Difference Between Fast & Slow Twitch Muscles: What it Means for Training & Performance

Muscles are a fascinating thing - aren't they? Even if you're not a physiology enthusiast like myself, it's impossible to ignore the fascinating intricacies of a muscle. What I aim to do in this post is to break down the different types of muscles - the "fast" and "slow" twitch - their differences, and what it means for performance and training. I'll try to keep it as simple as possible, but understand that when it comes to the muscle, there are so many factors that are inter-related, that it can often lead to confusion or even more questions. If you're strictly interested in how to train the different muscle fiber types, and want to skip all the science-y explanations and jargon - then jump straight to Training for Muscle Fiber Types. Otherwise, try to follow along, and by any means if you have any further questions, contact me - I can literally talk about this stuff all day!

Muscle Type
The first thing we need to understand is what muscle's consist of. 
Muscles are made up of hundreds of thousands of muscle fibers, which are composed of even smaller and smaller structures that do the majority of the "work" (i.e. contraction). These muscle fibers are then grouped into clumps called motor units. This is not entirely important, just know that YOU HAVE LOTS of muscle fibers in each muscle. What IS important to understand is that, although most muscle fibers are defined as either FAST twitch (also known as Type II), and SLOW twitch (Type I), each individual muscle is a collection of BOTH fiber types. That is to say that in any given muscle, there is a myriad of both fast and slow twitch fibers (see figure 1). Common categories of muscle fiber type that you may have seen or heard of include Type I (Slow Oxidative), Type IIa (fast oxidative), or type IIb (fast glycolytic) fibers, but in actuality each muscle contains a varying range of fiber types from "very slow" to "very fast". Regardless, some muscles (such as larger muscles like the quadriceps) have more fast twitch fibers, while some (like the muscles of the calf) are mostly slow twitch, but all muscles contain both fast and slow fibers. Make sense?

Figure 1: microscopic image of a muscle with a variety of slow twitch (darker) and fast twitch (lighter) muscle fibers. Based on the image, this specific muscle would be relatively more fast-twitch.

So what's the difference then?

Differences between Fast and Slow twitch
There are plenty of differences between fast twitch and slow twitch muscle fibers that affect its performance and ultimately how it should be trained. Yes, fast twitch muscle fibers are aptly named for their ability to contract QUICKLY, resulting in greater force production when compared to type I (slow) fibers. However, there are important factors that relate to WHY they can do this.

For example, fast twitch muscle fibers are larger (in diameter) than slow twitch fibers. This means they have more cross-sectional area, or the area straight through a muscle. The result of this is more space for contractile proteins (called Actin & Myosin) within the muscle to provide the force to CONTRACT the muscle. Actin and Myosin bind together (i.e. form cross-bridges) and physically pull the muscle every time we use them. The more of these structures present within a muscle, not only the more force it can generate, but also the quicker it can do so - hense the name "fast twitch". As muscles get bigger through training (called "hypertrophy"), the amount of cross-bridges and thus force they can (potentially) generate also increases. I say "potentially" because although muscles have the tools to be stronger, they still have to be trained to do so...which we'll get to shortly. But before we do, let's look at some implications of being "fast twitch".

Although bigger muscles are generally faster because of the fact they are larger and have more structures to make them contract at a quicker rate, there are some repercussions. 
Note: in actuality, there are many more reasons on a cellular level why fast twitch fibers are "faster" - such as the presence of more enzymes,  compounds such as creatine or glucose, and hyper-sensitivity to contract, but these are much more complex and just understand that they all work together regardless of muscle size. However, bigger isn't always better. Having bigger and thus "faster" muscles is great for generating a lot of force in a short amount of time, but the draw back is that they fatigue very quickly when compared to slow twitch because they simply don't have the "fuel" to produce energy fast enough.

Slow twitch fibers are generally smaller in diameter, and therefore have less structures and compounds within them that help them contract. However, they can maintain their contractions for a longer period of time (i.e. are "fatigue resistant") for many reasons, one of which being that they have more mitochondria present. Mitochondria are organelle's that produce energy (or ATP in the physiology world). More energy = more ability to contract for a longer period of time. Therefore, because slow-twitch fibers have more mitochondria present, they can produce more energy over a period of time to contract the muscle, and are thus termed fatigue resistant. Slow twitch fibers are more fatigue resistant for many reasons, others which include more myoglobin for carrying oxygen, lower threshold for activation among others.

Table 1: Breakdown of the different fiber types and their respective characteristics 
Muscle Fiber Type
Size (diameter)
Force Production
Type I:Slow-Twitch            (Oxidative)
Type IIa:Fast-Twitch
Moderate Speed
Type IIb:Fast-Twitch

Still with me? Let's move on to the fun stuff...

Training for Muscle Fiber Types
When it comes to training and performance, muscle fiber type can play a major role in determining how well an individual can perform at a specific task. For example, if two individuals who were equally trained were asked to perform a 24" box jump, we would expect the one with the more fast-twitch muscle fibers of the quadriceps (and hip extensors) to produce more force (for reasons discussed above), and thus out-perform their slow-twitch counterpart on the box jump. But just because the other individual (with less comparative fast-twitch fibers) can't jump with as much force, it doesn't mean they SHOULDN'T be doing box jumps. As with anything, it comes down to what the goal of the training session is, or what sport (if any) the individual plays. Regardless of the amount of fast or slow twitch fibers, if you're in a sport or training for a specific task that requires high amounts of force over a relatively quick period of time, such as in hockey, basketball, or during most combat sports then - guess what - you should be training primarily fast-twitch movements using the muscles that are required for your task or sport. I don't think it takes a physiology major to understand that if you're training for a marathon (or any long distance endurance sport), then you're emphasis should be on training slow-twitch fibers. Just remember that:

The most important factor in training your muscles is to focus on the ones that are required for your task or sport, and to do so in a way that closely mimics the sports requirements. 

To be more training-specific, if your goal is to increase your power or "explosiveness", then perform exercises that mimic this and stick to a 3-5 rep range for each exercise at about 80-100% of your 1RM, or 1-rep max (the maximum amount of weight that you can successfully lift one time). This will ensure you are training your fast-twitch muscle fibers.

If your goals are to increase your endurance or to perform in a long-distance activity, then you should focus on the 12-20 rep range for each exercise, using a weight that is about 65% of your 1RM. 

Although there are benefits of training both slow and fast twitch fibers together (stay tuned for more information on this), generally speaking, each individual has different and specific requirements and this should be taken into account when developing a training program. 

To end, let me answer a question that I'm sure you've been thinking of:

So how do I increase the amount of fast-twitch (or slow twitch) fibers that I have?

Well, not to sound like a Debbie-Downer, but you simply can't. The amount or proportion of fast to slow twitch fibers you have is primarily genetic. While we can train our muscles to be MORE fast (by following my advice above) or MORE slow and thus fatigue resistant, we just can't change a slow twitch fiber in to a fast twitch fiber or vice-versa - at least no research has shown the ability to do this. Yet, like everything in the human body, there is still so much to learn and the training is ever evolving to meet this. But don't worry, luckily for you, I am just crazy enough to keep up with it - and am more than happy and will continue to share it with all of you. Until next time!

Thanks for reading, 

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Kalan is a Combat Sports Strength & Conditioning Coach who brings an evidence-based approach with practical-based knowledge to the world of strength & conditioning. He is recognized as a Certified Strength & Conditioning Specialist (CSCS) with the National Strength & Conditioning Association (NSCA). and is currently finishing his MSc. Kinesiology degree at the University of Victoria. Through his masters thesis research, Kalan has established and implemented the Anglos Karate Athlete Fitness Test Battery which is used by both Karate BC & Karate Canada as their standard fitness test for  athletes across the country. Additionally, Kalan is an exercise physiology lab instructor at the University of Victoria, and trains individuals (including athletes) every day to help meet their fitness needs and goals. He has many years of experience both as an elite athlete and high performance coach and is knowledgeable in the many fields surrounding fitness and training for sports performance.