This month, we ask why is it that some people seem to be effortlessly fast and explosive, while others struggle to generate the same “pop” in their movements despite their best efforts. Is it the case that some people are naturally faster than others? And if so, to what extent can we change this?

Muscle fiber types

It has been known since the late 1800s that not all muscle fibers are the same. Anatomists at the time observed that muscles from a single animal could be either redder in color and slower to contract, or paler in color and faster to contract. By the 1960s, muscle fibers were being categorized into slow-twitch (type I) or fast-twitch (type II). These muscle types function in different ways. As the name suggests, slow-twitch muscle fibers are slower to respond, but have better endurance; fast-twitch fibers are the opposite – quick to contract, and quick to fatigue. Fast-twitch fibers have since been found to be subdivided into different types; the main types were originally known as IIA fibers and IIB fibers.
For somewhat convoluted scientific reasons, the fast-twitch fibers found in humans are now usually divided into IIA and IIX fibers, whereas the even faster IIB fibers are now known to be found only in rodents and small mammals. However, some sources still refer to the fastest muscle fibers found in humans as IIB. Confused yet? You’re not alone!  

Both IIA and IIX fibers are used during short duration activities involving more strength and power. The faster you are trying to produce the force, the more you rely on your IIX fibers. They are heavily involved in activities like sprinting and Olympic weightlifting. IIA fibers, on the other hand, are still able to produce force relatively quickly, though not as fast as the IIX fibers. They are crucial for activities like powerlifting where the aim is to lift the maximum weight possible, rather than lifting a smaller weight more quickly.   
Different muscles in the human body have different proportions of the three main muscle types, with the lower body having a greater number of slow-twitch fibers specializing in endurance to handle the everyday activities like maintaining posture, standing, and walking around.

There’s also a large amount of variation in the proportion of fiber types possessed by different people, though. In endurance athletes, for example, the proportion of slow-twitch muscle fibers in certain muscle groups may be 70 to 80 percent. In sprinters, on the other hand, the proportion may be as low as 25 to 30 percent. So where do these differences come from? Is it that sprinters are drawn to that activity because they naturally have fiber types that make them good at generating speed and power, or is it their training that makes them that way?

Nature or nurture?

Scientists think that somewhere around half of the observed variance  in muscle fiber ratios in humans is due to genetic factors, with environmental factors being responsible for a similar (or slightly lower) proportion. 

We don’t have a very good idea of which particular genes are involved, yet. Scientists have looked at some potential candidates, at least in mice, but it’ll be a while before a genetic test can predict our fiber ratios.

The effect of training

Although the genetic component to the variation is clear, scientists have also found that training can alter the performance of different fiber types in at least two ways.

Firstly, muscle fibers can become larger in response to training. If you use your fast-twitch fibers more, they will become larger, and therefore stronger.

Secondly, it appears that muscles fibers can, under the right circumstances, change type. Training programs that focus on a lower volume of high-speed lifts can change IIA fibers into IIX fibers, for example. There are limits to what we can say for certain about this, though. We don’t know the extent to which it is possible to change someone’s muscle type, and we don’t know exactly which training methods are best for doing so.

Whether muscle fibers can change from slow-twitch to fast-twitch or vice versa is more controversial. Although this has been observed in some unusual circumstances (such as spinal cord injury, for example), and there is some circumstantial evidence that endurance training may increase the proportion of slower fiber types, this has not been observed to happen directly as a result of training.
To complicate matters, there is also some evidence that slow-twitch muscle fibers can actually change structure and become faster with the right kind of endurance training. This happens not by changing the fiber type, but by changes to some of the structures within the muscle fiber. In other words, not all the fibers of a particular type are the same. There are also intermediate, or mixed, fiber types that have properties in between the main three types, and it is thought that fibers may change type by gradually moving from slow to fast, rather than it happening all at once in a single step.

The verdict: Partially TRUE

If your ambition is to be a world-class sprinter, then there’s good evidence that you should choose your parents carefully (lots of relevant factors appear to have a genetic component). Sadly, no amount of training will turn slow-twitch Sammy into an Olympic 100m gold medallist. However, a fair proportion of that variance is within your control – so if you want to be faster and stronger (or conversely, if you want to have better endurance), then the kind of training you do can make a real difference. Our natural tendencies are not set in stone!

In all of this, it’s important to remember that your ratios of different muscle fiber types are just one of a large number of different factors that will affect your speed, explosiveness, and athleticism. The extent to which training to be faster and more explosive will change your muscle fiber types may still be up for debate – but either way, there’s good evidence that it will make you faster and more explosive.
Finally, it may or may not come as a consolation to those of us who always struggled to outrun the faster kids in the class, but it turns out that having a relatively high ratio of slow twitch muscle fibers may come with some health benefits as we get older.