source for your slow-twitch fibres — those mitochondria-filled, oxygen-loving fibres — in your working muscles.

All about fast-twitch muscle fibres
If you go from a steady jog to an all-out sprint, very quickly you realize your oxygen intake can’t keep up with your muscles’ oxygen requirements. Even as you breathe faster and deeper, your muscles start to burn and you start to tire. When oxygen isn’t sufficiently available to fuel your slow-twitch muscle fibres, your fast-twitch fibres take over.

“Fast-twitch muscle fibres contain low levels of mitochondria and myoglobin, making them rely on anaerobic metabolism for energy production. Instead of oxygen, these fibres utilizs creatine phosphate to create quick and powerful muscle contractions that are shorter in duration and more sensitive to fatigue. These properties make fast-twitch muscle fibres useful to powerlifters, sprinters, and other athletes who require short bursts of energy,” Scott says.

All of your muscle groups have both fast- and slow-twitch fibres, but distribution varies
The fact that you can move from a sustained, oxygen-fueled jog to an all-out sprint is evidence that your muscles contain both types of muscle fibres. You’re using the same general muscle groups whether jogging slowly or running really fast, but the activity you’re performing determines which fibres play a predominant role in energy production.

“The proportion of slow-twitch to fast-twitch muscle fibres in a particular muscle varies depending on the function of the muscle and the type of training in which an individual participates. For example, muscles involved in maintaining posture tend to have more slow-twitch fibres while muscles involved in weightlifting will have more fast-twitch fibres,” Scott explains.

Fast-twitch versus slow-twitch muscle fibres?

Genetics
Just as genetically-determined height can play a role in which sports or activities you may be more inclined to excel at (shorter individuals in gymnastics, taller individuals in basketball), genetically-determined muscle fibre distribution can, too. If you have more slow-twitch fibres, for example, you might excel more at a marathon than a 50-metre dash. That said, unlike height, there’s evidence that specific types of training can change muscle fibre distribution.

“Genetics generally have the most control over your fibre distribution,” says Dr Hannah Dove at Providence Saint John’s Health Center’s Performance Therapy.  A fact that’s particularly true of those who are “blank slates” when it comes to sport-specific training. In other words, what you were born with may automatically predispose you to be better at one type of activity or another. “If you want to be a marathon runner, and your body already has more slow-twitch fibres, you might be better predisposed to be good at running long distances. But, if your muscles are composed of more fast-twitch fibres, you would have the capacity to be a better sprinter,” Dove explains.

This genetic predisposition may play a fairly significant role in high-level competition in sports. “Genetic predisposition is set in the developmental years and sets the overall composition of the muscle,” says  Dr Lem Taylor, director of graduate studies in the Mayborn College of Health Sciences at the University of Mary Hardin-Baylor. “Elite endurance athletes may be born with a 72% to 28% slow- versus fast-twitch composition, whereas most people are in the middle of the normal distribution—roughly 50/50 of each.”

The natural question, then, is what type of muscle fibre distribution are you working with? Well, it’s a little tricky to know for sure. For one thing, muscle fibre distribution varies between muscles themselves, so the exact distribution you have in your calves may be different than what you have in your deep core muscles.

Second, to get an accounting of your muscle fibre distribution, a muscle biopsy is the only truly accurate way to know. This is an invasive procedure that needs to be performed in a lab or medical setting… and if you don’t like needles, it may not be a test you want to get done.

There are other, non-invasive ways to get an idea of what your muscle fibre distribution might be based on multiple rep testing from a one-rep max (that’s how much weight you can lift for just one rep), but the accuracy could be limited. “I’m not a big believer in those fitness tests,” says Taylor. “It’s all based on rep ranges, and we know repetitions to failure may be influenced by [muscle fibre type], but it’s also highly influenced by training and overall current level of muscular endurance. They don’t have much application to administer from a standpoint that would predict success in a particular athletic event.”

If you’ve ever taken a genetic test to gauge your fitness abilities, you might have gotten an assessment of your muscle fibre breakdown. Taylor explains that these tests do have some validity, but results should also be taken with a grain of salt. “The most common test would be for the ACTN3 gene and its three genotypes. Expression of a particular genotype is more connected to the fast-twitch characteristics,” he says.

So if you happen to test high for that phenotype, there’s a reasonable probability that you have a higher number of fast-twitch muscle fibres, but there’s no good test for slow-twitch fibre distribution. “One could possibly work backwards… if they didn’t express the [fast-twitch] genotype, they could assume more slow-twitch [fibres]. But again, there are caveats to that gene, and research to support that athletes in high-level competition for anaerobic sports don’t always express the most fast-twitch version of that gene.”

The type of training you do
Genetics doesn’t tell the full story, though, especially in those who have been training or participating in sports for years. In these individuals, their training’s influence plays a role in their success in their chosen sport. “There is some genetic component that may cause some individuals to have more of one type of muscle fibre compared to others, but there is evidence that muscles are able to adapt and switch muscle fibre types with specific training,” says Scott. “An individual who is training for a marathon will have more slow-twitch muscle fibres in their leg muscles compared to an individual training for short-distance sprinting, who will have more fast-twitch muscle fibres.”

In these examples, it’s a little tricky to know which came first—the training or the muscle fibre distribution? In other words, did those genetically graced with more slow-twitch fibres naturally choose to train for endurance activities and events, and those graced with more fast-twitch fibres gravitate toward more fast, powerful events? Or did they follow their passion and experience muscle fibre changes based on their training to support their activities?

So far, the jury is still out. The science is still relatively new, and there exists a high between-person variability when it comes to training response. So where one person might respond significantly to training, another might not.

“We know the easier transition in fibre types can come with endurance training,” says Taylor. “It’s probably around a 35% to 55% range for the role that genetics plays in how much this can change, as we know there is a lot of variability [between individuals]. A ‘low responder’ to exercise training in general isn’t going to have much control over [the change in muscle fibre distribution], while a ‘high responder’ to exercise training will have the opposite effect.”

So if you’re someone who trains and trains and trains but struggles to see improvements (whether in endurance or strength), you likely fall in the low responder group, and would struggle to experience changes in your muscle fibre distribution.

The takeaway here is that you shouldn’t let your possible genetic distribution affect your choice of sports. “In each sport it is important to train for the demands of that sport, which a performance coach or strength coach can help you achieve if you don’t know where to start or how to train correctly,” says Dove. And this training plays a considerable role in how your muscle fibres change and develop, and how you’ll perform within the sport.

How can you train fast-twitch versus slow-twitch muscle fibres?
When it comes to training your muscle fibre types, especially if your goal is to accrue more of one type than another for a particular sport, you need to think about the specific needs of the sport itself.

The best activities for slow-twitch muscle fibres
“In order to enhance slow-twitch fibre performance, you would want to focus on endurance training,” says Dove. This may mean long runs, swims, or bike rides that challenge your muscular endurance. Keep in mind that “long” is relative. Regardless of muscle fibre distribution, you’re also working with your current level of cardiovascular and muscular endurance. If you haven’t been training for any type of endurance event, starting a couch to 5K program has the ability to improve your overall endurance, and with time and continued training, may also help alter your muscle fibre distribution based on your personal ability to do so.

It also may mean adding endurance-specific strength training to your routine. “Doing higher reps at a low weight or resistance [builds muscular endurance],” explains Dove. “The general rep schemes for endurance are 12 to 20 reps for 3 sets with a lightweight.”

The best activities for fast-twitch muscle fibres
If you’re training for an event that requires power and speed, whether it’s a 200-metre dash or a weightlifting competition, you’ll want to balance training for that sport with other strength training and power activities that further challenge your fast-twitch muscle fibres. “This generally means lower reps, but higher weight,” Dove says. “You could also work on explosive training such as plyometrics, jumping, and sprinting.”

To focus on fast-twitch fibres in your strength sessions, focus on a rep scheme of 6 to 8 reps for 3 to 4 sets at a heavier weight, closer to your one-rep maximum. “Sprinting, jumping, and plyometric exercises are also performed fewer times due to the high-intensity, short-duration nature of the moves,” Dove says.

The bottom line on fast-twitch versus slow-twitch muscle fibres
At the end of the day, you’re likely not going to get tested with a muscle biopsy to know whether you have a greater personal distribution of fast-twitch versus slow-twitch muscle fibres in a particular muscle group. Even if you were tempted to get tested, distribution varies between muscle groups, so knowing the distribution in your quadriceps, calves, or glutes certainly is interesting information to have, and likely gives you a snapshot of overall distribution, but may not tell the full story.

The main thing to remember is that everyone has both types of fibres, and most sports and activities actually do require use of both. Think about sports like soccer, tennis, or basketball —e ach requires endurance, speed, and power to varying degrees, which means all of your muscle fibres are needed to play their given roles.

Regardless, training does make a difference in how your muscle fibres develop, so if you want to try an activity, it shouldn’t matter what you’re genetically inclined to do. Enjoyment in sport is every bit as important as performance. And if you enjoy your sport, and you train accordingly, your performance will improve as your muscle fibres adapt to support your sport and training.

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