Slow-Twitch & Fast-Twitch Muscle Fibres

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Skeletal muscle groups of the body are made up of many individual muscle fibres. Each muscle fibre is further broken down into myofibrils which contain both actin and myosin myofilaments. These myofilaments contract to shorten and lengthen muscle tissue to allow movement and generate power.  This phenomenon is known as the ‘sliding-filament theory’ causing muscle contraction.

Each muscle group is comprised of two types of muscle fibre depending upon their energy production source, execution and expenditure. The two different muscle fibres are outlined below:

  1. Slow-Twitch (Type I; slow-oxidative) – These muscle fibres require oxygen as a fuel to allow energy production (ATP) and subsequent muscle contractions to take place. Slow-twitch fibres are recruited during sustained endurance activities such as prolonged running, cycling and swimming.
  2. Fast-Twitch (Type II) – These muscle fibres do not require oxygen to fuel the muscles for energy requirements. They mainly use the anaerobic metabolism to generate the required fuel for energy production. These muscle fibres are more suited to sports and activities that involve short bursts of strength, speed and power such as dead-lifting and short & middle distance sprints. Without the mandatory oxygen requirement, the fast-twitch muscle fibres tend to fatigue quickly. However, they are able to generate a similar muscle contraction force as slow-twitch fibres but at a more rapid rate.

Fast-twitch muscle fibres can be further divided into two categories:

  • Type IIa (fast-twitch oxidative) fibres can generate energy either with or without the use of oxygen. They utilise both aerobic and anaerobic metabolism energy producing systems to create energy. Examples of such muscle fibre recruitment include 400m sprint events.
  • Type IIb (fast-twitch glycolytic) fibres use anaerobic metabolism to generate energy and therefore do not require oxygen as an energy fuel. They are predominantly used in fast and powerful bursts during sports such as a 100m sprint. These types of muscle fibres contract muscles at the highest rate but fatigue quite easily within a short space of time.


The proportion of slow-twitch and fast-twitch muscles fibres within each muscle group is determined by genetics. Therefore, some people tend to be naturally gifted with certain specific types of fitness and sports. Previous research has suggested that Olympic sprinters possess approximately 80% fast twitch fibres, while elite marathon runners have 80% slow twitch fibres. No definite evidence suggests whether specific training can have an effect on the proportion of each muscle twitch fibre. However, various training programmes can influence the size, strength and capability of both slow and fast-twitch muscle fibres.

In summary, it is important that personal trainers and fitness instructors are aware of the body’s different genetic composition of both sets of muscle fibres between individuals and that they plan specific exercise regimes tailored to each individual and their sport most effectively & efficiently.

Please feel free to contact the Manchester G4 Clinic to speak to a member of the team for any questions that you may have.

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