Do You Have More Slow Or Fast Twitch Muscle Fibers?

Picture from Bodyworksprime

I always knew I had some decent fast twitch muscle fibers in me. How do I know you may ask? I could always sprint at the end of a race. That's how I won most of the track races I took part in while competing in school competitions. My strategy was always to sit and wait off the leader's shoulder until the last 80-100 m and outkick whoever was there.

So I was very surprised when a newly published paper (Lievens et al, 2024) suggested that only 50 percent of elite coaches and sports scientists (out of > 400) actually know whether their athletes have slow-twitch or fast-twitch muscle fibers. Yet 90 percent of them believe that knowing the dominant muscle fiber type of an athlete is useful for optimizing training and performance.

Humans on average have a roughly 50-50 mix of slow and fast twitch fibers. There can be a huge variation around this average as some people can be mostly slow-twitch while others can be mostly fast-twitch. I have written in more detail about the 3 different types of muscle fibers we have. 

Here is a quick recap. Type I or slow twitch muscle fibers which are the smallest and produce the least amount of forces, but once trained can go all day long without fatiguing. They also help to hold and stabilize our posture.

Type II or fast twitch muscle fibers which are further divided into Type IIa and Type IIx (also known as IIb). These are larger muscle fibers and produce a greater and quicker force (than type I), but have less mitochondria, myoglobin and capillaries (compared to Type I) and are prone to fatigue quicker.

Type IIa (also known as intermediate muscle fibers) is a mixture of Type I and IIx fibers. They use both aerobic and anaerobic energy systems and fatigue slower than Type IIx. Proper training of Type IIa fibers will increase their ability to utilize aerobic energy, translating to greater endurance.

Type IIx are the largest muscle fibers and produces the most forces but are inefficient and fatigue quickly as it has low oxidative capacity and relies on anaerobic energy.

If you have read this far, you may already know more about muscle fibers than 50 percent of the coaches in the article above.

Studies have shown that those with more fast-twitch muscles have a faster last lap sprint in the 1500 m while those with more slow-twitch fibers generally do better in races when they are more evenly paced.

Another study (Swinnen et al, 2024) showed that those with slow-twitch fibers tend to have better running economy (use less energy at a given pace) since slow-twitch fibers uses less energy than fast-twitch fibers. 

The gold standard to know your fiber type is a muscle biopsy which means taking a small piece of your muscle for analysis. It is a really painful process plus who wants to give away hard earned muscle?

That is probably why the coaches surveyed in the Lievens et al (2024) study said they do not know their athletes' fiber type, especially if they coach middle distance, team sport athletes or recreational athletes who were not active previously. 

Coaches in the Lievens (2024) study relied heavily on how high you can jump (or how long you stay in the air). It is a decent proxy for fast twitch muscle fiber testing. Men with a vertical jump in excess of 50 cm (20 inches) and women 35 cm (14 inches) have predominantly more fast-twitch fibers.

Another study found that fast-twitchers can usually do 5-8 reps at a given load (80 max in the study), while slow-twitchers did 11-15 reps. About a third in the study group did 9-10 reps and they had an even mix of fast and slow twitch fibers. Those with more fast-twitch fibers tend to be stronger but note that they fatigue more quickly too.

This has implications for strength training since the 1-rep max calculations used to find how many reps to do at a lower weight will be different for fast versus slow twitch fiber types. Slow twitchers need to do more reps to get a comparable stimulus.

Perhaps a good estimate is how you fare over long versus short distances. Note that it will be affected by factors like how you have trained. This is definitely worth considering when you are deciding what races to compete in or how many reps to do while lifting. 

What muscle fibers you have is not destiny and does not have to determine your athletic goals. It may tell you the best way to get there.

Reference

Lievens E, Van de Casteele F, De Block F et al (02024). Estimating Muscle Fiber-Type Composition In Elite Athletes: A Survey On Current Practices And Perceived Merit. Int J Sp Physiol Perform. 19(11): 1197-1208. DOI: 10.1123/ijspp.2024-0043