In any given group of cyclists you’ll find a wide disparity in strengths. There’ll be a guy who paperboys up every climb, but blows the pack’s doors off when a town line sprint is in sight. You’ll see a woman who yo-yos off the back when the pace gets brisk, but drops everyone like yesterday’s news after four hours on the road. One person will be dancing in a situation that has another dropping anchor. That’s because when it comes to the biology of bike riding, we’re all a little bit different, and it begins with our muscle fibres.

You’ve probably heard the old adage that there are two types of muscles, those that make you go long and slow and those that can make you go fast, but only for a short while. This is true… to a point. Like all human physiology, it’s a bit more complicated than meets the eye. Exercise physiologist and USAC certified coach Sean Burke gave us the lowdown on what’s going on beneath the Lycra when you apply power to the pedals.

Fat-Burners Fire Up for the Long Haul, Fast Twitch Fibres FTW!

Slow twitch muscle fibres (also called type 1) are very vascular, meaning they have greater blood flow, which allows faster delivery of nutrients and removal of waste products. Because of the greater blood flow, these fibres are sometimes also called red fibres. They also have many more mitochondria, which are the powerhouse of the cell. Slow twitch fibres are excellent at using stored fat (which you have in abundance) as fuel to create energy. That means they can be very fatigue resistant, so they’re the ones you use when you’re exercising for extended periods of time.

Fast twitch (or type 2) muscle fibres are a bit more complicated. They have been called “white fibres” as they have much less blood flow than slow twitch. These fibres tend to have very few mitochondria, and they are not as good at using fats, or other fuels delivered, to them via blood flow. This makes fast twitch muscles more easily fatigued, but they can create a faster, more powerful contraction when you really need them, such as in a sprint.

They Step Up as Needed, but Training Creates More Switch Hitters

Our muscles work on an “orderly recruitment of fibres.” This means that you always recruit the slow twitch fibres first, but when you need to produce more force (say in a sprint or on a steep hill), you’ll recruit the fast twitch fibres as well, kind of like the turbo boost in Mario Kart. Even when you do recruit those fast twitch fibres, you’re still using your slow twitch fibres; you just add to your output by recruiting more of the available muscle for a short period.

Seems simple enough, but there’s a twist: fast twitch fibres can be further classified into Fast Twitch A and Fast Twitch B. “Fast Twitch A fibres are a bit more like slow twitch fibres, in that they are more vascular and have more mitochondria than Fast Twitch B fibres,” says Burke. This makes them more useful for endurance exercise. Plus, it seems that, with enough endurance riding, you can actually train some of the faster fast-twitch fibres (Fast Twitch B) to be more like the slower fast twitch fibres (Fast Twitch A), increasing fatigue resistance and improving endurance performance.

That’s not to say we can expect to convert a pure sprinter into an endurance machine with the right training. Burke notes that studies about these conversions can be challenging to conduct. “There aren’t that many biopsy studies in trained athletes, and the changes that would occur are pretty small,” he says. But with enough training, you’ll experience adaptations. “If you ride four hours at endurance power output, you begin by using only a small percentage of your muscle fibres. As fatigue increases, it seems to be logical that deeper into the ride, and in a glycogen sparse state (i.e. nearly bonking) you’ll need to recruit more fibres and this could lead to conversion of those fast twitch fibres,” explains Burke.

It All Comes Down to Nature Plus Nurture

So there you have it, the rider smoking you for the town sign is recruiting a whole bunch of fast twitch B fibres, whereas the one pulling on the front after four hours is probably primarily slow twitch with some fast twitch A thrown in. The percentage of fast and slow twitch fibres you have is largely genetic, but training certainly influences their strength, power, and efficiency. “Ultimately, the muscle you train is the muscle you build,” says Burke, and the only way to get better at going fast or going long is to practice doing so. But if you find yourself much better at one than the other, keep at it. It’s always fun to play to your strengths.

This article originally appeared on bicycling.com.

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