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Fast & Slow Twitch Muscles Explained

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Twitch muscles are voluntary muscles that aid in movement. They are especially important for exercise.

There are two main types of twitch muscles:

  • Fast twitch: These muscles help with sudden bursts of energy involved in activities like sprinting and jumping.
  • Slow twitch: These muscles help with endurance and long-term activities like jogging or biking.

So let’s figure out why these are important for working out and how you can train them to optimize your fitness goals.

Fast Twitch Muscles

Fast twitch muscles have few blood vessels and mitochondria (unlike slow twitch muscles) because they don’t need to fuel quick, intense activities. They are anaerobic and use glucose in the body to fuel energy production.

There are a few different classes of fast twitch muscle fibers (Type 1 fibers are slow twitch):

Type IIa

These fibers can use oxygen and glucose to create energy. Because they can use oxygen, then have more Mitochondria then Type IIb fast twitch fibers. This gives them some similarities to slow twitch muscles in that they are not easily exhausted and they can recover more quickly.

A research study has linked Type IIa muscle and how big your muscles mass is.

Type IIb

Type IIb is the second type of fast twitch muscle. They rely soley on glucose to produce the energy needed for activity.

Type IIb muscles also have a much smaller number of mitochondria because they don’t use them to produce energy from oxygen like Type I and Type IIa. They’re also much larger around than other muscles and become worn out much faster than the other types of muscles.

Exercises that use Fast Twitch Muscles

  • Sprints
  • Powerlifts such as cleans
  • Plyometrics (jumping)
  • Strength training (low reps, explosive)
  • High intensity interval training (HIIT)

Slow Twitch Muscles (Type I)

These muscles have lots of blood vessels and mitochondria to keep them from exhausting for long periods of time. They are your body’s go-to source for movement before fast twitch fibers are engaged.

These muscles are aerobic, meaning they use oxygen to create energy. These fibers are important for endurance exercises that are often called “cardio” as they are important for heart health and muscle tone. Some examples are:

  • Long-distance running
  • Long distance cycling
  • Jump rope


It is important to exercise both your fast and slow twitch muscles. Slow twitch muscles are more important for endurance activities. Fast twitch fibers are more important for explosive activities and may be correlated with muscle size.

  • Jansson E, et al. (1990). Increase in the proportion of fast-twitch muscle fibres by sprint training in males. DOI:
  • Patel H, et al. (2017). Aerobic vs anaerobic exercise training effects on the cardiovascular system. DOI:
  • Talbot J, et al. (2016). Skeletal muscle fiber type: Using insights from muscle developmental biology to dissect targets for susceptibility and resistance to muscle disease. DOI:
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