• Courtney Ustrzycki

Understanding Types of Muscle Fibres

Ever feel like you’re getting stronger but you don’t see your muscle really getting bigger? That’s because of the different types of muscle fibres we are using (and building) when we exercise.


There are 2 types of muscle fibres:

  • Type I (slow twitch) – these muscle fibres have high aerobic capacity and low anaerobic capacity. The contract slowly and are able to hold a steady-paced twitch for long periods of time without fatigue. These fibres are used in endurance activities like long distance running, swimming and cyclists.

  • Type II (fast twitch, and can be further categorized into Type IIa and Type IIb) – these muscle fibres have a fast contract speed; a low oxidative capacity and are ideal for short bursts of power and output. These fibres are used in activities like sprinting and powerlifting. I like to consider these fibres more like the ‘dense’ muscles in the body; thick quality muscles, but offering more of ‘leaner’ looking physique.

In short:

  • Type I: longer timing of exercise, less dense (more shape and size)

  • Type II: shorter timing of exercise, dense muscle


Depending on the exercise or the required demand, the body will recruit the necessary muscle fibres in order to perform best. When the activity is light, it will recruit Type I muscle fibres, and when it becomes too intense, it will recruit Type IIa fibres, and then, Type IIb. The body will always recruit muscle fibres in this order.


You’ll need to know this moving forward:

Motor unit: the combination of a motor neuron (what stimulates the muscles to contract) and the grouping of muscle fibres.

Muscles can contain over 100,000 muscle fibres, depending on the size.

One motor neuron can control around 2,000 muscle fibres at one time.


Heavy strength training (with low repetition work, around 1-5 reps – like powerlifting, for example) requires the body to recruit more Type II muscle fibres (fast twitch.) The body needs to push a higher amount of weight in a shorter amount of time (similar to what a sprinter would do versus a long-distance runner. NOT that powerlifting and sprinting are similar though! Just an example.)


Training with more volume (higher repetition work – like bodybuilding, for example) requires the body to recruit more Type I fibres (slow twitch.) This is because when we are doing more repetitions in a single set, the body needs to fire more motor neurons to push through.


Here’s a small overview of average rep ranges and what muscle fibres are recruited:

Strength, 1-5 reps: Type II (more IIb)

Power, 5-8 reps: Type II (more IIa)

Hypertrophy, 8-15 reps: some Type I, some Type IIa

Endurance, 15+ reps: Type I


We can force the recruitment of more Type II muscle fibres to help break through plateaus or to gain strength while maintaining the same size.


No matter what type of training we perform, there are always going to be muscle fibres recruited and therefore muscle growth. But the type of muscle fibres that grow will differ. When we push for higher reps and a lot of hypertrophy (muscle growth) in training phases, then we can see more ‘growth’ in our size. This can often be seen as ‘gains’ or feeling thicker; it’s all in the types of muscle fibres. When we can see our strength going up but we don’t necessarily see many composition changes, that’s more of our Type II fibres being developed.


Just like with body composition, it’s less important to focus on specific numbers in terms of body composition to measure how much muscle we are developing, like the scale for example. Muscle fibres are muscle fibres, they are always going to grow when under resistance and if you want to develop more muscle on your body then you have to see the scale increase (unless we are only focusing on recomposition, but that’s another conversation.) Focusing on exercise performance progress is important, and that’s why we use tools like rep ranges and RPE (rate of perceived exertion) through different training phases.


Reference: Shane Giese, 2019

© 2019 by Courtney Ustrzycki. Photos credit of Georges Schemagin (video) & Workout Magazine Mexico. All rights reserved.