Get to know your muscle fibres
Stephen Garland describes the major structural, physical, biochemical, and energetic
characteristics of muscles.
All physical performance, especially peak performance, is entirely
dependent on the performance of skeletal muscle. There are many influences on
muscular output, such as oxygen supply, nervous stimulation, coordination,
and nutritional status. One or all of these might limit performance in some way,
but it always boils down to the muscles themselves performing the task. When
Jonathan Edwards, the world record holder in the triple jump, was asked why he
thought he had improved from contender to champion, he said it was due to
improved strength and speed, characteristics of muscle fibres themselves.
Understanding how muscles produce force is essential for any athlete or coach
to plan training to improve force production, whether it is to
build greater force or maintain a force for longer.
Why do muscles contract?
Muscle fibres are long, thin, tapered, cylindrical cells full of the mechanisms required to convert chemical energy into movement. They are large compared with other cells, with a length of a few millimetres to a few centimetres and a diameter of 50-100 micrometres. In comparison, a red blood cell has a diameter of about 7 micrometres. Fibres are arranged parallel to each other and usually lengthways, although sometimes they run diagonally to the length of the muscle ("pennate" muscles). A sheath of collagen, a connective tissue, for structural support, surrounds individual fibres. Bundles of tens of fibres and the whole muscle are surrounded by more connective tissue. Blood vessels, motor neurons (the sort of nerve that innervates muscle fibres) and other nerves wind in between the bundles. The contractile apparatus in each fibre is arranged in parallel, long, cylindrical strands, called myofibrils. Mitochondria and other organelles sit in the cell fluid (cytoplasm) surrounding these myofibrils. Actin and myosin are the contractile protein polymers contained in myofibrils, and they too are long and lie parallel and lengthwise. Using energy derived from ATP, the actin and myosin "filaments" attach via cross bridges and slide past each other in opposite directions, thus causing a contraction. This is the "sliding filament' and cross-bridge theory, which explains how muscles shorten.
Why are fast-twitch muscle fibres fast?
So far, so simple; now for some more specialisation. You no doubt
have heard of "white" and "red" or "slow" and "fast" muscles. There are seven or more types of fibre, but for simplicity, each fibre is usually put into
one of two categories.
Fibre type |
Slow-twitch |
Fast-twitch |
Contraction speed |
Slow |
Fast |
Energy source |
Oxidation |
Glycolysis |
Fatigue resistant |
Yes |
No |
Fast-twitch (FT) fibres shorten a given length in a shorter time
than slow-twitch (ST) fibres.
What is different about FT fibres that makes them contract
faster?
Force is generated by a cross-bridge between actin and myosin
filaments, causing them to slide past each other. The cause of this movement is
a conformational change in the cross-bridge molecule. Like an oar in a
rowing boat, it reaches out from the myosin filament (or rowing boat) and grabs
onto the actin (or water) and pulls the actin towards it, and then pushes it
away. The cross-bridge oar is then recycled so it can grab onto another bit of
actin (water) and so continue the contraction. The conformational change is
brought about by changes in concentrations of certain other molecules in the
fibre, and some biochemical reactions. Enzymes are large biological molecules
that speed up such chemical reactions. An everyday example of an enzyme can be
found in biological washing powder, which speeds up the breakdown of the
spaghetti sauce you got on your shirt.
One key difference between slow and fast-twitch fibres is the
ability of one of these enzymes, myosin ATPase, to speed up the cross-bridge
action. There is a small structural difference between FT and ST myosin ATPase,
which is enough to cause a difference in cross-bridging recycling rates. So fundamental is this enzyme to the characterisation of different fibre
types that one standard way for molecular biologists to differentiate the
fibre types in biopsies is to look at the particular type of myosin ATPase
present in the fibre. There is also a higher concentration of myosin ATPase in
FT, giving the fibre the ability to produce more rowing strokes per unit time.
The splitting of ATP to produce energy for muscle contraction is carried out by
this myosin ATPase at a site on the myosin "heavy chain" - part of the myosin
filament. There are slow and fast varieties of the myosin heavy chain
producing, not surprisingly, slow and fast contractions.
Last but perhaps most important, FT fibres contain a higher
density of filaments. There's none of this messing around with packing in
mitochondria or a fuel supply: FT is designed for maximum force, speed, and
power.
Why are slow-twitch fibres better for endurance?
To perform physical work, you need to burn a substrate, and
slow-twitch fibres contain plenty. They have more glycogen (carbohydrate)
granules and lipid (fat) droplets than fast-twitch, which allows them to
exercise for longer. Together with the substrate, you need oxygen to combust it
with, and slow-twitch fibres have both a higher density of capillaries
supplying oxygen to the muscle and high concentrations of myoglobin (the
intramuscular protein that can store oxygen). But it's not a case of
a burning substrate to produce the energy required for movement - there's a
complex series of reactions using different bits of machinery and enzymes. Slow-twitch fibres have lots of mitochondria, which contain the structure and
enzymes required for the oxidation of the substrate, and so are the engine of the
cell. Notice the word "oxidation" - it's also in the table showing the
differences in fibre types. A deficiency of mitochondria limits the capacity for FT to do aerobic work. If any more demand is put on FT fibres, more
and more energy is derived from the anaerobic glycolytic pathway, whereas the
ST can increase their aerobic activity. Anaerobic activity results in
the production of hydrogen ions and rapid fatigue. Which of all the muscles in
the body has the greatest need to be non-fatiguing? The cardiac muscle of the
heart is different from skeletal muscle in many ways. Which muscle has
the highest mitochondrial density? - The heart.
FT fibres, even when they are metabolising aerobically, are also
less efficient at using oxygen, requiring more oxygen molecules to produce the
same number of ATP molecules.
Why do we have both types of fibre in the same muscle?
There are apparent evolutionary advantages to being able to move
quickly or for long periods, and there is an even greater advantage in being
able to do both. You can escape from your quick predators (or surprise your
prey) by using FT and then keep ahead (or wear the prey down) by using your
ST. You won't be surprised to hear that there are more adaptations for just
that sort of thing. FTs are generally recruited for significant, powerful movements. One
motor neuron innervates more than one muscle fibre by branching off towards
its end, but always innervates the same fibre type. The sum of the neuron and
the fibres it innervates is called a motor unit. FT motor units are larger
(i.e. they contain more fibres) than ST and are used in significant movements. The
speed of conduction of the signal along the motor neuron is also greater in
FT, which leads to quick reactions to the lion that surprised you and also,
just as important, since FT are the fibres recruited, to a quick getaway. ST
fibres come in smaller motor units. This allows for smaller, more controlled
movements such as handwriting or eye movement. These are longer-term everyday
tasks, and so recruiting fatigue resistance is a positive advantage. Migrating
birds have ST flapping muscles predominantly; cheetahs have FT running muscles.
Human leg muscles, which are usually the ones involved in locomotion, typically
have a mixture of FT and ST. As a power athlete, I am sure you would prefer to
be a cheetah, rather than a goose.
Article Reference
This article first appeared in:
- GARLAND, S. (2003) Get to know your muscle fibres. Brian Mackenzie's Successful Coaching, (ISSN 1745-7513/ 2 / June), p. 8-9
Page Reference
If you quote information from this page in your work, then the reference for this page is:
- GARLAND, S. (2003) Get to know your muscle fibres [WWW] Available from: https://www.brianmac.co.uk/articles/scni2a7.htm [Accessed
About the Author
Brian Mackenzie was a British Athletics level 4 performance coach and a coach tutor/assessor. He has been coaching sprint, middle distance, and combined event athletes for the past 30+ years and has 45+ years of experience as an endurance athlete.