Muscle Fiber Type and What It Means for Strength Training

Fast Twitch, Slow Twitch, or In Between?

Bench Press
Bench Press. (c) Paul Rogers / Cooloola Fitness

Human skeletal muscle is made up predominantly of two types of muscle fiber: red fiber and white fiber. Red fibers (type 1) are also known as slow-twitch fibers, and white (type 2) are called fast-twitch fibers. White, fast fibers can also be broken into two types -- 2A and 2B. 2A fibers sit in between the slower red fibers and the ultimate fast 2B white fibers.

Fiber type predicts, to a large extent, the athletic activity for which you may be best suited, bearing in mind that many activities demand characteristics of both fiber types.

Humans have a combination of both types of fibers, but one may predominate. An Olympic sprinter for example may have around 80% fast-twitch, white fibers, and a good marathoner the reverse. Fiber type propensity may also determine to some extent your ability to lift heavy weights with speed and power.

Facts About Muscle

  • Muscle exists as 3 types: cardiac muscle, smooth muscle, and skeletal muscle. Skeletal muscle is the target of strength and conditioning training. Smooth muscle makes up blood vessels and certain organs.
  • Skeletal muscle makes up approximately 45% of total body weight.
  • Skeletal muscle attaches to two bones and crosses a joint between them.
  • Muscle cells are elongated and cylindrical in shape, and are called fibers. Muscle cells and fibers are synonymous.
  • Muscles can contract and shorten, thus creating a pulling force on bones and the attachments to bones (tendons and ligaments)
  • Muscles are organs, which means they have more than one type of tissue. Muscle contains muscle and fibrous connective tissue (fascia).
  • Muscles also incorporate blood vessels and nerves.
  • The nerves process messages from the central nervous system to the muscle, triggering contraction. Blood vessels supply nutrients and the energy required for movement, and remove waste products.
  • A motor unit consists of a motor neuron (nerve cell) and the muscle fibers that it controls. Motor units are often referenced in relation to muscle activation in weight training.

Fiber Types and Resistance Training

Fast-twitch fibers favor speed and power activities like sprints and throwing events that take only tens of seconds at most. Slow-twitch fibers favor endurance competitors like marathoners and triathletes. Having some transition fibers like the moderately fast and moderately enduring 2A fibers can be useful for middle distance runners where speed and endurance are useful.

When lifting weights, 2B fibers help you lift heavy with great power. 2B, fast-twitch fibers drive explosive power when doing 1RM or sets of low, heavy repetitions. Type 1, slow-twitch fibers are more suited to muscle endurance training, for example, sets of 20-30 repetitions.

Can fiber types be converted? The short answer is no, they cannot. However, you may be able to "train up" the fibers you have of a particular type.

For example, if you have 70% slow fibers and 30% fast fibers, there is some evidence that training heavy, at 5-8 RM for example, will theoretically boost the cross-section size of the 30% of type 2B fibers, if not the number. The reverse may also be true. For example, a predominantly fast-twitcher, a sprinter, may be able to emphasize his slow fibers by running regularly for an hour or more in order to compete in long distance racers, or by doing sets with a high number of reps in the gym. Regular full-body weight training in the range 10-15 reps per set is likely to hit your type 2A intermediate fibers.

In summary, if you're a gym rat, being blessed with white, fast fibers (2B and 2A) will probably give you a lifting edge in total weight lifted. If you have slow, type 1 fibers predominantly, you may not win a lifting competition anytime soon, although there is no reason why you should not be able to bulk up substantially.

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