Your muscles are made up of different types of fibres, each with their own functions. Although there may be some minor overlap between muscle fibre types, you can basically break them down into three major types: Type I, Type IIA and Type IIB/IIX (1, 2a and 2b/x). But what do these names even mean? Different muscle fibres are made of slightly different proteins. The names of the muscle fibres are based on the names of the protein they are made from.
So what’s with the IIB/IIX?
Those muscle fibres were originally called Type IIB. On examining muscle fibres more closely, they realised that the protein that forms that muscle is closer to the IIX found in smaller mammals. However, many people just chose to keep the old name (Type IIB) while others are using a name that corresponds more accurately to the type of protein that exists in us (IIX). So now that you know a little bit more about the names, let’s explore each type of muscle fibre a little bit more closely.
What are Type I Muscle Fibres?
These can loosely be seen as your endurance fibres. They are known as slow-twitch muscle fibres because they contract the slowest of the 3 main fibre types. Their main purpose is to sustain activity for a long period of time. Their downfall is that they do not generate much power. They can sustain energy levels because they contain the most mitochondria (powerhouses) as well as the highest density of blood capillaries of all the muscle fibre types. They primarily use the Oxidative Metabolic Pathway.
How Do You Train Them?
Now that we know what they are, why and how do we train them? These muscle fibres shine in any kind of prolonged physical activity, whether it be for strenuous work, for competitive sport or just for the running or cycling enthusiast.
Training them depends primarily on your goal. If you work in an environment that involves a lot of heavy lifting over a long period of time, you may want to use the weightlifting approach. Focus on moderate weights for a high number of repetitions (reps). However, if you are a distance racer or hiker (or anything to that effect), LISS (Low Intensity Steady State) Cardio would be your best bet — go for long runs, rides, swims, etc.
What are Type IIX Muscle Fibres?
These are you fast-twitch muscle fibres. They are great for explosive power and strength. These fibres are going to be most useful for powerlifters, Olympic weightlifters, weightlifting enthusiasts, movers, sprinters — anyone who requires the ability to generate high amounts of strength in a very short period of time. The drawback with this is that they fatigue very quickly since they primarily utilise the Glycolytic Metabolic Pathway. They tend to be thicker and stronger than Type I fibres and can generate energy much quicker.
How Do You Train Them?
Two words: explosive exercises! Training involving plyometrics or heavy olympic lifts for a maximal or close to maximal load will help to train these fibre types. Shorter sprint distances (100m and below) are also great ways to train these muscle fibres.
What are Type IIA Muscle Fibres?
This is a little more complicated. They are often described as combination fibres or intermediate fibres. This is because they generate high levels of power in a short space of time while being more resistant to fatigue than the IIX fibres. This is because, although they primarily utilise the Glycolytic Pathway for energy, they utilise oxygen to generate the glycogen that is used. They are also large fibres, but not as large as the IIX. They are used for longer sprint distances or anything that requires power over a few minutes instead of a few seconds.
How Do You Train Them?
You want to get into the gym! This is not the only way, but it is one of the best ways to get those Type IIA muscle fibres working. Lifting a heavy but submaximal load (~70% of your 1-rep max) for moderate reps (10-12) and sets (~5). Another great way to train would be to engage in some of the longer-distance sprints (200-400m) regularly.
Can You Change Muscle Fibre Type?
This is a topic that’s up for a lot of debate. The evidence that this can happen is limited. Changing between Type IIA and Type IIX has been documented, but Type I to Type II (and vice versa) has only been suggested. Various studies and observed data were able to show the change between I and II, but the debate has not yet been settled. However, if you train a certain fibre type, it stands to reason that other muscle types will be less prominent. For example, training type II muscle fibres would result in an increase in size of the fibres which would also mean that more of your muscle mass consists of those muscle fibre types.
Due to the high specificity of muscle fibre types, it’s important to understand what these types are and what they do so that you can target your training to increase the efficiency of the fibres that you need for work, your sport or just aesthetic reasons. It’s good to note that, as mentioned above, these are the 3 main types. Because a single muscle fibre may have different types of protein in it, there are many muscle fibres that appear to be a combination of 2 of the main types. This leads to fibre types that occur in smaller quantities but are there nonetheless for a total of 7 types.