Muscle Fiber Break Down – Fast & Slow Twitch Training Tips

Have you seen someone who runs hours every day? Now compare them to a guy who does bodybuilding. They look completely different. Both are training for their sport of choice and both are working out, so how can they look so different? There are two types of muscle fiber and depending on how you train, genetics, etc… they can have an impact on athletic performance goals.

Skeletal muscles are made up of individual muscle fibers. Not every muscle and muscle fiber is the same. There are two types of skeletal muscle fibers (fast-twitch and slow-twitch) and they each have different functions. It is important to understand what makes them different when it comes to movement and exercise programming. 

So, you go for your 100km job in the morning, your daily thing. You’ll be using more Slow-twitch muscle fibers. These fibers are fatigue-resistant and focused on sustained, smaller movements, and postural control. They are also aerobic, compared to the faster fibers we will talk about next. The slow-twitch fibers are normally referred to as type I muscles (or red fibers). They use oxygen more for fuel and that is a huge difference compared to their counterpart. But maybe you’re the guy who benches 400kg, you might not be a type I muscle guy. What are the other muscles? Those are your fast-twitch muscle fibers. They are bigger and more powerful, but for shorter durations and fatigue quickly. The fast muscles are more anaerobic, meaning they need less blood supply. Referred to as white fibers or type II more commonly. You have both types of skeletal muscles inside you. However, the ratios can differ depending on a variety of factors including muscle function, age, and training. 

Fast-twitch type II muscle fibers are also divided into Type IIb (or type IIx)  and Type IIa. Type IIb fibers produce the most force but are incredibly get tired at a super-fast rate. We are talking 3 to 15 seconds. That is not a lot of reps. The type IIa fibers are a mix of both type I and type IIx.  This is because they are able to use both aerobic and anaerobic energy systems (oxygen are carbs to keep in simple).

So now that you know what the different types of muscle fibers are, you might be wonder, are my hands type 1 or type 2. My chest, back, legs, etc… well they are all of them. Let me explain, your muscles are not comprised of one type of muscle fiber. All of your muscles are a mix of fast-twitch and slow-twitch muscle fiber types. To say you are more type or the other comes down to genetics, age, training you have done over your years, and much more. Before you start training (at a young age), you are around 50/50 of the fiber types. However, when you look at the higher-level athletes you’ll see a much different ratio. Some can be 70-80/20-30, depending on their sport.1 Age is also a big factor. As we grow older in our years, loss of muscle can happen. It is not a split of both types I and type II, uncertainly is it more of our type IIb fibers. Resistance training can help this and prevent loss of muscle. 

So we are here, now we understand the different types of muscle fibers and their roles in athletics and exercises. Try not to think of yourself as only one type. Even big strong guys need to be able to take a job or go for a long walk. We are human after all. 

  1. Powers SK, and Howley ET. (2012). Exercise Physiology: Theory and Application to Fitness and Performance, (8th Edition). New York, NY: McGraw Hill.