Fast and Slow Twitch Muscle Fiber Types

Are you a better sprinter or a distance runner? Can you be great at both? Many people believe that having faster or slow twitch muscle fibers may determine what sports athletes excel at and how they respond to training.

Fast and Slow Twitch Muscle

Muscle Fiber Types

Skeletal muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction.

It is generally accepted that muscle fiber types can be broken down into two main types: slow twitch (type I) muscle fibers and fast twitch (type II) muscle fibers. Fast twitch fibers can be further categorized into type IIa and type IIb fibers.

These distinctions seem to influence how muscles respond to training and physical activity, and each fiber type is unique in its ability to contract in a certain way. Human muscles contain a genetically determined mixture of both slow and fast fiber types.

On average, people have about 50 percent slow twitch and 50 percent fast twitch fibers in most of the muscles used for movement.

Slow Twitch Muscle Fibers (Type I)

The slow twitch muscle fibers are more efficient at using oxygen to generate more adenosine triphosphate (ATP) fuel for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons and bicycle for hours.

Fast Twitch Muscle Fibers (Type II)

Because fast twitch fibers use anaerobic metabolism to create fuel, they are better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction as slow muscles, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to a sprinter since she needs to quickly generate a lot of force.

Fast Twitch Muscle Fibers (Type IIa)

These fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of type I and type II muscle fibers.