Fast and Slow Twitch Muscle Fibre and What This Means.

The natural makeup of your muscle fibres may influence what sports you are naturally good at and how well you respond to training. Fast-twitch muscle fibres contract quickly but tire rapidly. Slow-twitch muscle fibres contract slowly but also tire slowly. Having a greater proportion of fast-twitch fibres can help you excel at activities that require bursts of power and strength and may make you a better sprinter or weightlifter. Having more slow-twitch fibres can help you become an accomplished endurance athlete, such as a distance runner or cyclist.

How Do Muscle Fibres Differ?

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 (also known as IIx fibres). There are also hybrid fibres with more than one type.

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, people have about 50% slow-twitch and 50% fast-twitch fibres in most of the muscles used for movement.

Anaerobic vs. Aerobic Metabolism in Exercise

There are two types of metabolism that the body uses to turn fuel (the food you eat) into energy and understanding these explains how the differing muscle fibres function so differently.

Aerobic Metabolism

During aerobic metabolism, your body creates energy through the combustion of carbohydrates, amino acids, and fats in the presence of oxygen. Combustion means burning, which is why this is called burning sugars, fats, and proteins for energy.

Aerobic metabolism provides energy for exercise and other body functions (like breathing). Examples of activities that use aerobic metabolism include walking, running, or cycling with sustained effort.

Anaerobic Metabolism

Anaerobic metabolism creates energy by burning carbohydrates in the absence of oxygen. This occurs when your lungs cannot put enough oxygen into the bloodstream to keep up with the demands of your muscles for energy. It is generally used only for short bursts of activity, such as when you sprint when running or cycling, or lift heavy weights.

When there isn't enough oxygen in the bloodstream, glucose and glycogen cannot be fully broken down into carbon dioxide and water. Instead, lactic acid is produced, building up in the muscles and degrading muscle function.

Your body will often switch between aerobic and anaerobic metabolism during sports and exercise activities that require short bursts of sprints as well as sustained jogging, such as in soccer, tennis, and basketball.

Slow-Twitch Muscle Fibres (Type I)

The slow-twitch muscle fibres are more efficient at using oxygen to generate more) 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 can fire more rapidly.

Having more fast-twitch fibres can be an asset for sprinting because they allow you to quickly generate a lot of force.2

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. In this way, they are a combination of type I and type II muscle fibres.

Fast-Twitch Muscle Fibres (Type IIb or IIx)

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.

Hybrid Muscle Fibres

Research suggests that some muscle fibres are pure, or one type, and others are hybrids that express more than one type of fibre. Hybrid fibres may be a combination of both IIa and IIb fast-twitch muscle types or be any combination of both slow-twitch and fast-twitch types. It's also possible that they may shift over time. More research that examines single fibres is needed to better understand the role of hybrid fibres and how they may change over time.

What Happens During Anaerobic Exercise

Anaerobic metabolism produces lactic acid, which can build up in the muscles to the point where you "feel the burn." This burning sensation is a normal side effect of anaerobic metabolism.

Fast-twitch muscle fibres rely more on anaerobic metabolism for quick contractions, but they fatigue more quickly.

High-intensity intervals turn a normally aerobic exercise like endurance running into an anaerobic exercise. Anaerobic metabolism is needed once you exceed 90% of maximum heart rate.

Does Muscle Fibre Type Affect Sports Performance?

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.

Research on sports performance within families also suggests that genetic factors may influence about 30% to 80% of the differences among individuals.

However, genetics are just one aspect. Fibre type is part of a great athlete's success, but it alone is a poor predictor of performance. Environment and training matter too. 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.

Can Training Change Your Muscle Fibre Type?

There is some evidence showing that human skeletal muscle may switch fibre types from "fast" to "slow" due to training or transition from hybrid to pure fibres.2 This is not entirely understood, and research is preliminary and still looking at that question.

One very limited study showed the muscles fibres of a set of twins. One had trained as an endurance athlete for decades and the other was sedentary for decades. Researchers found that the endurance athlete had 55% more slow-twitch fibres than the sedentary twin. Also interestingly, the untrained twin had more fast-twitch and hybrid fibres.

Although genetic differences may be dramatic at the elite levels of athletic competition, following the scientific principles of conditioning can dramatically improve the personal performance of a typical athlete.

With consistent training, muscle fibres can develop more and improve their ability to cope with and adapt to the stress of exercise. With consistent effort, they may even have the potential to shift types to favour your sport over time.