What are Slow Twitch Muscle Fibers?

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Slow twitch muscle fibers contract slowly with less force production good for low intensity endurance exercise. They have more mitochondria and capillaries than fast twitch fibers.


  • Also called Type I fibers. Slow twitch fibers rely on aerobic metabolism and are fatigue resistant, ideal for endurance sports.
  • They fire more slowly due to less sarcoplasmic reticulum versus fast twitch fibers but sustain activity for longer periods.
  • Marathon runners, triathletes and cyclists tend to possess high levels of slow twitch fibers while sprinters have more fast twitch fibers.
  • The fibers can shift somewhat between slow and fast in response to training. Weight training increases fast twitch fiber size and strength.


  • Activated during a marathon or century bike ride.
  • Used in pilates, yoga and other sustained lower intensity exercise.

Related Terms

  • Fast Twitch Fibers – Type II muscle fibers that contract rapidly and powerfully, but fatigue quickly. Good for strength and speed.
  • Type II Fibers – Another term for fast twitch muscle fibers. Produce forceful contractions helpful for bursts of speed and power.
  • Skeletal Muscle Fibers – Long multinucleated cells that make up skeletal muscle tissue. Can be categorized by contraction speed – slow twitch and fast twitch.
  • Sarcoplasm – The cytoplasm of a skeletal muscle fiber containing glycogen, myoglobin, enzymes, and organelles. Surrounds the myofibrils.

Common Questions

  • Are slow or fast twitch fibers better? Neither – different sports benefit from each. A mix provides the best athleticism and muscle development.
  • How much can fibers change? Moderately. Slow twitch fibers can take on some fast twitch characteristics and vice versa.

Do Not Confuse With

  • Lactic acid – Metabolic byproduct associated more with fast twitch fiber activity.
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