Can Slow-Twitch become Fast-Twitch?

A lot of athletes have this question, often coming up, “Can I increase my type II or fast-twitch muscles?”. Athletes often look for type II as they are faster and perfect for high-intensity workouts and sprints.

What if we say that it is possible for us to replace slow-twitch with fast-twitch. The blog deals with types of muscles, their differences, and how to change the composition.

Muscle fibers

Consisting of single muscle cells, muscle fibers help us to control the physical forces of the body. They help in organized movement and together constitute the muscular system.

Types of muscle fibers

Muscle fibers are of two types mainly, slow-twitch (type I) and fast-twitch (type II). Fast-twitch fibers can be divided into two types further, type IIa and type IIb fibers.

1. Slow twitch or Type I fibers

These fibers are more efficient at using oxygen (aerobic) and have extended muscle contradictions. As a result, they generate more ATP or Adenosine Triphosphate. Moreover, as they burn less energy, they have a lesser rate of fatigue.
Therefore, slow Twitch fibers are usually helpful for long durations of exercise like marathons, cycling, and jogging.

2. Fast Twitch or Type II fibers

These muscles use anaerobic metabolism and are responsible for generating short bursts of energy. These muscles usually generate lesser ATP. As a result, they are subjected to more fatigue.
These muscles are important for sprint exercises and allow to generate huge force and speed.

The two sub-types of fast-twitch muscles are as follows:

A. Type IIa Fibers

These muscles are equally responsible for creating energy through aerobic and anaerobic metabolism. Therefore, they are a combination of both slow and fast-twitch muscles.

B. Type IIb Fibers

This type completely uses anaerobic metabolism to create energy. It has the highest rate of contractions and thus, generates the most fatigue. Type IIb fibers are helpful for 100m races and fast sprints.

How can I change my muscles from slow to fast-twitch?


Fast-twitch muscles are required mostly by athletes and powerlifters. According to Professor Trappe at Ball State University, on average, everyone has 50% of both muscles. However, it’s the training and the sport they excel in which the athlete excels.

Consequently, changing muscles would mean changing the entire metabolism from aerobic to anaerobic. However, resistance training can change from slow-twitch to fast-twitch. The process includes focusing on heavy loads, more reps, and high sets. This would lead to muscles depending on anaerobic glycolysis instead of oxygen. As a result, lactic acid is produced which brings more fatigue and lactic acid.


We all have different combinations of muscle fibers, depending on genetics and our physical factors. However, training allows us to increase or decrease the combination of muscles. But we advise expert supervision.

Read more about Bowflex and does it really help to increase mass and gain?

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