Muscle Fibers


Muscle cells are arranged in fibers There are two types of fibers, slow-twitch ones and fast-twitch ones

Slow-twitch fibers contract slowly and are used during jogging, cycling and swimming. These fibers are more efficient users of oxygen while generating energy.

This makes them resistant to fatigue.

Although they lack the power and force of fast-twitch fibers, slow twitch fibers are critical to endurance.

Slow twitch fibers are used for most movements because they are more efficient in meeting the demands of working muscles.

Fast-twitch fibers contract quickly and fatigue rapidly.

This is due to depending on anaerobic metabolism to provide the energy.

Fast-twitch fibers are used in explosive activities like weight training and sprinting. 


The ratio of fast-twitch to slow-twitch muscle fibers is though to be genetically determined but the size of the muscle is not.


    Ratio of fast to slow-twitch fibres is thought to be genetically determined.
    The metabolic capabilities of both types of fibres can improve through specific strength and endurance training.
    Sprinters and weight lifters have a large percentage of fast-twitch fibres.
    Marathon runners generally have a higher percentage of slow twitch fibres.
    Muscles that primarily maintain posture against gravity (core strength) require more endurance and generally have a higher percentage of slow-twitch fibres.
    Muscles that produce powerful, rapid, explosive strength movements



The adaptations that can occur in response to training are shown in the table below.
Aerobic training - in slow-twitch fibres     Anaerobic training - in fast-twitch fibres

    hypertrophy of slow-twitch muscle fibres
    increased capillary supply to muscle fibres, improving gaseous exchange & movement of nutrients and waste products
    increased number and size of mitochondria (energy factory of cells) enabling more efficient energy production
    significant increase in myoglobin content (transports oxygen from the cell membrane to mitochondria)


    increase and efficiency of ATP/PC supply
    increased glycolytic enzymes which improve functioning of the cell
    hypertrophy of red-twitch muscle fibres
    increased tolerance of lactic acid, allowing performance to be sustained for longer periods
    muscle contractions can be made more forcefully and quickly as there are a greater volume of fast-twitch fibres