Want to be explosive, agile, and strong? Then you want a high ratio of fast-twitch muscle fibers, what we call "athletic muscle." And yes, you CAN adjust your training to shift that ratio in the right direction.
The body has two broad categories of muscle fibers: slow-twitch and fast-twitch. These two categories have several complex hybrid subtypes.
Fast-twitch fibers produce a lot more force, contract faster (producing more speed and power), and have higher growth potential, but they're a lot less resistant to fatigue. That means having a lot of fast-twitch fibers is not only beneficial for athletes but for bodybuilders and powerlifters too.
Some of the benefits of having a higher ratio of fast-twitch muscle fibers will surprise you:
- More speed, power, strength, and development potential of all three
- Higher degree of muscle growth and muscle tone/definition
- Shorter time to reach maximal muscle tension
- Requires less training volume to get a maximal training effect
- Better carb tolerance (less likely to store carbs as fat)
- Quicker adjustments to sudden changes of direction
- Better force absorption
Having a higher ratio of fast-twitch fibers is vital if you want to get jacked, strong, and powerful.
But what if you don't? What if you were born with a lot of slow-twitch fibers? Are you genetically doomed?
The old belief among sports scientists was that, even though it was possible to increase the ratio of slow-twitch fibers through endurance training, increasing fast-twitch fibers was less likely. Luckily, this was debunked.
The studies using a method called "single fiber SDS-PAGE" say it's possible to increase the ratio of slow-twitch or fast-twitch fibers. But older methods, like "histochemistry" or "SDS-PAGE homogenous," failed to see the conversion largely because they couldn't measure hybrid fibers.
Confused yet? No biggie. What's important is that the most current literature indicates that it is possible to increase the ratio of fast-twitch fibers. Even better? You can do it in a matter of weeks.
With the proper training methods – like a combination of heavy and explosive work – you can convert a significant amount of slow-twitch (and hybrid) fibers into fast-twitch fibers. If you were to stop that type of training and switch to endurance training, you'd decrease the ratio of fast-twitch fibers while increasing the ratio of slow-twitch ones.
The main mechanism is the switch of hybrid fibers into pure types. For example, a sedentary person will have 20-40% hybrid fibers, an active person only 10-20% (so more "pure" types based on their activity), and elite athletes have almost no hybrids.
Most muscle fiber researchers agree that you can increase your fast-twitch fiber ratio by at least 10% in a short period – 4-8 weeks.
Is that significant? Well, the average human has around 50% fast-twitch fibers. The highest measured fast-twitch ratio (in a top sprinter) was 71%. So yeah, if a 10% shift brings someone from 50 to 60% fast-twitch, it's a big deal.
And it's possible that longer periods of specific training, or a better training protocol specifically aimed at targeting the fast-twitch fibers, could increase the ratio even more. I'll use a real-life example.
There was a Canadian bobsleigh athlete who was an endurance runner until he was 19 years old. The bobsleigh is an extreme power and speed sport.
He completely changed his performance matrix after switching to sprinting and then bobsleigh and became the most explosive athlete I've ever trained. I'm talking 4.19 seconds for the 40-yard dash and a 41-inch vertical jump. He also posted a 425-pound bench press at a bodyweight of 181, and he power-cleaned 335.
The question is not if it's possible to increase your fast-twitch muscle fiber ratio, but HOW to do it optimally. There's a way to do it, but it might not be what you expect.
It's not quite as simple as "endurance training will increase slow-twitch fibers and weight lifting will increase fast-twitch fibers."
Well, it is for the endurance part. Elite endurance athletes can have as much as 80% slow-twitch muscle fibers. But when it comes to fast-twitch, it's not as simple as just lifting weights.
In fact, a typical "bodybuilding" approach – reps in the 8-20 range, not focusing on accelerating and doing lots of volume – will shift you toward a slightly slower profile.
The super fast-twitch fibers (MHC IIX) will convert to the fast-twitch fibers (MHC IIA), and some fast-twitch fibers will convert to slow/fast hybrids (MHC I/IIA). It won't increase the slow-twitch fiber ratio, but you're still moving toward a less powerful profile.
Even pure strength training will have that effect – a decrease in MHC IIX fibers and an increase in MHC IIA ones with no change in MHC I fibers.
However, when researchers used a combination of strength and explosive work, there wasn't much of a decrease in IIX fibers, but there was still a significant increase in IIA fibers. But, it came with a decrease in type I fibers.
So, strength training by itself can lead to a slightly slower profile, whereas the use of strength work AND plyometrics will increase the conversion of slow-twitch fibers to fast-twitch fibers (1).
By themselves, plyometric or stretch-shortening cycle exercises increase the ratio of fast-twitch fibers (2). The same was found with sprint training (3). So, right off the bat, we can say that anything done explosively will help increase your fast-twitch fiber ratio. Examples:
- Loaded Jumps
- Olympic Lifting Variations
- Dynamic Effort Lifting (40-60% of your 1RM for 2-3 reps done explosively)
- Anaerobic Power Work (like 10-15 seconds all-out on an air bike)
- Drop and Catch Method
- Overspeed Work
If you haven't heard of those last two, here they are:
While studies looking at strength work conclude there's a move toward a slightly less-fast profile, the studies normally include sets where participants use 70-80% for "strength" work.
Heavier work with less fatigue accumulation works better. Sets using 70-80% will still allow 6-10 reps, so the fast-twitch fibers might not come into play until the last half of the set. The set will still lead to some accumulated fatigue, which could stimulate the shift toward a slightly slower profile.
However, suppose you're using sets of 1-3 reps with 90%-plus or even supramaximal partial lifts or eccentric overloads. In that case, you could likely switch to a faster profile, especially if you do not have to grind these reps. Why? Because you need to rely on the fast-twitch fibers and a high firing rate along with very little fatigue accumulation.
In fact, single efforts (one rep per set) are likely one of the best approaches to develop the fast-twitch fibers and increase their ratio.
Here are some of my favorite heavy methods for that purpose:
Heavy Weight Strategy 1
Do heavy sets of 1-2 reps while dominating the weight. This means using a load that's roughly 90 to 92.5% of your maximum. Go as heavy as you can while still dominating the weight – no grinding and still having the feeling that you can accelerate the weight. Do 3-5 sets in a session.
Heavy Weight Strategy 2
Do heavy partials for 2-3 reps. Obey the same rule as above, but instead of using a full range of motion, use roughly half – from the middle to completion. Use lifts from pins, blocks (bench or deadlift), or box (squat). The weights will be heavier than your max on the full-range lift, but still choose a load you can dominate.
Heavy Weight Strategy 3
Use the eccentric overshoot method. You load the bar with a moderate load, one that you can accelerate. It'll be anywhere between 60 and 80% depending on your level of explosiveness. Add extra weight during the eccentric phase of the lift via weight releasers. Try going as heavy as possible on the releasers, provided you can keep lowering the weight slowly through the whole range of motion.
When the releasers unhook from the bar, lift the weight as fast as you can. Do sets of singles. Ramp up the releaser weight over 4-5 work sets, then perform a back-off set with 10-pounds less per side.
Heavy Weight Strategy 4
Do accelerative strength-skill work. Do 2-3 reps with 80-85% with an intent to accelerate. Move as explosively as possible during the concentric. Do 4-5 work sets.
Isometrics mean you're producing force with no movement. On the surface, such methods seem counterproductive for increasing fast-twitch fibers. After all, these fibers are associated with speed, and isometrics are completely devoid of speed!
But three applications of isometrics can increase the recruitment, stimulation, development, and possibly the ratio of fast-twitch fibers:
Isometric Strategy 1
Use maximal effort overcoming isometrics. "Overcoming" means you're trying to move an immovable resistance. For example, pushing or pulling against pins with an empty bar. Maximal refers to pushing or pulling as hard as possible. This is only sustainable for around 6 seconds, then there's a dramatic force drop-off.
Your sets should either be 6 seconds all-out or 9 seconds; the first 3 are a smooth ramp-up in force production followed by 6 seconds all-out. Do this for 2-4 sets.
Or do it using "reps" in which you push against the pins as hard as possible for 3 seconds, then rest for 5-7 seconds before your next rep begins. Push for 3 seconds; rest for 5-7 seconds. Do 3-5 of those per set. Use two sets of safety pins so you can rest the bar on the bottom pins during the recovery period. Do this for 2-3 sets.
Isometric Strategy 2
Do functional isometrics. You need two sets of safety pins. The highest pins would be 8-10 inches above the lowest ones. Press the bar into the highest set of pins and push as hard as possible for 6 seconds. The difference here? The bar is loaded.
How much weight do you use? It doesn't matter much since you'll be producing maximum force by pushing against the pins, but use a weight that's closer to your 1RM (around 85-90%) on the full-range lift. The purpose of using a loaded bar is to get a feel that's similar to a regular lift (better dynamic correspondence). The pins act like a sticking point. This teaches you to maximize force production when you reach a sticking point. The second set of pins should be set around where your sticking point is in a lift.
Isometric Strategy 3
Do loaded stretching. Loaded stretching is an anomaly. Even though there's no movement and the duration is fairly long, it emphasizes fast-twitch fibers.
To understand why, consider blood flow restriction (BFR) training. One study found that the occlusion effect it causes (restricting blood from coming in or out of the muscles) leads to preferential recruitment and development of fast-twitch fibers (4).
The reason? Occlusion prevents oxygen from coming inside the muscles, depriving the slow-twitch fibers of their primary fuel source and shifting the recruitment toward the fast-twitch fibers in the process.
Loaded stretching also causes an occlusion effect, so the same conclusion likely applies. Both stretching and muscle contraction reduces the amount of blood that can come in or out of a muscle. So loaded stretching, combining both, will have a very strong occlusion effect, leading to an increase in fast-twitch fiber recruitment.
For loaded stretching with the purpose of building muscle and strength, do sets of 1-1:30 minutes for a total time under load of around 3 minutes – so 3 sets of 1 minute or 2 sets of 90 seconds.
To make it maximally effective for the fast-twitch fibers, you want to:
- Go to the most extreme position you can reach.
- Contract the muscles as much as you can and maintain the tension.
The more these are respected, the greater the occlusion effect will be and the better fast-twitch stimulation you'll get.
When it comes to designing your training program specifically to maximize the conversion to, and development of, fast-twitch fibers, I'm reminded of something that track coach Boo Schexnayder said:
"You must give the body an unambiguous message to what you want it to become."
It applies to increasing the ratio of fast-twitch fibers. The more "unidirectional" your training is, the more likely you are to get the adaptation you're after. So if you want more fast-twitch fibers, you want the vast majority of your training stimulus to be done with methods favoring the conversion toward a faster profile.
Any method that would, by itself, lead to a conversion toward a slower profile should be avoided. Don't get me wrong, if 80-90% of your volume is done on fast-twitch methods and 10-20% is high-rep bodybuilding work or even some cardio, it'll still work. But if you try to be a "jack of all trades," you have no shot at becoming a fast-twitch machine.
- Use fast-twitch methods for as much of your workload as possible.
- Use a limited number of methods in a training phase. All of these methods are neurologically demanding. It's better from a recovery, performance, and learning perspective to use two methods in a workout and no more than five to six in your training block.
- Stick with a set of methods for 3-6 weeks. Most of these are heavily neurological and will give you the fastest progression for the first three weeks. The more advanced or explosive you are, the more frequently you need to change the methods.
- Keep overall volume lower than you'd think. Creating too much fatigue can hurt the adaptations toward a faster profile. And due to the nature of the methods, it can make recovery harder. The biggest mistake? Approaching fast-twitch training the same way you'd approach bodybuilding training. Use the "training economy" principle of doing the fewest exercises possible to get the job done. The less fatigued you are while still giving a strong stimulus to your body, the more likely you'll be to transition to a faster profile.
- Opt for a whole-body approach. It will allow you to stimulate each muscle (at least indirectly) 3-4 times a week. Each stimulation promotes the conversion of muscle fibers while still allowing you to recover properly. Avoid workouts with a significant amount of fatigue.
- Stick to one category per workout. Choose to either go heavy, use isometrics, or get explosive on each workout.
- Use a low density of work. This means using ample rest between sets. This is contrary to what I recommend when trying to maximize muscle mass or lose fat. But this is the way to do things that favor the conversion toward a faster profile. Basically, you don't want to do your sets while being in a fatigued state.
- Remember the Schmidtbleicher principle. The last thing you do has the greatest impact on motor learning. Never allow yourself to finish an exercise (last rep of the last set) with a grinding, slow, or technically incorrect execution. If you feel like the last rep of an exercise wasn't optimal, rest a few minutes and perform 1-2 violent reps, even if that means decreasing the weight. In fact, it's smart to program a "back off" set with a slight load reduction but performed with the intent to be violently explosive.
Increasing your ratio of fast-twitch fibers is not only possible, it happens pretty rapidly, provided you give your body the correct stimulus. An increase in fast-twitch fibers of at least 10% (possibly 15 or even 20%) is achievable.
But remember, the body becomes whatever you ask it to become. The "clearer" the adaptation message is (by using almost exclusively fast-twitch methods), the more likely you are to create a maximal conversion.
Furthermore, if you stop using those methods in favor of more traditional hypertrophy work and endurance work, some of the newly converted fast-twitch fibers will convert back to what they were prior.
You can still do things like conditioning and hypertrophy work. But if you want to stay a fast-twitch machine, the methods here should make up as much of your training load as possible.
- Liu Y et al. Different effects on human skeletal myosin heavy chain isoform expression: strength vs. combination training. J Appl Physiol (1985). 2003 Jun;94(6):2282-8. PubMed.
- Malisoux L et al. Stretch-shortening cycle exercises: an effective training paradigm to enhance power output of human single muscle fibers. J Appl Physiol (1985). 2006 Mar;100(3):771-9. PubMed.
- Andersen JL et al. Myosin heavy chain isoforms in single fibres from m. vastus lateralis of sprinters: influence of training. Acta Physiol Scand. 1994 Jun;151(2):135-42. PubMed.
- Krustrup P et al. Heterogeneous recruitment of quadriceps muscle portions and fibre types during moderate intensity knee-extensor exercise: effect of thigh occlusion. Scand J Med Sci Sports. 2009 Aug;19(4):576-84. PubMed.