An Introduction to Muscle Fibres and How to Train Them

Skeletal muscle in humans, such as those that we use to exercise, are a mosaic of different types of muscle fibre with many diverse structural properties and functional capabilities. So, let’s start with a quick anatomy lesson (the most important bit). I’ll try to keep it all as brief as possible.

The three types of muscle fibre

The muscular mosaics are made up for three muscle fibre types, the first of which is called our ‘Type 1’ or ‘slow-twitch’ type. These type of muscle fibres are slow in force production and have a high oxidative profile, which means that they have high levels of mitochondria, a high capillary supply and high levels of oxidative enzymes.

The second of these is termed ‘Type 2a’ and they have similar profiles to the Type 1 fibres however, they have the ability to contract much quicker – these are often the forgotten muscle fibre out of the three as they float in between the two and which have potentially more impact on our performance. The third, ‘Type 2b’, are the fastest contracting fibres and have what we call a glycolytic metabolic profile, which means that they have low levels of mitochondria present, poor capillary supply and are high in glycolytic enzymes.

The different percentages of these types of fibres within our muscles has a huge impact on how we perform as athletes, but it is also hugely useful for us all to know and understand the outlines of our muscle fibre make-up. By understanding our muscle fibre make-up we are able to capitalise on this during training and get the best out of the hard work we put in when in a gym setting.

Which type do you have more of?

From the information above, I hope that it is easy to see that the higher levels of slow-twitch muscles we have, the greater performers we will be at endurance events or lower intensity based exercise. Therefore, the opposite is true if we have high levels of the fast-twitch muscle fibre types. People with high levels of fast-twitch muscle fibres will better suit explosive exercises with very high intensities, but low durations. The questions you may be asking yourselves are, how do I know which I have more of? And, how does this relate to my training?

First of all, your genetics will have a large impact on which side of the fence you sit on, but major dominance from one particular fibre type is very rare and would be evident by the fact you may be either running world class marathon times or pushing the 10-second boundary for a 100m sprint. For the rest of this article I want to address the bulk of you that play amateur level sport or simply train in the gym for your own personal benefit, whatever that may be for.

Let’s start by talking through the well-known observations as well as my own understandings from the half decade that I have been working with amateur athletes and the general public. Females generally have a higher percentage of slow-twitch fibres than men and men generally have a higher percentage of fast-twitch fibres than women do, but this in itself does not really help us when applying it to training, so let’s look deeper.

Individual muscle group and training application

Individual muscle groups and even different areas of the same muscle groups can differ with their percentage of fast to slow-twitch fibres, this is now where it becomes training specific. Here are some examples of the major muscle groups that may help you. I have personally observed that:

  • in general, the quadriceps have a higher percentage of slow-twitch fibres;
  • the lower portion of the hamstrings (used for knee flexion) have a higher percentage of fast-twitch fibres;
  • the top portion of the hamstrings (used for hip extension) have a higher percentage of slow-twitch fibres;
  • the glutes have a good 50/50 split of each type;
  • the lats have a higher percentage of slow-twitch fibres; and
  • the pecs have a higher percentage of fast-twitch fibres.

To maximise the use of this information, I would suggest primarily training the fast-twitch dominant muscle groups within the 1-6 rep range, with either speed or maximal strength based exercises. I would further suggest training the slow-twitch dominant muscle groups with rep ranges of 7-15, with tempo’s which will maximise time under tension (“TUT”) (I have another excellent blog on this topic if you need to brush up on your knowledge of tempo based training!).

Now, this is not to say that all muscle groups should not be trained through all different types of training modalities and performance characteristics, but these are a good way to structure the majority of your training to get the most out of it consistently.

Thanks for reading.

Keep training,


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