In the previous lesson, we considered the diversity of human movements with reference to the classical guitarist Karen Schaupp and surfer Molly Picklum. Associated with this variability in types of movement is diversity in skeletal muscle fibres types.
Detailed analyses of skeletal muscle fibres have revealed quite marked variation in their structural, metabolic and mechanical properties. Not surprisingly, this variation has fairly significant consequences for their function.
The relative proportions of these types of muscle fibres varies between muscles and from person to person, and indeed may play a role in determining the types of sporting and other activities that we choose to participate in.
There are two major types of skeletal muscle fibres classified originally upon the rate at which they developed force when contracting (as shown in the diagram opposite):
Slow-Twitch (Type I) Fibres
As their name suggests, these fibres develop force at a comparatively slow rate. They are typically smaller in diameter, have a rich capillary network surrounding them and have particularly high concentrations of myoglobin (an oxygen-binding protein) in their sarcoplasm.These types of fibres appear to be fairly resistant to fatigue and particularly well suited to movements requiring low force but sustained contractions (e.g. distance running).
Fast-Twitch (Type II) Fibres
These fibres are significantly larger in diameter and develop force roughly three times faster that slow-twitch fibres. Some fast-twitch fibres are fairly resistant to fatigue and have a fairly high myoglobin concentration (and are known as Type IIa fibres) whilst other are more easily fatigable and have lower concentrations of myoglobin (Type IIb fibres).Type II fibres are better suited for short duration, high force type activities (e.g. sprinting or pole-vaulting).
In humans, individual muscles contain a mix of slow-twitch and fast-twitch muscle fibres, so what determines when these different muscle fibres are recruited? You will recall that a motor unit consists of a motoneurone together with all the muscle fibres that it innervates. Well, it turns out that the muscle fibres that form a motor unit are always of the same type. Consequently, the nervous system can select to activate slow-twitch or fast-twitch fibres (or even their sub-types), simply by activating the appropriate motoneurone.
Which skeletal muscle fibre types are activated depends entirely upon the type of activity being undertaken:
- Slow-twitch fibres are recruited when the muscle is required to produce any force at all but are fairly limited in the maximal force that they can generate.
- Type IIa fast-twitch fibres are recruited when intermediate forces are required.
- Type IIb fast-twitch fibres are recruited when higher intensity activities are undertaken that require even more force to be generated.
| Runner | Slow-Twitch | Fast-Twitch |
|---|---|---|
| Endurance | 60-80 % | 20-40% |
| Sprinters | 25-40 % | 60-75 % |
Given the functional diversity of skeletal muscle fibre types it is not surprising that exercise physiologists have considerable interest in the relative number of these fibre types in different athletes.
Clearly muscles that have a higher proportion of fast-twitch fibres will produce more power than muscles containing predominantly slow-twitch fibres. Such muscles would be particularly suited to high-intensity (power) sports.
On the other hand, muscles with a predominantly slow-twitch fibre type would be more efficient in endurance sports.
As shown in the table on the right, analysis of the relative proportion of slow-twitch and fast-twitch fibre types in elite athletes has revealed a predominance of slow-twitch fibres in endurance runners and fast-twitch fibres in sprinters.