Muscle Fibres
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 fibre types can be broken down into two main types:
- slow-twitch (type I) muscle fibres
- fast-twitch (type II) muscle fibres
Fast-twitch fibres can be further categorized:
- type (IIa)
- type (IIb) fibres.
Muscle Fiber Types
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 fibre types.
On average, people have about 50% slow-twitch and 50% fast-twitch fibers in most of the muscles used for movement
Slow-Twitch Muscle Fibres (Type I)
The slow-twitch muscle fibres 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 fibres and can go for a long time before they fatigue.
Because of this, slow-twitch fibres are great at helping athletes run marathons and bicycle for hours.
Fast-Twitch Muscle Fibres (Type II)
Because fast-twitch fibres 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 fibres 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 fibres can be an asset to sprinters because they allow you to quickly generate a lot of force.
Fast-Twitch Muscle Fibres (Type IIa)
These fast-twitch muscle fibres are also known as intermediate fast-twitch fibres. They can use both aerobic and anaerobic metabolism almost equally to create energy.
Fast-Twitch Muscle Fibres (Type IIb)
These fast-twitch fibres use anaerobic metabolism to create energy and are the "classic" fast-twitch muscle fibres that excel at producing quick, powerful bursts of speed.
This muscle fibre has the highest rate of contraction (rapid firing) of all the muscle fibre types, but it also has a faster rate of fatigue and can't last as long before it needs rest.
Does Muscle Fibre Type Affect Sports Performance?
Your muscle fibre type may influence what sports you are naturally good at or whether you are fast or strong. Olympic athletes tend to fall into sports that match their genetic makeup. Olympic sprinters have been shown to possess about 80% fast-twitch fibres while those who excel in marathons tend to have 80% slow-twitch fibres.
Fibre type is part of a great athlete's success, but it alone is a poor predictor of performance. There are many other factors that go into determining athleticism, including mental preparedness, proper nutrition and hydration, getting enough rest, and having appropriate equipment and conditioning
