This may not be a topic often covered by most personal trainers or by your training partner’s bro-science, but knowing your muscle fiber type and training specifically for it can lead to greater gains in lean muscle.

Muscles are made up of 2 different fiber types. While all of us have both fiber types in our bodies, the ratio of fast-twitch and slow-twitch can vary tremendously from person to person. Knowing your personal muscle fiber genetics can be invaluable when designing a training program.

In this article I am going to help you determine what muscle fiber make-up you have and explain how to train to maximize your growth potential.



Slow twitch muscle fibers are known as Type 1 or red muscle fibers. This type of muscle fiber is responsible for long-duration, low intensity activities. These include walking and endurance activities like long distance running and cycling.


Fast twitch muscle fibers are known as Type 2 or white muscle fibers. These are then divided into two groups, namely Type 2A and Type 2B. These muscle fibers are responsible for short-duration, high intensity activity. Type 2B fibers are designed for explosive, very short-duration activities like Olympic lifts and Type 2A fibers are built for short-to- moderate duration, moderate-to-high intensity activities like most weight training activities.

If you consider, for example, a marathon runner, you will generally see an individual with a slow-twitch muscle make-up. These endurance athletes can have as much as 80% or more slow-twitch muscle fibers which makes them extremely efficient in long-distance endurance activities. Now if you consider a 100m sprinter, you will see an individual who has fast-twitch muscle fiber make-up of up to 80% or more. This makes them extremely fast, strong and powerful but with a limited endurance capability.


To find which muscle fibers your muscles comprise of, you are going to need to test the repetition limits of your muscles when compared to your max strength. The first thing you need to do is determine your 1 rep max (1RM) for an isolation exercise for each specific muscle. An example to test your bicep would be to do a dumbbell curl with the maximum possible weight for which you can only perform 1 rep. The reason for the use of an isolation exercise is that we only want to test that specific muscle group. Using an exercise that makes use of other muscle group simultaneously will alter result accuracy.

Now that you have found your 1RM, take a weight that is 80% of it and do as many reps as possible. To calculate 80%, take your 1 rep max weight and times it by 0.8. Example- say you do a 1 RM dumbbell curl with 20kg, you will take 20 x 0.8 which equals 16kgs.


4 to 7 reps – you have mostly fast twitch fibers in that muscle. This means that you should be able to lift heavy but you can’t do a large number of reps because your muscle fiber type lacks endurance capability.

8-10 reps – You have roughly a 50/50 mix of slow and fast twitch muscle fibers. This is a typical muscle fiber mix and means you will have medium strength and medium endurance capabilities.

12 to 15 reps plus – You have a slow twitch muscle fiber make-up. This means that you probably won’t be as strong as your fast twitch counterparts but you will be able to perform a lot more reps.

Because each muscle’s make-up may be different, it is necessary to repeat this process for all muscle groups. By ascertaining what muscle fiber you have in each muscle, you will be able to tailor your training in order to develop them to their utmost potential.


No matter what type of muscle fiber you predominantly have, when weight training, the goal is to work as many muscle fibers as possible. The more muscle fibers you are able to affect, the greater gains and strength will be achieved.


For those of you who have predominantly slow twitch muscle fibers, in order to engage the largest number of fibers, you are going to need to train with high repetitions, higher volume and with shorter rest periods. The reason for this is that your muscles take longer to fatigue, they recover faster and they require more work to maximize growth. Because slow twitch fibers don’t have a large potential for growth, you should also include some low repetition training to exploit the fast twitch fibers you do have. To maximize muscle growth, an ideal rep range for slow twitch fibers is 12-15 reps with 4-6 sets and a rest period of 30-60 seconds.


For the fast twitch group, you will find it much easier to build muscle as fast twitch fibers have a greater potential for growth. These muscles are generally the strongest and quickest to develop. . To maximize muscle growth, an ideal rep range for fast twitch fibers is 4-8 reps with 3-4 working sets and a rest period of 60-120 seconds.


Training your muscle to recruit your fiber make-up will help you to get results faster. This is achieved because you will be specifically targeting your training according to the exact specifications of your muscles. But remember, no matter which fiber group you fall into, you are going need to train as heavy as possible without compromising on form. If you don’t go heavy, your muscles won’t have a reason to grow.

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