Can you Change your Type of Muscle Fiber?

​I am writing this blog after being asked an interesting question from one of my colleagues – “Can you increase your type II muscle fibers by following a strength or hypertrophy programme?” Well, the short answer is YES. By following either of these training programmes, you are likely to increase your percentage of type II muscle fibers. 

I am aware that my ‘short’ answer is very simplistic, therefore in this blog I shall go into further details about;

• The different types of muscle fibers
• The sporting demands linked to muscle fiber types
• Is it down to genetics or recruitment?
• Can varying types of training change muscle fibers

Types of Muscle Fiber​

There are three types of muscle fiber within skeletal muscle. Type I (slow-twitch), Type IIa and Type IIb (fast-twitch). Each has different qualities in the way they perform and how quickly they fatigue. Type II muscle fibers have been further categorised as Type IIa and Type IIb as they also have different qualities and how quickly they succumb to fatigue.
 
Type I fibers are also known as slow twitch fibers and red in colour. Due to large volumes of myoglobin, oxygen and high numbers of mitochondria, this is what gives them this significant colour. These large volumes and quantities also mean they are very resistant to fatigue and are capable of producing repeated low-level contractions by producing large amounts of ATP through an aerobic metabolic cycle. Long distance level athlete(s) would have large percentages of slow-twitch as opposed to fast-twitch fibers due to these significant qualities.
 
Type IIa fibers are also known as fast oxidative fibers. These fibers contain high amounts of mitochondria and myoglobin and they are also red in colour. They produce and filter Adenosine Triphosphate (ATP) at a fast rate by the use of the aerobic and anaerobic metabolism. This produces fast, strong muscle contractions although they are more prone to fatigue than type I fibers. These fibers are used more during sustained power activities such as 400m or even 800m and are more resistant to fatigue than Type IIb.
 
Type IIb fibers are also known as fast glycolytic fibers (also known as Type IIx). They are white in colour due to a low level of myoglobin and also contain few mitochondria. They produce ATP at a slow rate but break it down very quickly by the use of the anaerobic metabolism. This produces short, fast bursts of power, but which results in a rapid fatigue rate. These fibers are recruited for very short-duration high-intensity bursts of power such as maximal and near-maximal lifts and short sprints.

Genetics or Recruitment?

Based upon the researched carried out by Professor Trappe of Ball State University, he discovered that on average, people are born with about 50% of slow-twitch and fast-twitch muscle fibers. Any varying amounts depend primarily on genetics.

Although an athlete may have a predominance of one type of muscle fiber, this does not totally dictate which sports and training programme they must follow. With the use of structured training, with high amounts of reinforcement to bring about physiological and neuromuscular adaptations, athletes can change the characteristics of their sport going against their dominant muscle fiber type.

For example, a 400m runner who would largely have higher percentages in Type IIa muscle fibers could train and refocus their efforts to suit running in a 200m and 100m races, therefore displaying having higher percentages of Type IIb fibers. I very much doubt there is any research out there to determine the exact amount of time before a change in muscle fiber type would come about (as I certainly haven’t come across any), this is largely dependant on the athlete’s genetic make-up, and of course, everyone is genetically unique.

It is common however, that at the elite level of competition, you will tend to find that those athletes who do possess the certain characteristics in their muscle fiber are competing in the sport that is most suited to their muscle fiber type dominance.

One of the main difficulties in a possible muscle fiber type change is from slow-twitch (Type I) to fast–twitch (Type II). This is because slow-twitch fibers make use of the aerobic metabolism for the production of ATP, which uses oxygen whereas, fast-twitch fibers rely on anaerobic glycolysis to produce ATP. During this process, lactic acid begins to accumulate and a condition called acidosis occurs which brings about muscular fatigue.

​Therefore, an athlete would not be simply looking to alter their type of fast-twitch fibers (oxidative to glycolytic), they would have to change energy metabolisms. For example, resistance training can turn Type IIb fibers into Type IIa due to an increase in the ability to utilise the oxidative cycle.

Training for Change

If an athlete is looking to change from fast-twitch to slow-twitch, the process is fairly self-explanatory. The athlete would need to incorporate long, slow durational runs, which are solely targeting their aerobic metabolism.
 
If an athlete wants to move from Type IIa to Type IIb (therefore becoming more powerful e.g. powerlifting), they would need to focus on heavy loads, low reps with high sets. Similarly, if an athlete wants to move from Type IIb to Type IIa (becoming faster for short periods of time e.g., basketball or rugby), they would need to focus on lighter loads (60% max RM – strength-speed), higher reps and fewer sets. 

Conclusion

​Every person is genetically unique. We all have varying combinations of different muscle fiber types. This unique combination of the varying types of muscle fibers tends to dictate which sports and activities we take part in, and ultimately best achieve in. That’s why it's easier for one person to change their body shape by ‘hitting the weights’ to display typical mesomorph body characteristics, while others find ‘smashing’ marathons just as easy.
 
It is easier to change between fast-twitch fibers and from fast-twitch to slow-twitch, than it is to go from slow-twitch to fast-twitch. Along with a person’s unchangeable genetic make-up, there will also be a change in energy metabolism. 

You're probably already going through some ideas in your head, right? Tell me what type of muscle fiber you or your athlete(s) are displaying and how you are planning on training them to best suit their genetic make up. I'd love to help out!