It is not the human skeletal muscles that are fast or slow. Rather, each skeletal muscle (physically and chemically) consists of different types of muscle fibers. Ranvier (1873) was the first to describe the differences in skeletal muscle fibers. He distinguished between “red, tonic” and “white, phasic” muscle fibers and their contractile properties. Step by step, further research resulted in more and more differentiation possibilities of the muscle fiber types, which opened up a wealth of new insights.
The properties of the individual muscle fiber types
On the basis of today’s modern measuring methods, two types of muscle fibers are distinguished in the human skeletal muscle: Type I and Type II, whereby Type II fibers are again divided into Type IIa and Type IIx (formerly IIb).
Type I muscle fibers
- slow muscle fibers, so-called slow twitch fibers ST fibers
- Slow small motor units, contract and slacken relatively slowly (approx. 80 ms).
- the slow and permanent pace of work
- Primary posture control
- Many mitochondria and capillaries as well as a lot of myoglobin, thus the “red color”
- can use fat burning quickly and extensively for energy supply if required
can use fat burning for energy supply if required
- Hardly tiring
- Good regeneration
Type IIa and IIx muscle fibers
- Fast muscle fibers, so-called FT fibers or Type II fibers
- Fast large motor units, they contract in a relatively short time (approx. 30 ms), high working speed, as required.
- Primary ballistic movements of the extremities
- Little mitochondria, little myoglobin, thereby the “white color”
- Fast fatigue
- poor regeneration
Type IIa muscle fibers
- Force development (how much stress in which time) higher than type I
- about 3 to 5 times faster than Type I fibers
Type IIx muscle fibers
- force development (how much stress in which time) significantly higher than type I and higher than type IIa
- About 10 times faster than Type I fibers
The genetic component plays a major role in the proportion of individual muscle fiber types in human skeletal muscles. Basically, all skeletal muscles have all muscle fiber types. However, with great individual weighting. A 50:50 distribution of type I fibers and type II fibers is the norm here (Saltin and Gollnick, 1983).
The weighting of the muscle fiber composition becomes clear with athletes of the different disciplines: While endurance-trained people partly have highly dominant type I fibers, it is the type II fibers with sprinters and high-speed athletes.
In 1990, Eriksson operated on a cross-country skier and controlled the muscle fiber composition of the thigh in a sequence of muscle biopsies.
- On the day of surgery, the muscle showed 80 % type I fibers.
- One month after the inevitable immobilization 57 %.
- After six months of training 84 % again.
Therefore, the question of the training-induced transformation from one type of muscle fiber to another is of considerable practical importance. In further studies (Bouchard 1994) it was found that apparently only 20% of the muscle fiber composition can be changed by training.
The main meaning of the training, however, lies in the targeted enlargement of the cross-section and thus the surface of the muscle, not in a transformation from fast to slow muscle fibers or vice versa.
Types of contraction of skeletal muscle fibers
A stimulus from a nerve lead to a so-called action potential (AP). The AP spreads via the muscle fiber into the T-system. The gradation of muscle strength is achieved by different recruitment (= number of excited motor units) of the musculature and the change of the action potential frequencies.
A single stimulus from a nerve always leads to a maximum Ca ++ release and thus to a maximum single twitch of the skeletal muscle fiber (all-or-nothing rule). A further shortening is only achieved if a second stimulus occurs during this single twitch. The summation of such stimuli causes filament slippage and thus an increase in tension or movement in the skeletal muscle fiber.
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