Can you really alter your fiber types?

By Cat James

We all like to think that if we train hard enough, we can excel at any sport we put our mind to. But in reality, some people do have a genetic advantage before they even start training.

Our skeletal muscles are made up of individual fibers. Did you know that most Olympic sprinters have around 80% fast-twitch muscle fibers, whilst most marathon runners have 80% slow-twitch muscle fibers?

Slow-twitch fibers can work for longer periods of time without running out of energy. They use oxygen to generate adenosine triphosphate so that they can perform slower, sustained movements like long- distance running or cycling.

Fast-twitch fibers are often thought to be stronger than slow-twitch fibers, but that’s not the case. They actually produce the same amount of force, but they fire more rapidly. This makes them best for generating short, sharp bursts of strength and speed.

There can be further categories between Type IIa, which uses both anaerobic and aerobic metabolism to create
energy and type IIb, that use anaerobic energy only. Type IIb is the fastest firing muscle fiber, but it also fatigues faster than any other.

What kind of muscle fibers do you have?

The average person tends to have a 50/50 split of fast and slow-twitch fibers in their muscles that are used for movement. It’s hard to know definitively how much of each fiber you have, but you can use this test to determine if you lean further towards one way or another.

1. Choose a classic strength exercise – like bench press or back squat
2. Load the weight to 80% of your 1 rep max
3. Complete as many reps as you can until failure, or you lose your form

If you score…
• 7 reps or less – you have more fast-twitch muscle fibers
• 8-10 reps – you are equally balanced between fast-twitch and slow-twitch
• More than 10 reps – you have more slow-twitch muscle fibers

What determines which kind of muscle fibers you have?

There are two genes that have a large influence over whether you are fast or slow-twitch dominant: ACTN3 and ACE.
The ACTN3 gene gives your body the instructions to make a protein called alpha (a)-actinin-3, that’s mostly found in fast-twitch fibers. If you have the R577XX variant of this gene, you’ll produce very short (a)-actinin-3, which is broken down quickly and results in more fast- twitch fibers. If you have the R577XX variant of this gene, you won’t produce any (a)-actinin-3 at all, which results in more slow-twitch fibers.

ACE gives your body the instructions to make a protein called angiotensin-converting enzyme, which converts a hormone called angiotensin I to another form called angiotensin II that helps control blood pressure and skeletal muscle function. There are three variants of this gene: II, DD and ID. The DD pattern is associated with having a higher ratio of fast-twitch to slow-twitch muscle fibers.

Can you train your body to produce more fast or slow-twitch muscle fibers?

Whilst you’re not likely to be able to push your body to become significantly more fast or slow-twitch dominant, you can train your muscle fibers to be more efficient with the right exercise program.

For more aerobically efficient slow-twitch fibers:

  • Perform high rep, low weight exercises
  • Reduce your rest period to 30 seconds between sets
  • Incorporate exercises with sustained isometric contractions and no join movement, like planks, wall sits and push-up holds

For more explosive fast-twitch muscle fibers:

  • Lift heavier weights; the heavier the weight, the more fast-twitch fibers will be used
  • Limit reps to 2-6 per set, and increase rest periods to 60-90 seconds in-between
  • Focus on power moves using equipment like bar- bells, kettlebells and medicine balls

Want to dig deeper into the science behind your gainz? Check out this fascinating article on VOLUME OR INTENSITY?

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