Fast-twitch muscles

Fast-twitch muscles: Twitch and you're gone - all you need to know about developing fast-twitch muscle fibre for speed, power and strength

Let’s get out of the blocks straight away, with our fast-twitch fibres blazing; on the ‘B’ of the bang, as Colin Jackson once put it!

There are more than 250 million muscle fibres in our bodies and more than 430 muscles that we can control voluntarily. Fibres are, in fact, 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.

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In order to perform a sport skill numerous muscles and muscle fibres have to interact. The process is controlled by the brain, which sends out electrochemical messages to the muscles via the spinal cord. These signals are received in the muscles by ‘anterior motoneurons’, whose role is to stimulate muscular contraction. Muscular force is generated through the interaction of two protein filaments that constitute the myofibril: actin and myosin.

Anterior motoneurons and motor units can be likened to a car’s starter motor, while the brain is like the key; the former kicks the muscle fibres into action (or rather ‘contraction’) after the latter has been turned.

Some muscles have large numbers of motor units and relatively few fibres, which enables them to execute highly precise movements. One such muscle is the eye, which has one motor unit for every 10 muscle fibres. By contrast, the gastrocnemius (calf muscle), which performs larger, more powerful movements, has 580 motor units to 1.3 million fibres.

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The interaction that occurs at muscular (and tendon and joint) level is two-way, since there are built-in feedback and control mechanisms to prevent muscles from damaging themselves by over-contracting. Proprioceptive (feedback mechanism) components of motor units, joints and ligaments continually monitor muscular stretch and swing into action if, for example, a limb is moved beyond its normal range. This is achieved by muscle spindles ‘pulling back’ on muscle fibres to reduce the stretch. This ‘stretch/reflex’ is a vital component of our body’s muscular safety mechanism, but it can also play a significant role in developing greater fast-twitch muscle power (see table 2 below).

Fast-twitch fibres, also known as ‘white’ or ‘type II’ fibres, contract two to three times faster than their slow-twitch counterparts, producing 30-70 twitches per second, compared with 10-30 for slow-twitch.

There are two basic types of fast-twitch fibre:

  • Type IIa, aka ‘intermediate’ fast-twitch fibres or ‘fast oxidative glycotic’ (FOG) fibres because of their ability to display, when exposed to the relevant training stimuli, a relatively high capacity to contract under conditions of aerobic or anaerobic energy production;
  • Type IIb fibres, the ‘turbo-chargers’ in our muscles, which swing into action for a highperformance boost when needed. These are also known as ‘fast glycogenolytic’ (FG) fibres, since they rely almost exclusively on the short-term alactic/glycotic energy system to fire them up.

Slow-twitch fibres, aka type I, red or slow oxidative fibres, are designed to sustain slow but long-lived muscular contractions and are able to function for long periods on aerobic energy.

Most coaches and athletes will be familiar with type IIa and type IIb fast-twitch fibres, but it should be noted that other types have been identified. Former national athletics coach Frank Dick has described a further seven sub-divisions, although the differences between these are not considered significant enough for them have a crucial effect on sports conditioning.(1)

Fast-twitch fibres are thicker than slow ones and it is the former that grow in size (hypertrophy) when activated by the ‘right’ training.

Activating fast-twitch motor units is the key to improved strength, speed and power. Unlike slow-twitch motor units, which are responsible for most of our day-to-day muscular activity, fasttwitch units are quite lazy and tend to slumber until called to action.

While typing this article, the slow-twitch motor units of my fingers and wrists were getting a good workout. As indicated, they are designed for repeated submaximal, often finite, contractions. It was only when I picked up the computer, the desk it sat on and the 30 reference books I was using to help me write this piece, and hurled the whole lot out of the window in abject frustration at my writing ability, that my larger fast-twitch motor units contributed anything!

The role of mental energy

To recruit these units takes powerful movements, possibly fuelled by an excited hormonal response associated with increased adrenaline and neural stimulation (as with my desk throwing).

In terms of producing more power, this works because the increased mental energy boosts the flow of electrical impulses to the muscle, generating increased muscular tension.

It should be pointed out that extreme levels of this ‘neuronal stimulation’ can lead to impaired sports performance. For example, a golfer relies on the synchronous firing of fast-twitch motor units during the ‘swing’; but if he becomes overly aggressive and ‘tries too hard’ a poor stroke usually results, even though his fast-twitch motor units could be capable of expressing more power because of their increased state of tension.

Fast-twitch muscle fibre is recruited synchronously – ie all at the same time – within its motor unit. This is, in part, a physiological manifestation of a neural activity – sports skill learning. Let’s use sprinting to explain this. Carl Lewis had a wonderful silky sprint action. His finely-honed technique allowed his fast-twitch motor units to fire synchronously and apply power. The end result was championship and world record-breaking form. In short, Lewis’s neural mastery of sprinting form allowed his fasttwitch motor units to fire off smoothly, operating like cogs in a well-oiled machine. It also allowed him to recruit the largest, and therefore most efficient, power-producing units. This latter ability is a further key element in developing optimum fast-twitch motor unit power.

By contrast, slow-twitch muscle motor units are recruited asynchronously, with some resting and others firing when carrying out endurance activity.

Fast-twitch motor units are recruited according to the ‘size principle’, in that the more power, speed or strength an activity requires, the larger the units called in to supply the effort. It would, however, take a flat-out sprint or a near PB power clean to fully activate them. This means that power athletes have to be in the right frame of mind to get the most out of their fast-twitch motor units. There is no such thing as an easy flat-out sprinting session or power-lifting workout.

By contrast, the endurance runner could go for a 60-minute easy ‘tick-over’ effort and drift mentally away from the task while still giving his or her slow-twitch motor units a decent workout.

It is often assumed that those blessed with great speed or strength are born with a higher percentage of fast-twitch muscle fibres, and that no amount of speed work (or neuronal stimulation) will turn a cart-horse into a race horse. But, in fact, fast-twitch fibres are fairly evenly distributed between the muscles of sedentary people, with most possessing 45-55% of both fast- and slow-twitch varieties.

Thus few of us are inherently destined for any particular type of activity, and how we develop will depend mostly on two factors:

  • The way our sporting experiences are shaped at a relatively early age;
  • How we train our muscle fibres throughout our sporting careers.

The table below compares fast-twitch muscle percentages in selected sports activities with those of sedentary individuals – and a very speedy animal. 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!

Table 1: Fast-twitch muscle percentages compared


Fast-twitch muscle fibre (%)



Distance runner


Middle distance runner





83% of the total fibres examined in the rear outer portion of the thigh (vastus lateralis) and nearly 61% of the gastrocnemius were fast-twitch

Adapted from Dick page 109(1) and Williams (97)(2)

Ross et al studied motor unit changes in sprinters and concluded that positive adaptations of muscle to sprint training could be divided into:(4)

  • Morphological adaptations, including changes in muscle fibre type and cross-sectional area – ie the ability of fast-twitch muscle fibres to exert more power by increasing in number and/or size;
  • Metabolic adaptations to energy systems to create more speed – eg a greater ability to complete short repeated maximal efforts, acquired through an improvement in the shortterm alactic/glycotic energy system which is, in turn, gained from the creation and replenishment of high-energy phosphates.

Similar finding were made by Abernethy and his team, who compared sprint training methods with those used by endurance athletes.(4)

Table 2 summarises the best methods for enhancing fast-twitch motor units. Conversely, the wrong training – and even what might in some cases seem to be the ‘right’ training – can compromise their development.

Table 2: The best training methods for fast-twitch motor units



Lifting weights in excess of 60% 1RM

The heavier the weight, the greater the number and size of fast-twitch motor units recruited. A weight in excess of 75% 1RM is required to recruit the largest units

Performing a physical activity flat-out – eg sprinting, swimming, rowing or cycling as fast as possible

Good recoveries are needed to maximise effort. The short-term anaerobic energy system will positively adapt. The minimum speed needed to contribute towards absolute speed development is 75% of maximum

Training your muscles eccentrically

Research indicates that this form of training increases fast twitch motor unit recruitment.(6) An eccentric muscular contraction generates force when muscle fibres lengthen (see plyometric training, below)

Plyometric training

These exercises utilise the stretch-reflex mechanism, allowing for much greater-than-normal force to be generated by pre-stretching a muscle (the eccentric contraction) before it contracts. A hop, bound or depth jump is an example of a plyometric conditioning drill; a long jump take-off is an example of a plyometric sport skill.

Complex training

This can induce greater recruitment of fast-twitch motor units by lulling the protective mechanisms of a muscle into reduced activity, allowing it to generate greater force. Complex training involves combining weights exercises with plyometric ones in a systematic fashion (see PP 114, Feb 1999). A good example is: 1 set of 10 squats at 75% 1RM followed, after a 2-minute recovery, by 10 jump squats, repeated 3 times

Over-speed training

This will have a transferable neural effect only if the athlete consciously moves his own limbs at the increased pace. It includes downhill sprinting and hitting or throwing sports using lighter implements

Good recovery

24-48 hours’ recovery should be taken between very intense plyometric/complex training and speed work sessions. A further 24-36 hours’ recovery will result in an over-compensatory peak – ie opportunity for a peak performance

Sport specific warm-up

This will reduce the risk of injury, increase the receptivity of the neuromuscular system to the ensuing work and reduce the potentially contradictory effects of non-specific preparation on fast-twitch motor units

Mental preparation

Maximum fast-twitch motor unit recruitment can result from specific mental preparation before and during competition


Let’s return to the sprint training research of Ross and his team.(3) They believed that volume and/or frequency of sprint training beyond what is optimal for an individual can induce a shift towards slower muscle contractile characteristics. Basically, this means that if a sprinter were to perform too many under-speed track reps, his top speed would be impaired.

What’s best for power athletes

For 100% power athletes (such as 100m sprinters) and even those involved in sports where occasional maximal or near maximal quick flashes of power are required, such as golf, baseball (pitching and batting) and football (goal keeping), it may well be that high-intensity training sessions, interspersed with long periods of rest, are best for the optimum development of fast-twitch motor units, particularly in-season.

This can make the conditioning process very difficult. In the England cricket team, for example, batsmen are often encouraged to develop their aerobic fitness by running during down times in matches, and during pre-season. Although a general level of aerobic fitness is useful, it is possible that too much steady state work, particularly in-season, could blunt the batsmen’s sharpness and dull their fast-twitch motor units.

In-season it may be far better for them to condition themselves using sprints, medicine ball work and autogenic training (a form of mental conditioning). Think of the cheetah in our muscle fibre distribution table. What does this fastest land animal do? It lies around all day, exploding into action every now and again: fast-twitch fibre development heaven – but hell for its prey!

In support of this point, Ross’s team noted that detraining appeared to shift the contractile characteristics of fast-twitch motor units towards type IIb, thus providing them with more potential oomph. This effect can often be seen in power athletes who sustain minor injuries after a good period of training and are then obliged to train lightly for 2-3 weeks. Afterwards, to their complete surprise, they often produce a PB because the enforced rest has facilitated the fibre shift and upped their fast-twitch potential. Other research has indicated that a decrease in weight training after a prolonged period of training can have a similar effect.(5)

Note, though, that too long a lay-off can produce less positive effects, due to muscle shrinkage (atrophy) in sports where muscle size is also important, eg for shot putters and American football line-men.

John Shepherd

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  1. Dick F Sports Training Principles (4th edition) A&C Black 2002
  2. J Comp Physiol [B] 1997 Nov;167(8):527-35
  3. Sports Med 2001;31(15):1063- 82
  4. Sports Med 1990 Dec;10(6):365-89
  5. Acta Physiologica Scandanavica, 151, 135-142
  6. J of Strength and Conditioning Research vol 16 (1), 9-13

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