Can a Muscle Fiber Type Be Changed?

"Can you increase your type II muscle fibers by following a strength or hypertrophy program?" posed an intriguing question to one of my coworkers, prompting me to write this blog. In a nutshell, the answer is YES. You are likely to raise your percentage of type II muscle fibers if you follow either of these training programs.

 

I recognize that my 'short' answer is oversimplified, so in this blog, I'll go through the following topics in further depth:

 

 

Within skeletal muscle, there are three types of muscle fiber. Type I (slow-twitch), Type IIa, and Type IIb are the three types of muscle contractions (fast-twitch). Each has distinct characteristics in terms of how they operate and how soon they tire. Type II muscle fibers are further divided into Type IIa and Type IIb, based on their characteristics and how rapidly they succumb to fatigue.

 

Slow twitch fibers, often known as Type I fibers, are red in color. This is what gives them their distinctive color: enormous amounts of myoglobin, oxygen, and high numbers of mitochondria. They can also produce repetitive low-level contractions by creating enormous amounts of ATP through an aerobic metabolic cycle, owing to their large volumes and quantities. Because of these important traits, long-distance athletes would have a higher percentage of slow-twitch fibers than fast-twitch fibers.

 

Fast oxidative fibers are another name for Type IIa fibers. These fibers are crimson in color and contain a large number of mitochondria and myoglobin. They use aerobic and anaerobic metabolism to create and filter Adenosine Triphosphate (ATP) at a rapid rate. Although type II fibers are more prone to fatigue than type I fibers, they produce quick, powerful muscular contractions. These fibers are more resistant to fatigue than Type IIb and are employed more during sustained power events like as 400m or even 800m.

 

Fast glycolytic fibers are another name for Type IIb fibers (also known as Type IIx). Due to a lack of myoglobin, they are whitish in color and have few mitochondria. They create ATP slowly but use anaerobic metabolism to break it down quickly. This results in short, powerful bursts, but it also leads to a high rate of exhaustion. These fibers are recruited for high-intensity bursts of power that last only a few seconds, such as maximal and near-maximal lifts and sprints.

 

Recruitment or Genetics?

 

Professor Trappe of Ball State University discovered that people are born with around half of their slow-twitch and fast-twitch muscle fibers. Any variations in quantities are mostly determined by genetics.

 

Although an athlete's muscle fiber composition may favor one kind over another, this does not completely define which activities and training programs they should pursue. Athletes can change the characteristics of their sport by going against their dominant muscle fiber type using systematic training and significant amounts of reinforcement to bring about physiological and neuromuscular adaptations.

 

A 400m runner, for example, who has a high percentage of Type IIa muscle fibers, could train and refocus their efforts to suit running in 200m and 100m races, resulting in a higher percentage of Type IIb fibers. I doubt there is any research out there to pinpoint the exact amount of time it takes for a change in muscle fiber type to occur (I haven't seen any), as this is primarily dependent on the athlete's genetic make-up, and everyone is genetically unique.

 

At the elite level of competition, however, you will often discover that those athletes who do have certain muscle fiber characteristics compete in the sport that best suits their muscle fiber type supremacy.

 

The transition from slow-twitch (Type I) to fast-twitch (Type II) muscle fibers is one of the most difficult aspects of a probable muscle fiber type change (Type II). This is because slow-twitch fibers use aerobic metabolism, which uses oxygen to produce ATP, whereas fast-twitch fibers use anaerobic glycolysis to produce ATP. Lactic acid builds up during this process, resulting in a condition known as acidosis, which causes muscle weariness.

 

As a result, an athlete would have to shift their energy metabolisms as well as their type of fast-twitch fibers (oxidative to glycolytic). Resistance exercise, for example, can transform Type IIb fibers into Type IIa fibers by improving their ability to use the oxidative cycle.

 

Train to Change

 

The process of switching from fast-twitch to slow-twitch is rather self-explanatory for an athlete. Long, slow durational runs that are purely focused on the athlete's aerobic metabolism would be required.

 

If an athlete wishes to progress from Type IIa to Type IIb (and hence become more powerful, for example in powerlifting), they should focus on heavy loads, low reps, and high sets. Similarly, if an athlete wants to transition from Type IIb to Type IIa (getting quicker for short periods of time, such as in basketball or rugby), they should concentrate on lower loads (60 percent max RM – strength-speed), higher reps, and fewer sets.

 

Conclusion 

 

Every individual is genetically diverse. We all have different combinations of muscle fiber types in our bodies. This specific blend of muscle fiber types tends to govern the sports and activities we participate in, and eventually get the best results in. That's why it's easier for some people to change their body shape by 'hitting the weights,' while others find 'smashing' marathons just as easy.

 

It's easier to switch between fast-twitch and slow-twitch fibers than it is to switch from slow-twitch to fast-twitch fibers. There will be a change in energy metabolism in addition to a person's immutable genetic make-up.