Muscle fibers: fast and slow – how to train them

Every muscle fiber in your body does slightly different work. Slow fibers keep you afloat in long-distance running, while fast fibers come into play when the bar gets heavy. In this article we look at how these fibers respond to intensity and how to choose the right repetition range so that your training actually delivers results.

Slow and fast muscle fibers — what’s the difference?

Broadly speaking, our muscles fall into two fiber types: slow and fast. Slow fibers are labeled type 1A. They are smaller and don’t develop great strength, but they endure for a long time. Whenever an activity runs longer than 90 seconds, the body hands the main work over to these fibers. That’s why they matter in long-distance running and long bike rides.

Fast muscle fibers (type 2) are larger and produce much more force, but they tire quickly. These are further divided into two subtypes: 2 A and 2 B. Type 2 A fibers carry the main load when an effort lasts roughly 25–89 seconds. Type 2 B fibers switch on during the shortest and most explosive efforts, 0–24 seconds. Sprinting, jumping and throwing all rely on fast fibers.

The time ranges are of course approximate — a lot depends on resistance. In strength training, it’s the weight of the bar that dictates how long a set lasts.

How muscle fibers kick in during training

Fiber recruitment is driven by intensity. The heavier the bar, the more fiber types get to work. In a strength training context specifically, the picture looks roughly like this:

• Slow type 1 A muscle fibers are recruited at an intensity of 58–69%, with a set lasting over 90 seconds.
• Fast type 2 A muscle fibers come into play at an intensity of 70–84%, with a set lasting 26–89 seconds.
• Fast type 2 B muscle fibers switch on at an intensity of 85–100%, with a set lasting 0–25 seconds.

An important point: whether you’re on a tennis court, lifting a bar or running 200 m, all fiber types are working together to some degree. One fiber type simply carries the main training load while the rest provide support.

When you lift heavy weights, your muscles develop high strength, which means mainly type 2 fast muscle fibers are at work. They contract faster and produce more force than slow ones. This has a practical implication: heavy weights won’t make you slow. On the contrary — your speed qualities improve.

Repetition range and muscle fiber selection

Repetitions are a simple tool for choosing which type of fiber gets called on in training:

• 12–20 (and more) repetitions — mainly type 1 slow muscle fibers.
• 6–12 repetitions — predominantly type 2 fast muscle fibers.

The 6–12 repetition range is indeed the most effective when your goal is muscle mass. The reason is simple: the same type 2 fibers are at work, and they have the greatest hypertrophy potential. If you’re building mass, it’s worth keeping the bulk of your training in this window.

Genetics and muscle fiber distribution

An average person’s muscle tissue is made up of about 50% type 1 slow muscle fibers, 25% type 2 A (oxidative-glycolytic) fibers and 25% type 2 B fast glycolytic fibers. This distribution isn’t uniform, though — different muscles have different compositions, depending on use, body part and region.

Muscles that are constantly at work — for example the abdominal and back muscles that maintain posture — consist largely of type 1 A slow fibers. Muscles that contract less often but develop great strength (the deltoid, for example) contain more type 2 fibers.

Genetics plays a big role here. The fiber composition of skeletal muscles varies enormously from person to person. Those with more type 2 fibers can add strength and mass more easily. The practical takeaway:

• If you have more type 1 slow fibers, use a base range of 10–12 repetitions in the gym.
• If you have more type 2 fast fibers, stick to a range of 6–8 repetitions.

Finding out your exact fiber type would require a muscle biopsy, which the average gym-goer is unlikely to undergo. An easier way: experiment with both windows for a couple of months and see which one your body responds to faster in terms of mass and strength.

FAQ about muscle fibers

Can slow fibers be trained to become fast?

Short answer: to a limited extent. The basic split is genetic and stays fixed. However, the ratio of 2 A to 2 B does change with training — heavy strength sets shift the balance toward 2 B, while endurance training shifts it the other way.

Will heavy weights make me slow?

No. On the contrary — the fast muscle fibers you activate by lifting heavy weights contract faster. Sprinters’ gym training is the best example of this.

How many times a week should I do heavy sets?

For a beginner, 2–3 strength training sessions a week are enough, with the heavier day for a given muscle group coming once every 5–7 days. For advanced lifters, volumes grow, but quality has to outrank quantity.

Translated by Janar Rückenberg

Source: WHO – physical activity.

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