Fast or Slow: The Muscle Fibre Test That Could Improve Your Fitness

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Helen Foster
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Did you know that we have two types of muscle fibre in our muscles – fast and slow – and that targetting the right type for the sports you love to do could improve your performance? Neither did I until I read about it the other day…

So there I was doing my monthly read of health magazines when I found it. A piece called What’s Your Superpower by Catherine de Lange in New Scientist (sorry you might have to pay for it).

In it, was a section on whether you were born to exercise, and what makes some people just naturally talented when it comes to certain sports – and within that was a fairly simple comparison test to determine whether you have predominantly fast-twitch muscle fibres (which makes you good at power sports like sprinting or weightlifting) or slow-twitch ones that make you better at endurance exercise like, erm running half marathons perhaps. I was riveted…

girl in the wall sit test position wearing bright gym kit

The Types of Muscle Fibre

Muscles power our body, and the exercise we do – and they are made up of individual fibres. Specifically two types of fibre – fast-twitch muscle fibres (which also have two types but let’s not complicate things too much) and short-twitch ones.

We all have both types of fibre in our muscles. However, if you do tend to have considerably more of one type of muscle fibre you’ll naturally be better at sports using those fibres.

It’s been found, for example, that elite marathon runners tend to have a higher percentage of short-twitch muscle fibres while elite sprinters have a greater propensity to fast-twitch ones.

But as well as naturally being born with more of one type of fibre, you can also create more of one type with the right training.

Why Fibre Type Matters

The reason that the fibre type can make a difference is that they each behave differently.

Slow-twitch fibres firstly twitch more slowly when they are stimulated (the clue is in the name!). For this reason, they tend to last longer before they run out of energy.

They also produce fewer waste products than fast-twitch fibres which also bodes well for endurance – and lastly, they are better at pulling on your fat stores for fuel. This produces a longer-lasting source of fuel than the sugar stored in your muscles – again, very helpful if you’re going to be running 10 miles or more.

Fast-twitch fibres do the opposite, moving quickly when you switch them on. They are good at producing short bursts of intense power – but, that power can’t be sustained. They are larger than slow-twitch fibres and packed tighter in the muscle but they also produce higher levels of waste- and the lactic acid build-up associated with this can make the muscles burn and tire quickly. 

So, how do you know which you have? Well, the only sure way is biopsy – which, probably isn’t appealing unless you’re Usain Bolt (who apparently has 80 % fast-twitch fibres in his legs), but there are a couple of simple tests that might give you a clue that you can do at home.

The Fast Twitch, Slow Twitch Test

Finding out if you favour either fibre is actually pretty simple and involves two simple tests…

The Wall Sit Test
This just sees you doing a simple wall squat for as long as you can.
If you don’t know what that is. Stand up straight against a wall, now walk your feet forward and squat down as if you were going to sit in a chair.

Stop when your thighs are parallel with the floor and make sure your knees and ankles are in a straight line, support your back against the wall and hold it until your thighs cry.

Anything over 30 seconds is good and probably means you have a high percentage of slow-twitch fibres

The Vertical Jump Test.

Stand side-on against the wall and reach up. Mark that point. Now, with your hand in the air jump as high as you can from a standing position. Measure that point. Now calculate the difference. Anything over 20 inches (50cm) means you probably have mostly fast-twitch fibres,

I, of course, couldn’t wait to try this out, My score….
Wall squats – 55 seconds
Reach test – a pathetic 15cm (poor).

Hmmm, so according to this, I have a higher proportion of slow-twitch fibres that are good for marathon running. However, I do seem to have missed out on whatever genetic gifts allow you to run said half marathons in less than 2 hours.

In fact my fastest ever, ever time for a half was 2.06 – I was thrilled. Then the winner of the men’s marathon, which started at the same time as the half crossed the finish line and threw up by my trainers! Thanks, you’ve just proved my time was actually hopeless – and I also now need to wash my shoes!

woman in orange top and shorts doing box jumps

How to Train Your Muscle Fibres

So should you panic and hand in your marathon entry if you get the ‘wrong’ score for your sport – well, no.

While our propensity to a certain type of fibre is determined by factors including age and genetics, and yes elite athletes get a clear advantage by being born with more fibres of the right type, the rest of us, less elite types, can train ourselves to power up the type of fibre we need to call on during our favourite sport.

The more you train in ways that stimulate each muscle fibre, the more trained the become – and it’s also theorised that some unused fibres may also convert to the type you use most with the right training.

Also, don’t worry if you scored poorly on both tests. This does not doom you to a life of couch potato-dom.

Most people, especially if you’re new to exercise or don’t train hard at one particular discipline have roughly 50:50 of each type of fibre. Not getting an overwhelming score on either test just means you’re normal, but might need to train things up a bit if you want to get better at whatever sport you’ve chosen to do.

So, if you want to train short-twitch fibres for endurance work run long slow distance (some of our running tips might help here) and do lighter weights for more repetitions – ideally 12 or more per set of reps. You want to help the fibres build endurance to help you do the same.

If you want to train fast-twitch muscle fibres for sports like short distance running or team sports like tennis or football when you need to accelerate over short distances, then do lots of sprints, intervals, HIIT training and explosive power-lifting and fast moves like kettlebell swings.

Plyometric work like jumping on boxes also helps train fast-twitch fibres.

So there you have it, the simple guide to the different types of muscle fibre and how to improve your fast-twitch or slow-twitch response. Now, I know you want to go and do those tests now don’t you…

What to Read Next

There’s also a simple home test that can predict how prone you are to shin splints? If that sounds interesting then you’ll find our piece on the simple shin-splint test here.

Also, did you know that you can workout for your blood type. It’s an intriguing theory and we look into it more in our longer post on exercising for your blood type.

Also, if you do need to increase the amount of weight you lift, have a look at how to pick the best weight for you here.

Who is The Wellness Nerd?

My name is Helen Foster, and I’m a health journalist and wellness author. Publications I’ve written for include Women’s Health, Reader’s Digest, Body and Soul, Good Health at the Daily Mail, and more. I have also written 16 books on health and nutrition.

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