How to Activate Fast Twitch Muscles

Co-authored by Laila Ajani and Eric McClure

Last Updated: April 11, 2024 References

If you’re looking to bulk up or improve your one-rep max at the gym, activating your fast twitch muscle fibers can be a serious game changer. Every muscle in your body is made up of a combination of fast and slow twitch muscle fibers. Fast twitch fibers are activated during explosive movements, while slow twitch fibers help with repetitive, slower movements. As a result, you can target fast twitch fibers by making a few minor changes to your exercise habits. Read on if you want to know more about how your muscles work and what you can do to get those fast twitch fibers twitching even faster!

Question 1 of 5:

What are fast twitch muscle fibers?

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Every muscle contains a combination of fast and slow twitch fibers. There are no muscles that are exclusively fast twitch or slow twitch muscles.^{[1]
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Research source} Inside of each muscle, there are thousands of individual fibers that contract and relax to perform different movements. The fast twitch fibers are the parts of your muscle that perform rapid movement, while the slow twitch muscles perform slower movements that require more endurance.
- Fast twitch muscles are also known as type 2 (or glycolytic) muscles. Slow twitch muscles are also known as type 1 (or oxidative) muscles.
- Fast/slow twitch fibers are only found in skeletal muscles, which are what most people picture when they hear the word “muscle.” Skeletal muscles include things like biceps, abs, and calf muscles, but not your heart muscles or the smooth muscles in your organs.^{[2]
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Question 2 of 5:

Can you develop fast twitch muscle fibers?

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You can strengthen fast twitch fibers, but you can’t make more of them. Your genes determine how many fast twitch muscle fibers you have in each muscle, but they may not be particularly developed or strong depending on how you exercise. You can change the way you work out to target fast twitch fibers. Unfortunately, you can’t turn slow twitch fibers into fast twitch fibers and vice versa, though.^{[3]
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- Since fast twitch muscle fibers are designed for explosive, fast movement, they tend to get bulkier when they’re developed. Slow twitch fibers are designed for endurance, so they tend to get leaner over time. This is why bodybuilders are bulky while marathon runners are lean. They can both be healthy and strong, but their workouts target different muscle fibers.^{[4]
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Question 3 of 5:

Which exercises activate fast twitch fibers?

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Lifting heavy weights and sprinting are your best bet.^{[5]
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Laila Ajani
Fitness Trainer

Expert Interview} Basically, the faster your muscles fatigue, the harder your fast twitch fibers are working. So, if your 1 rep max on the bench press is 200 pounds (91 kg), doing 4 reps at 180 pounds (82 kg) is going to activate your fast twitch fibers a lot better than doing 20 reps at 80 pounds (36 kg). Sprinting as hard as you can for 30 seconds will activate fast twitch fibers, while jogging leisurely for 20 minutes will activate your slow twitch fibers.^{[6]
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- Literally any exercise where you lift heavy weights or rely on quick bursts of energy to perform the exercise are going to strengthen your fast twitch fibers.^{[7]
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  Expert Interview} Bicep curls, box jumps, kettlebell swings, wind sprints, deadlifts, and barbell squats will all develop fast twitch fibers.
- As a rule of thumb, if you can only perform 7 reps or fewer at a given weight, you’re primarily working your fast twitch fibers out.^{[8]
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Question 4 of 5:

How do you know if you have fast twitch fibers?

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Everyone has fast twitch fibers! Unfortunately, short of getting a muscle biopsy, there’s no way to know how many fast twitch fibers you have.^{[10]
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Go to source} As far as the ratio between fast twitch and slow twitch fibers goes, this comes down almost entirely to your genetics. Don’t worry about this, though. Your athletic ability and strength aren’t predetermined or limited by your genes. Anyone can strengthen and develop their fast twitch fibers if they work out.^{[11]
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- As a general guide, the bulkier you are, the more developed your fast twitch muscle fibers are. Again, this isn’t really indicative of anything other than the type of exercise you do. It’s not like lean athletes with stronger slow twitch muscles aren’t healthy or strong.^{[12]
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Question 5 of 5:

Can you lose fast twitch muscle fibers?

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They can atrophy and weaken, but you can work out to keep them healthy. If you spend a lot of your day being sedentary, your fast twitch fibers will weaken over time.^{[13]
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Research source} But if you get back into a regular exercise routine that incorporates explosive calisthenics, short-burst running, and resistance weightlifting, there’s no reason you can’t strengthen those fibers back up!^{[14]
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- You do naturally lose muscle fiber as you age. Luckily, strength training 2-3 days a week will naturally maintain your fast twitch fibers, even as you get older.^{[15]
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Different muscles perform different functions, so certain muscles may naturally have more fast twitch or slow twitch fibers. For example, your soleus muscle in your lower leg is mainly responsible for sitting and maintaining your posture. These are mainly endurance-based activities, so the soleus doesn’t have a ton of fast twitch fibers. This doesn’t really impact anything from an exercise perspective, though.^{[16]
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