Most muscles are made up of two kinds of muscle fibers that help you move your body:

  • slow-twitch muscle fibers, which move more slowly but help to keep you moving longer
  • fast-twitch muscle fibers, which help you move faster, but for shorter periods

“Twitch” refers to the contraction, or how quickly and often the muscle moves.

Slow-twitch muscle fibers are all about endurance or long-lasting energy. In comparison, fast-twitch muscle fibers give you sudden bursts of energy but get tired quickly.

Slow-twitch fibersFast-twitch fibers
Type 1 muscle fibersType 2 muscle fibers
Activate firstActivate for sudden bursts
Use slow, even energy Use a lot of energy, quickly
Engaged for low-intensity activitiesEngaged for big bursts of
energy and movement
Have more blood vessels
(for more oxygen and longer use)
Create energy anaerobically
(without oxygen)

Let’s take a closer look at the ways slow-twitch muscles differ from fast-twitch muscles:

  • Type 1 and type 2 muscle fibers. Your body normally uses slow-twitch fibers to power muscles first. Fast-twitch muscle fibers are mainly only used when the body needs to make sudden, more powerful movements.
  • Energy use. Slow-twitch muscles use energy slowly and fairly evenly to make it last a long time. This helps them contract (work) for a long time, without running out of power. Fast-twitch muscles use up a lot of energy very quickly, then get tired (fatigued) and need a break.
  • Intensity and duration. Slow-twitch muscle fibers power low-intensity activities. This is because they need a steady, even supply of energy. In comparison, fast-twitch muscles fibers work when you need a big burst of energy.
  • Blood vessels. Muscles with more slow-twitch fibers have more blood vessels. This is because they need a good and constant supply of blood and oxygen to let them work for a long time without getting tired. Fast-twitch muscle fibers don’t need as much blood because they make their own quick source of energy.
  • Oxygen needs. Slow-twitch muscle fibers use an aerobic energy system. This means that they run on oxygen. Fast-twitch muscles mainly run on an energy system that doesn’t need oxygen. This is called an anaerobic energy system.
  • Appearance. The bigger blood supply in slow-twitch muscles fibers can make them look redder or darker. On the other hand, muscles that have more fast-twitch fibers look lighter because they have less blood.

To visualize the differences, think of slow-twitch muscles as “plugged in” to the heart. On the other hand, fast-twitch muscles mainly run on a “battery.”

Muscle fiber type 2a

One kind of fast-twitch muscle fiber can also act like a slow-twitch muscle fiber. It’s also called an intermediate muscle fiber or type 2a.

This muscle fiber can use its own energy and be powered by oxygen from blood. It switches depending on the kind of activity you’re doing.

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Most of the muscles in your body have more than one kind of muscle fiber. But some muscles have more slow-twitch fibers because they have to do the same job for a long time.

For example, the muscles in the back of your lower legs and the muscles in your back are mostly made up of slow-twitch fibers. This is because they have to help you stand and hold your posture for long periods of time.

Fast-twitch fibers wouldn’t be able to do this because they can’t keep working for that long. Muscles that need speed rather than endurance will have more fast-twitch fibers. For example, the muscles in your eyelids that help you blink are all fast-twitch fibers.

Your slow-twitch muscle fibers are working hard whenever you’re doing an activity or exercise that needs muscles to work — or even stay still — for a long time. These include:

  • sitting up
  • standing
  • walking
  • slow jog
  • running a marathon
  • biking
  • swimming laps
  • rowing
  • many yoga positions
  • some pilates exercises

Fast-twitch muscle fibers are working more if you’re doing high impact activities like:

  • running
  • sprinting
  • jumping
  • boxing
  • skipping rope
  • lifting weights

You can only do this for a relatively short while before you tire out.

Most people are born with about the same amount of slow-twitch and fast-twitch muscles fibers in their bodies. Some people may be born with more of one kind of muscle fiber, which might make them better at a certain sport.

For example, if you naturally have more slow-twitch muscles fibers, you might be better at long-distance running. This is rare, and more research is needed on this.

If you train hard enough at one sport, you may “change” the muscle fibers in your body. For example, if you’re a marathon runner and train for a long time, some of your slow-twitch muscle fibers will grow longer. This gives you long, leaner muscles.

Similarly, if you lift weights or sprint a lot, your fast-twitch muscle fibers will grow bigger. This builds your muscles.

Slow-twitch muscle fibers help you move (or stay still) longer. They need a rich blood supply because they use oxygen for energy. This is why slow-twitch muscle fibers are also called “red” muscles.

Fast-twitch muscle fibers help you move when you need sudden and at times reflexive movements, like hopping, sprinting, and blinking your eyes.

Some muscles like those in your back have more slow-twitch fibers because they have to work tirelessly to help you stand and sit up.