Fast and Slow Twitch Muscle Fibres (POST 36)
April 2, 2012
Fast and Slow Twitch Muscle Fibres
Are you a better sprinter or distance runner? Many people believe that having more fast and slow twitch muscle fibres may determine what sports athletes excel at and how they respond to training.
Skeletal muscle is made up of bundles of individual muscle fibres 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 and fast twitch (Type II) muscle fibres. Fast twitch fibres can be further categorized into Type IIa and Type IIb fibres.
These distinctions seem to influence how muscles respond to training and physical activity, and each fibre 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, we have about 50 percent slow twitch and 50 percent fast twitch fibres in most of the muscles used for movement.
Slow Twitch (Type I)
The slow muscles are more efficient at using oxygen to generate more fuel (known as ATP) 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. Therefore, slow twitch fibres are great at helping athletes run marathons and bicycle for hours.
Fast Twitch (Type II)
Because fast twitch fibres use anaerobic metabolism to create fuel, they are much 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 a sprinter since she needs to quickly generate a lot of force.
Type IIa Fibres
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. In this way, they are a combination of Type I and Type II muscle fibres.
Type IIb Fibres
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 much faster rate of fatigue and can’t last as long before it needs rest.
Fibre Type and Performance
Our muscle fibre type may influence what sports we are naturally good at or whether we 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 percent fast twitch fibres, while those who excel in marathons tend to have 80 percent slow twitch fibres.
Can Training Change Fibre Type?
This is not entirely understood, and research is still looking at that question. There is some evidence showing that human skeletal muscle may switch fibre types from “fast” to “slow” due to training.
These studies and journal articles offer more insight on muscle fibre research:
What can I do to improve my performance?
Keep in mind that genetic differences may be dramatic at the elite levels of athletic competition. But following the principles of conditioning can dramatically improve personal performance of a typical athlete.
With consistent endurance training, muscle fibres can develop more and improve their ability to cope with and adapt to the stress of exercise.
Is fibre type the number one factor that makes an elite athlete elite?
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.