Slow Twitch and Fast Twitch

You have likely heard fast twitch and slow twitch as terms used in athletic training. But what exactly are they? And how do they shape what you should be doing with your physical preparation for football?

Lets get the general outlines of each fiber out there straight away. I will keep this stuff as brief and simple as possible, but having a general base of basic knowledge of this is good because it will help in understanding why certain training methods are better than others, and allow you to make decisions in the future. So;

Motor units & Muscle fibers

Our skeletal muscles (the muscles that you think of when you think of ‘muscles’ – the ones responsible for movement) are made up of both slow twitch and fast twitch muscle fibers. That is, every skeletal muscle has a combination of both types of muscle fibers. When you break each muscle down to individual parts, you get to a point where you have individual muscle fibers, and each individual fiber is controlled by a motor neuron or motor nerve. Refer to the picture below (I chose this simple amateurish looking drawing as it is the most simple to follow and point out the concept I am trying to get across – just google image ‘motor unit’ and you will understand what I mean.)


Each motor neuron controls many individual muscle fibers, however the number it controls will vary, sometimes as many as into the hundreds. So you have motor neurons controlling muscle fibers, and together they form what is called a motor unit. Each motor unit is composed of entirely fast twitch or slow twitch muscle fibers. So whilst a whole complete muscle will contain both slow twitch and fast twitch muscle fibers, individual motor units within the muscle are made up entirely of one or the other. This point will become important later.

Slow twitch

Slow twitch fibers (often also referred to as Type I) are efficient and fatigue resistant, and also respond better to aerobic style conditioning. They have a lower capacity to grow in size than fast twitch fibers do. They also contract slower than fast twitch fibers, and are a lot less capable of producing force. In short, these muscle fibers cope well with long-distance repetitive endurance style training or tasks.

Fast twitch

As you would expect, fast twitch fibers (also known as Type II) are the opposite of slow twitch. They have the capacity to contract far quicker, produce greater force and grow larger than slow twitch fibers. However on the downside, they are far more prone to fatigue. Fast twitch muscle fibers are also further categorised into Type IIa and Type IIb fibers, with the best way to think of the difference between the 2 being that Type IIa being closer along the continuum toward slow twitch fibers (slightly more fatigue resistant and not quite as powerful, etc, as Type IIb.) But this isn’t really that important for this discussion. Fast twitch and slow twitch, and their qualities is all you need to know.

It helps to think of 2 different types of athletes here when thinking of these concepts. I have chosen a sprinter for a fast twitch athlete, and a marathon runner for a slow twitch athlete, as this is a simple and easy comparison to make, and highly relevant when considering running in football.


Different muscles and individuals

It is important to note that different muscles have different proportions of slow twitch and fast twitch muscle fibers – that is certain muscles are more slow twitch dominant (such as Soleus in lower leg) and others are more fast twitch dominant (hamstring group), while others have a more even spread in general, such as quadriceps.

Another point here though is the obvious potential for genetic differences, where certain individuals are more fast twitch or slow twitch dominant than others. This means that they will respond differently to the identical training stimulus of another person with a different fiber composition, and be better suited to certain sports. For example, if you are very slow twitch dominant, being an Olympic level sprinter probably isn’t going to lead anywhere successful.

Ok you have had the boring (but useful to know) quick anatomy brush up. Now, how does this relate to footy training?

So what category does a footy player fit into? Activities like tackling and jumping and kicking are all certainly fast twitch in nature. As is sprinting, decelerating and changing directions. In fact, all the running you do in a contested situation in footy is fast twitch dominant. Far from just setting off running at a continuous pace once the ball is bounced, you will explode into a fast twitch dominant effort, followed by intermittent periods of rest and then more short fast twitch bursts (be it sprinting, tackling or whatever.) This is a very important thing to consider – footy is not a slow twitch sport in nature – the key moments in games when things happen are fast twitch in nature.

At this point I would like to say that we are going to talk purely about the running component at this stage. This topic is already one that can become very long and drawn out, and the other areas of tackling, contested body work, etc, have already been covered in detail in Functional Strength Training for Australian Rules Football where the strength, power and plyometric training is designed to develop fast twitch ability in these key tasks.

If footy is such a fast twitch dominant sport, why is so much of the conditioning work out there still so slow twitch dominant? (5-10km runs and these sorts of things.) The most common answer to this question will be ‘to build endurance – because endurance is important for footy.’ Indeed endurance is important for footy – there is no point being the fastest bloke on the field for the first 3 minutes, but then being absolutely useless after that because you cant sprint again because you are knackered. However, the endurance you are looking for isn’t this long duration/slow-to-moderate speed form of endurance. The type of endurance you require in a game of footy is the ability to sprint and rest and repeat this to a high level all game long. This is often why guys comment that they feel super fit from all the 10km runs they have done, yet 5 minutes into the first quarter of a trial game they feel spent – the carryover simply isn’t there. Of course game situations are something unique with all the other elements to them, but suffice to say that long duration running will not develop running ability for a game of footy – and certainly not as efficiently.

So where do fiber types fit into this discussion? Whilst you cannot convert fast twitch fibers to slow twitch fibers, or vice versa (or at lest no conclusive evidence has proven this to be physiologically possible), you can train the existing fibers to exhibit certain characteristics depending on how you train. This ties in with a concept that Mike Boyle discusses in Advances in Functional Training, in which he talks about ‘intermediate fibers.’ Intermediate fibers aren’t actually a 3rd categorisation of muscle fibers as such, but rather fibers that may be either slow or fast twitch, but which will be greatly influenced by the types of stresses or activities they are put under. In other words, if you do lots and lots of long duration running, you are progressively becoming more slow twitch dominant. If you are doing more of your conditioning via repeat fast twitch style training, you are developing your conditioning base in a way that will carry over to a game of footy, where you are required to be fit – but to exhibit this fitness in a fast way, for repeated bouts.

We are going to expand on this point and tie it in with anaerobic/aerobic training principles, as well as discuss different methods for achieving the goal of appropriate conditioning for footy season in the next couple months. This article has already dragged out a little longer than I would have liked, but we have now achieved the important aim of introducing the concepts of fast twitch and slow twitch fibers – in order to understand that there is a lot more to appropriate running training than just energy systems, but the development of certain muscle properties also comes into play. We must condition the muscle properties in a way that they will be stressed in a game of footy.

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