What’s the Difference Between Fast- and Slow-Twitch Muscles?

It’s not uncommon for athletes to hear certain phrases like “You look like a distance runner,” or “You’re built for sprinting!”.

These comments don’t necessarily hold any malicious intent, but they’re based on the assumption that someone’s athletic build represents their athletic caliber.

Oftentimes, this kind of thought process is based on the observation that different kinds of athletes develop and hold muscles differently — but those within the same sport (or same subset of a sport) tend to have similar characteristics in their builds.

This led people to believe that certain musculature was more likely to develop in certain kinds of athletes as a result of their main method of training, namely between high-intensity, explosive exercise versus long-duration endurance work.

And thus began the conversation about fast versus slow-twitch muscle fibers.

What are they exactly, and do certain athletes benefit from one muscle type more than the other? If so, how do you know whether or not you have them? And if you don’t have them, how do you get them?? (Is it even possible??)

The deeper you dive into the enigma of muscle fibers, the more questions that seem to arise. And when our clients come to us with burning questions about the latest research, it’s only fair that we take the time to address what they’re asking.

But before we wade knee-deep into the nuances of how to train for specific muscle fibers, let’s review some of the basics, first.

The Muscle Complex and Its Composition

Skeletal muscles are basically big bundles of contractile tissue anchored to two points on the skeleton. When the tissue contracts or relaxes, it creates movement (or stability) by manipulating the spatial relationship between those two anchor points (either stabilizing, increasing, or decreasing the relationship in some way). 

If you want to get more specific about it — and we always do, right? —  those bundles of connective tissue can be separated out even further. Each complete muscle (like your bicep, for example), is made up of fascicles. Going one step deeper, each fascicle is a group of muscle fibers, and each muscle fiber is made up of myofibrils, which are made out of protein and are where the chemical process of contraction occurs. 

The muscle fiber level is where the fast twitch and slow twitch distinction occurs, and you can have different types of muscle fibers in one fascicle.  Depending on the type of muscle fiber, there will be differences in functionality going even deeper than the fiber itself, down to the specific chemical process of how that fiber contracts and what it requires to do so. 

Muscle fibers are queued to contract by a motor neuron, and multiple neurons are controlled by a motor unit. This is where it gets a little complicated — a motor unit (and its neurons) control many muscle fibers, and those muscle fibers aren’t all in the same fascicle. But, remember that fascicles can mix fiber types? Motor units don’t — they only serve one fiber type per motor unit.

Additionally, the number of muscle fibers in each motor unit can vary greatly depending on the function of the muscle itself. For instance, the human eye can sometimes have a ratio as low as 1:1 between fibers and neurons to allow for highly precise and controlled movement coordination. But with larger, less pinpointed parts of the body, like your quadriceps or latissimus dorsi muscles, you’ll often have significantly higher ratios of fibers to neurons. Oftentimes, these bigger muscle groups can function with hundreds of muscle fibers per one motor neuron.

Now, that may all seem overly detailed, but it comes back around later — promise. Before that, though, let’s review what the two main muscle fiber types are.

Fast-Twitch Versus Slow-Twitch Muscle Fibers

Thankfully, while the conversation about the anatomy of muscle fibers can feel incredibly complex, the fiber types themselves have quite straightforward names.

Fast-twitch muscle fibers are those that are capable of quickly generating and releasing force in a short period of time, resulting in a rapid “twitch” of the fibers in the motor unit. In the context of your training, you typically recruit fast-twitch muscle fibers for more explosive or plyometric movements. (And yes, there are subsets to fast-twitch fibers, but more on that later.)

On the other hand, slow-twitch muscle fibers are — you guessed it — slower in speed when it comes to generating and releasing force. The total time it takes for these motor units to process force and relax again is significantly longer.

Not too complicated, right?

…Well, theoretically.

But hold onto your hats; we’re about to explain why they’re so different.

How Do Fast- and Slow-Twitch Fibers Work?

Of course, the answer couldn’t be as simple as “fast muscle fibers are fast, and the slow ones are slow.” But hey, we’re not in it for the easy answers; we’re all about the nitty gritty details.

So here it is: the rate at which a muscle fiber can “twitch” is determined by its ATP and ATPase content and production rate.

ATP (more officially known as adenosine triphosphate) is the primary molecule that carries the energy to fuel our muscle fibers; ATPase is its dedicated protein enzyme that breaks down the energy for our muscles to use during physical activity. ATPase is also directly correlated with myosin, which is another protein that aids the process of muscle contraction. 

All together, these three substances determine how quickly a muscle fiber is able to respond to forces and physical activity.

That, and one other factor… mitochondria!

A higher presence of mitochondria actually leads to slower ATP production and breakdown, but it’s a more sustainable process long-term. That’s because mitochondria utilize oxygen to produce ATP aerobically, meaning that they’re limited by the rate of incoming oxygen, but that supplies can be continually replenished as you breathe. 

Slow-twitch fibers tend to have a higher quantity of mitochondria, which means that it takes much longer for them to break down the ATP to create energy, allowing your muscle fibers to prolong its energy and power production over longer periods of time. Hence the connection between slow twitch muscle fibers and endurance events such as distance running. Similarly, most of our postural muscles have a high ratio of slow twitch fibers, allowing them to support us throughout the entire day without muscles fatiguing and giving out on us. Otherwise we would see people literally dropping like flies as their fast twitch muscle fibers ran out of the ATP necessary to function. 

Fast-twitch fibers tend to have a lower quantity of mitochondria and a higher density of glycogen which is an energy substance that’s comparable to high octane fuel — it can be utilized very quickly and can produce high levels of energy and force.  Muscles can only store a certain amount of glycogen, and it can’t be replaced fast enough to keep up with the demands of activity. In combination, though, both the ATP and glycogen can create a high burst of energy, but not at a sustainable rate for longer durations.

Phew! That was a lot to take in. In summation, the rate of energy production and utilization is what correlates slow-twitch fibers to endurance and fast-twitch fibers to explosive training.

We’ll get into more detail later about how you can leverage that information for your training. But first, let’s see what the data shows about muscle fiber types in relation to athletes and their particular sports.

Are Muscle Fibers Determined by Training?

This is one of the burning questions a lot of people have asked us.

Many studies suggest that elite level athletes often have a higher percentage of the fiber type that’s considered more advantageous to their sport. One study focused on the muscle fiber types between endurance athletes and powerlifters/weightlifters, where the endurance athletes had 40% fast-twitch fibers compared to the lifters’ 60% fast-twitch composition.

However, while research indicates that specified training can alter your muscular composition, it’s not quite as cut and dry as that.

The Two Types of Fast-Twitch Muscle Fibers

Yes, you read that right: there’s more than just the two main types of muscle fibers. There’s also tiered types within fast-twitch muscle fibers known as Type IIa and Type IIb.

Perhaps it feels tedious, but the differentiation between these two yields an important nuance — both are categorized as fast-twitch, but they have varying degrees of aerobic capacity.The greater your aerobic capacity, the longer your muscles can function without fatiguing or burning out.

Type IIa fibers have more potential for increased aerobic capacity, whereas Type IIb fibers produce the highest energy forces within the quickest window of fatigue. So, as we think about motor unit recruitment with specific types of training, we can observe how higher force motor units like Type IIb will be the primary fibers used during higher intensity activities like powerlifting, sprinting, or jumping to achieve maximal activation and power. (And the same goes inversely with slow-twitch and Type IIa fibers.)

Can I Actively Change Fiber Types?

Many athletes wonder if you can adjust your workouts to change the composition of your fiber types to best suit your training needs. And it’s a fair question to ask, considering how specific the fiber recruitment can get.

Unfortunately, most recent research indicates that it’s not physiologically possible to change one fiber type to another… It is possible, though, to shift the ratio between the different fiber types by building more of one or the other.

We’ll address some of the methods to tailor your training in the next section, but we did want to take a second to acknowledge another way your muscle fibers can adjust over time: excessive periods of rest.

Prolonged periods of inactivity or detraining has been associated with a fairly rapid decline in efficiency for our muscle fibers. This is particularly notable between Type IIa and Type IIb fibers, where you can quickly lose out on aerobic capacity if you’re unable to maintain sufficient, consistent training.

So the bottom line is that you can’t change the genetic makeup of muscle fibers, but you can potentially alter how well you can recruit said fibers through targeted training.

So, What Can I Do to Target Specific Muscle Fibers?

Since slow-twitch fibers are less likely to fatigue (and allow for longer durations of activity), they can often be honed by practicing endurance-based exercise. To target these particular muscle fibers, you can focus on lower intensity training and workouts, like running, walking, cycling or swimming.

Interestingly enough, though, you don’t have to limit yourself to exclusive methods of training to recruit specific muscle fibers (i.e., you don’t have to stick solely to endurance workouts for slow-twitch, nor do you have to stick with explosive exercise for fast-twitch).

One of the most common forms of activity that research suggests is high resistance training, like powerlifting, as this kind of exercise recruits a large majority of motor units. High resistance workloads will activate and train your fast-twitch fibers while also recruiting slow-twitch fibers for prolonged endurance!

Take this one study as an example: in a group of elite level cyclists, one group maintained endurance training, while the other paired their endurance work with supplementary resistance training. The results indicated that both groups maintained sufficient levels for VO2 max, but that the group with additional resistance training had experienced a large improvement in force development and maximum muscle strength. Upon further inspection, this improvement had also presented an increase in fiber proportions for Type IIa fibers.

So, for those of you who don’t think that resistance training can contribute to improved endurance, think twice before you dismiss the idea! While both groups of athletes had performed well, those with resistance training under their belts showcased a substantial improvement in their long-term endurance capacity.

Recruit Those Muscle Fibers!

Alright, let’s take a beat — that was a whole lot of detailed information to take in.

In the grand scheme of things, training based on muscle fiber type is still a relatively new concept in the research field. There are some fascinating theories and studies that suggest the possibility of improved performance, and honing your endurance capacity is no easy feat.

Keep in mind, though, that the research we currently have still only indicates a minor adaptation in muscle fiber types. Without further evidence, it will likely take years of training to witness any large, measurable changes in your performance.

But there’s no reason to let that stop you from still reaping the benefits, right? Even if the results aren’t immediately astounding, there’s still lots to gain from complementary resistance training; as long as you focus on staying consistent and accurate in form, you’ll still experience improved endurance in both your fast- and slow-twitch muscle fibers.

So get to training, athletes!


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  3. Andersen JL, Aagaard P. Myosin heavy chain IIX overshooting in human skeletal muscle. Muscleand Nerve 2000: 23: 1095–1104.
  4. Wilson JM, Loenneke JP, Jo E, Wilson GJ, Zourdos MC, Kim JS. The effects of endurance, strength, and power training on muscle fiber type shifting. J Strength Cond Res. 2012 Jun;26(6):1724-9. doi: 10.1519/JSC.0b013e318234eb6f. PMID: 21912291.
  5. Aagaard P, Andersen JL, Bennekou M, Larsson B, Olesen JL, Crameri R, Magnusson SP, Kjaer M. Effects of resistance training on endurance capacity and muscle fiber composition in young top-level cyclists. Scand J Med Sci Sports. 2011 Dec;21(6):e298-307. doi: 10.1111/j.1600-0838.2010.01283.x. Epub 2011 Mar 1. PMID: 21362056.

By Dr. Josh Tatsuno DPT

Dr. Tatsuno is passionate about high-level sports training and rehabilitation and is dedicated to educating his clients of proper maintenance and care for their bodies. After graduating from the Chapman University Doctorate of Physical Therapy Program, Josh explored his passion for developing and implementing vertical jump, strength and conditioning, and speed and agility programs for all levels of athletes. Josh stays active through his personal passions of backpacking or playing a variety of sports, though he also enjoys traveling, photography, and being a foodie in his spare time.

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