Slow Twitch vs Fast Twitch Muscle Fibers

Eric November 5, 2010



Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction.

The types of muscle fibers can be broken down into two types: slow twitch (Type I) muscle fibers and fast twitch (Type II).  Fast twitch fibers can be further categorized into Type IIa and Type IIb.

The differences between the muscle fibers effect how they react to physical activity, and each fiber type is unique in its ability to contract in a certain way.  All skeletal muscle contains a mixture of both slow and fast fiber types.  On average, we have about 50 percent slow twitch fibers and 50 percent fast twitch fibers, with different muscles that we use more or less frequently containing different amounts of each fiber type.  In comparison, Olympic sprinters have on average 80 percent fast twitch fibers, while Olympic marathoners possess 80 percent slow twitch fibers.  Your genetic makeup determines which of the muscle fibers are more dominant in each of your muscles, which helps to explain why training a certain way results in better results than another.

So how do you train and develop the different types of muscle?

Slow Twitch (Type I)
The slow fibers are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue.   Muscles that contain higher concentrations of slow twitch fibers are muscles that you are constantly using.  For example, some people, no matter how much they train their chest, all that seems to grow is their triceps and shoulders. This is because their body is designed in such a way that it is very hard to put a couple inches on the chest.

Your calves, for example, are used in every step that you take throughout your day, and therefore they are loaded with slow twitch fiber and will have a difficult time putting more size on them because of the ratio of slow to fast twitch fibers in them.

Then there are people who use their muscles more when compared to other people. For example someone with a cushy desk job would have less slow-twitch fibers than the person in construction because they obviously use their muscles more. This would be of course if they didn’t train or do any other physical demanding activities.

So you have to ask yourself¦ What muscles do you use the most? What sports do you play and what muscles are used the most during play? Where do you spend most of your time, and what muscles do you strain in those different places?

Fast Twitch (Type II)
Because fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since he or she needs to quickly generate a lot of force.

  • Type IIa Fibers
    These fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibers.
  • Type IIb Fibers
    These fast twitch fibers use anaerobic metabolism to create energy and are the “classic” fast twitch muscle fibers that excel at producing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but it also has a much faster rate of fatigue and can’t last as long before it needs rest.

Fast-twitch fibers are then obviously easier to grow because they are used less. When you are working out it is like they are being challenged for the first time. That is the reason why people see the most gains with these muscles when they first start working out.

They will need lower reps per set when compared to your slow-twitch muscles because less is required to pump them.

That’s why when most people start training they have a lot of fast-twitch fiber gains and they make the most gains in the first months.

They keep the same rep, rest time and set amount and can’t understand why they cannot grow like they did when they first started. Implement these muscle gain tips and you will see amazing results.

Your muscle fibers become more slow-twitched and experienced while you train and therefore your set and rep amounts need to be increased while your rest time between sets needs to decrease.

Which muscle fibers should I train and how?

If you want to stay lean, focus on your slow twitch fibers as they are less bulky than fast twitch.  Do exercises that include movement that lasts longer than 60 seconds, so that you’re creating an endurance-type workout.  Do higher reps, the weight won’t matter.  Lift whatever you are able while maintaining good form all the way to the last rep.

If you want to gain muscle mass, focus on your fast twitch fibers by focusing on fewer reps.  But do more weight while doing those fewer reps so that you can still get a pump very quickly without incorporating your slow twitch fibers.  Generally, exercise enthusiasts looking to gain size have adopted a 3-4 sets rule of anywhere from 4-12 reps each.  Keep in mind that the more you use your muscles, the more the slow twitch muscles will take over.  You will have to gradually increase your set and rep amounts and also decrease the rest time between sets to continue to see growth.  “Overtraining” is basically when you train so much that even when trying to develop your fast twitch fibers, you’re using the muscle so much that inadvertently your slow twitch fibers take over and don’t allow your fast twitch fibers to grow and develop.  Keep workouts to under 60-70 minutes to avoid this.

There’s a lot of info in this post.  I hope you made it this far, because there’s a wealth of knowledge that will help you train through hurdles created by training incorrectly.

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