For those of you who didn’t know, we have 3 main muscle fibers types in our muscles and they each serve a different function! Understanding which muscle fibers you are dominant in can help orient you to a more efficient style of training for your body.

Type I muscle fibers, also known as “slow-twitch”, are primarily found in activities such as long-distance running, where there is a high aerobic demand and lasts for a long time. Athletes who regularly perform this type of activity are usually very skinny and have a small/lean look to them, just like the ectomorph.

Because slow twitch fibers generate ATP predominantly through aerobic energy transfer. They can produce contractions that are steadier, more repetitive and at low intensity with long duration. To better perform to this type of activity and develop more type I fibres, you have to incorporate mostly aerobic and endurance exercises in your training, such as practicing to run a half-marathon (21 kilometres) or doing high rep training.

There are two types of fast-twitch fibers. The first is type IIa. These muscle fibres are larger in size and are stronger than type I fibers. People who participate in  start and stop sports such as football or basketball are good examples of athletes that are dominant in type IIa fibers.

They look bigger and more muscular than the long-distance runners, like mesomorphs compared to ectomorphs, due to their larger size of muscle fibres. These type of fibers have fast shortening speed and a good capacity for energy transfer from aerobic and anaerobic sources.

This is why in order to train these people I would make them work on their aerobic and anaerobic skills simultaneously. I would do this through interval training such as circuits and HIIT (high intensity interval training), making them lift heavy weights and then performing an aerobic activity to make sure to get the most work out of that type of muscle fiber.

Lastly, there are the type IIx or “couch potato” fibers. They are called that due to their prevalence in people who are sedentary, and thus, those people will have the more “beefy”, round-shaped strongman look, like the endomorph.

People with very dominant type IIx fibres will perform activities such as olympic lifting. This is because type IIx are extremely strong and big, yet they have almost no resistance to fatigue or injury. They are so susceptible to injury that they are often damaged beyond repair and lose themselves in the amino acid pool. In fact, in most cases sedentary people lose their type IIx fibers immediately when beginning a training program.

These fibers have the most rapid shortening velocity and the greatest anaerobic potential. Therefore, to train someone for type IIx muscle fibers I would make them train with heavy lifts in short bouts of time, because the fibers don’t contract for a long time. The workout would be comprised of many power movements, olympic lifts, short sprints, etc.

Understanding muscle fibers will allow you to train with the right type of activity. For example, if I’m training to be an olympic lifter, I won’t be doing long distance running because that wouldn’t be beneficial to what I want to accomplish. By training in the appropriate muscle fiber parameter that aligns with your goal, you will be able to get better and faster results.

If you want to get bigger, Fast-Twitch muscle fibers will be your goal since these are a lot larger, meaning you have to train heavier and more explosively. If you want to get skinner and leaner, Slow-Twitch style training will be beneficial and higher reps and cardio should be your focus.

Personally, I prefer working on Fast-Twitch fibers (mostly IIa) because it is more athletic. Every sport you play will involve quick and explosive movements and require power in those movements. Personally, I find the more athletic type 2 style is the most useful and the best looking aesthetically for both men and women, but thats up to preference.

Ioannis Gauthier