What Are Fast and Slow Twitch Muscles?

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Know any dunkers who can’t run a mile? How about a marathoner who can’t clear a phone book? The difference might lie in the makeup of their skeletal (aka foundational) muscles. Humans have two general types of skeletal muscle fibers: slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscles help enable lower intensity, higher endurance movements such as distance running, while fast-twitch muscles fatigue faster, but are used in more powerful movements such as sprinting.

All Mixed Up — The Need-to-Know

Photo by Collin Orcutt

Fast-twitch muscles break down into two categories: “moderate fast-twitch” (type IIa) and “fast-twitch” (type IIb). Moderate fast-twitch muscles are larger, thicker, quicker to contract, and wear out more rapidly than slow-twitch. Fast-twitch, the most powerful and lowest in endurance, are activated when the body nears its maximum exertion.

Here’s how it works: During aerobic exercises such as running or swimming, slow-twitch fibers are the first to contract. When the slow-twitch fibers become tired, fast-twitch varieties begin to take over. During high-intensity exercises that require quick, powerful movements (like jumping or sprinting), fast-twitch muscles contract first.

There are significant benefits to mixing high- and low-intensity workouts (and in the process working fast and slow twitch muscle fibers!). Aerobic slow-twitch exercises increase stamina and oxygen supply, allowing the body to burn energy— i.e. calories— for longer periods of time. Fast-twitch exercises burn considerably more energy in less time, and can lead to gains in muscle mass that may help trigger weight loss, lower blood pressure, and improve strength [1]. A well-rounded workout routine includes a combination of intense and endurance exercises and can help the body adapt and perform in a variety of scenarios [2].

I Like the Way You Move — Your Action Plan

There’s no conclusive evidence that muscle fibers can naturally transform from fast- to slow-twitch or vice versa. Still, fast-twitch fibers can be converted into moderate fast-twitch, or vice versa, through increases in endurance or resistance training [3]. To work on endurance, spend 30 minutes doing an activity like walking, jogging, dancing, swimming, tennis, or distance running. For resistance exercises, try weight training or bodyweight exercises.

But before getting carried away, it’s important to set realistic goals. Fast-twitch muscle depletes with age considerably more than slow-twitch, so increasing muscle power is less feasible later in life [4]. Fortunately, both resistance and endurance exercise slow the loss of muscle mass and force due to age [5], and resistance training can build muscle in older adults.

Generally, when muscles are forced to work differently, the body is taken out of its comfort zone, and the result is a better workout. Recognizing the most challenging levels of endurance or resistance training is key to boosting workout efficiency and performance.

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Works Cited

  1. Slow-twitch fiber glycogen depletion elevates moderate-exercise fast-twitch fiber activity and O2 uptake. Krustrup,P., Söderlund,K., Mohr, M., et al. Copenhagen Muscle Research Centre, Institute of Exercise and Sport Sciences, Department of Human Physiology, University of Copenhagen, Copenhagen, Denmark.
  2. The determinants of skeletal muscle force and power: their adaptability with changes in activity pattern. Fitts, R.H., McDonald, K.S., Schluter, J.M. Department of Biology, Marquette University, Milwaukee, WI 53233.
  3. Muscle mechanics: adaptations with exercise-training. Fitts, R.H., Widrick, J.J. Department of Biology, Marquette University, Milwaukee, Wisconsin, USA.
  4. Human aging, muscle mass, and fiber type composition. Lexell, J. Department of Rehabilitation, Lund University Hospital, Höör, Sweden.
  5. Effects of age and training on skeletal muscle physiology and performance. Thompson, L.V. Department of Physical Medicine and Rehabilitation, University of Minnesota, Minneapolis.

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