Get to know your muscle fibres
Stephen Garland describe 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 and coordination, or nutritional status. One or all of these might limit performance in some way, but it always boils down to the muscles themselves to perform 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 with the aim of improving force production, whether it is to produce 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 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 lengthways. 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 theories 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 in fact seven or more types of fibre, but for simplicity each fibre is usually put into one of two categories.
Fast-twitch (FT) fibres shorten a given length in a shorter time than slow-twitch (ST).
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. Just like an oar in a rowing boat, it reaches out from the myosin filament (or rowing boat) and grabs on to 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 on to 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. In fact, 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 are 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 allow 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 intra-muscular protein that can store oxygen). But it's not simply a case of 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 substrate, and so are the engine of the cell. Notice that word "oxidation" - it's also in the table showing the differences in fibre types. The capacity for FT to do aerobic work is limited by a deficiency of mitochondria. If any more demand is put on FT fibres, more and more energy is derived from the anaerobic glycolytic pathway, whereas the ST can simply 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, which 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 obvious 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. FT are normally recruited for big, 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 big 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 predominantly ST flapping muscles; cheetahs have FT running muscles. Human leg muscles, which are normally the ones involved in locomotion, usually 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.
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