Speed and power training: understanding your fast twitch muscles will boost performance

The role of muscles and muscle fibre in sprinting

Muscles and muscle fibre

There are more than 250 million muscle fibres in our bodies, and more than 430 muscles that we can voluntarily control. Muscle fibres are bundles of cells, held together by collagen (connective tissue). Each fibre consists of a membrane, numerous nuclei and thousands of myofibrils (inner strands) that run the length of the fibre. In order to perform sports skills, numerous muscles and muscle fibres have to interact. These are ‘controlled’ via messages sent from the brain through the spinal cord and out to the muscles. When these signals reach the muscles, they are received by what’s known as anterior motor neurons – at this junction reside the collective of muscle motor units and muscle fibres. The chemical acetylcholine changes the electrical signal into a chemical reaction within the motor units, and muscular force is generated through the interaction of two protein filaments that constitute the myofibril: actin and myosin.

Some muscles have large numbers of motor units and low numbers of fibres – this enables them to execute highly precise movements like the eye, which has one motor unit to every 10 muscle fibres. This contrasts with other body parts that perform much more powerful, larger movements, such as the thigh and calf muscles. As an example, the gastrocnemius (largest calf muscle) has 580 motor units to 1.3 million fibres.

Fast-twitch muscle fibres

Fast-twitch muscle fibres contract two to three times faster than slow-twitch muscle fibres. They have a ‘twitch rate’ of 30-70 twitches per second. These fibres are also known as white, or type II fibres.

There are two types of fas-twitch fibre - Type IIa and Type IIb
Type IIa or ‘intermediate’ fast-twitch fibres are also termed, ‘fast oxidative glycolytic’ (FOG), because of their ability to display, when subject to the relevant training stimuli, a relatively high capacity to contract under conditions of aerobic or anaerobic energy production. This means they are relatively enduring and contribute to longer-lasting sports activities, such as 400 and 800m running, and tennis rallies.

Type IIb fibres are the turbo charger muscle fibres. These fibres are also known as ‘fast glycogenolytic’ (FG) fibres. They rely, almost exclusively, on the immediate energy system to fire them up (see sports science made simple, body chemicals, parts 1 and 2).

Fast-twitch muscle fibre is thicker than slow-twitch fibre. And it’s these fibres that are primarily responsible for increases in muscle size when subject to the relevant training stimuli.

Activating fast-twitch motor units is the key to improved strength, speed and power

Correctly and consistently training fast-twitch fibre is the key to improved strength, speed and power. Paradoxically, these fibres are quite lazy. They need to be prodded into action by considerable mental input. This ups the flow of electrical impulses to the motor units and literally switches them on. If you are really focused and in ‘the zone’ the flow of electrical impulses from the brain can be virtually continuous –
this continuity results in target muscles being unable to relax, which in turn results in increased muscular tension and power capability. However, it should be noted that extreme levels of this ‘neuronal stimulation’ can lead to impaired sports performance. To clarify: if a golfer were to become overly aggressive in an attempt to drive the ball as far as they could from the tee, a poor stroke usually results. This is the result of increased tension and its impairment of skill.

Fast-twitch muscle fibre is recruited asynchronously within its motor unit
Fast-twitch muscle fibre is recruited asynchronously (meaning in a step-by-step pattern) within its motor unit according to the ‘size principle’. As an activity requires more power, speed or strength, increasingly larger units are called into action to power the activity. It will take a flat-out sprint, or a near PB power clean to fully activate them. And, as noted, this requires the power athlete to be in the right frame of mind. There is no such thing as an easy flat-out sprinting session. This contrasts with the endurance runner who, for example, could go for a 60-minute easy ‘tick over’ effort and drift mentally away from their running, while still creating a positive training effect on their slow-twitch muscle fibre and CV system.

Do the percentages of fast-twitch muscle fibre vary between people?
It’s often thought that those who are blessed with great speed, or great strength, are born with a higher percentage of fast-twitch muscle fibres, and that no amount of speed and power training will turn a ‘carthorse into a racehorse’. Although this may be true within certain parameters, the distribution of fast-twitch fibres is actually fairly evenly distributed between our muscles at birth. Most of us will actually possess somewhere between 45-55% fast-twitch and 45-55% slow-twitch fibres. So many of us without training; will be neither super-endurance, nor super-fast, strong or powerful, potential athletes. The path towards a speed, or power orientated, sport, or an endurance one, will reflect genetics to a certain degree, but more pertinently for the majority:

•    The way our sporting experiences are shaped at a relatively early age.
•    Crucially, how we train our muscle fibres throughout our sporting career.

Table 1: Fast-twitch muscle percentages in selected sports activities, compared to the sedentary (and a very speedy animal!)


Individual Percentage fast-twitch muscle fibre
Sedentary 45-55%
Distance runner 25%
Middle-distance runner 35%
Sprinter 84%
Cheetah 83% of the total number of fibres examined in the rear outer portion of the thigh (vastus lateralis), and nearly 61% of the calf (gastrocnemius) were comprised of fast-twitch fibres.

Note: the extremes of muscle-fibre distribution. The right training will positively develop more of the fibres needed for either dynamic or endurance activity, although the cheetah may not be aware of this!

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