Muscle fiber is the morphological unit of skeletal muscle or, more simply, one of the many cells that make it up. Each muscle is in fact formed by a certain number of fascicles, in turn made up of cells called, precisely, muscle fibers. Thanks to these cylindrical units, the chemical energy released by metabolic reactions is transformed into mechanical energy which, by acting on the bone levers, realizes the movement.
Muscle fibers, grouped in bundles, have a variable length from a few mm to several cm, with a diameter ranging from 10 to 100 µm (1 µm = 0.001 mm). For this reason the anatomy describes them as long cylindrical cells, polynucletate, because they contain numerous nuclei near their surface. Instead they contain thousands of filaments, called myofibrils, containing contractile units called sarcomeres. Skeletal muscle fibers are the largest cells in the body.
Physiologists dealing with muscles tell us that the various fibers differ, not only from an anatomical point of view, but also for some specific physiological characteristics:
inside each muscle different types of fibers can be recognized, classified according to the speed of contraction and fatigue resistance.
Fast-contracting white fibers
Fast-twitch fibers (white, type II or FT, from the English "Veloce twitch"), intervene in rapid and intense muscular actions. Inside we find a high concentration of the enzymes typical of the anaerobic alattacid and glycolytic metabolism.
The fast-twitch fibers are innervated by α motoneurons, very large and with large-caliber axons, specialized in the fast transmission of nerve impulses.
The density of the capillary bed is rather low, especially when compared with the second type of fiber that we will describe in a few lines; also reduced the content in myoglobin, mitochondria and oxidative enzymes. The speed of contraction and the developed strength are however from two to three times higher.
Fast fibers are recruited during short duration exercises that require a large neuromuscular effort. They are activated only when the recruitment of slow-twitch fibers is maximum.
In response to an intense physical effort the smallest motor units are activated first and, as the intensity increases, there is a progressive greater recruitment of the fast fibers
Alongside purely fast fibers, which develop high forces but quickly become fatigued (type IIb or FF, from the English Veloce fatiguable), there are other fibers with a slightly lower contraction speed but with greater resistance (type IIa or FR, from English Veloce fadigue resistant). Because of these transition characteristics, the IIa fibers are also known as "intermediate fibers", a sort, that is, of a point of transition from fast to slow ones. This transition can be stimulated, in one way or another, through specific long and repeated training sessions for a sufficiently long period of time.
In adult skeletal muscles there is a third type of fiber, called IIx, with intermediate characteristics between IIa and IIb.
The sprinters' muscles have a high percentage of type IIb white fibers.
Red fibers with slow contraction
Slow-twitch muscle fibers (red, type I or ST, from the English "slow twitch"), are recruited in low-intensity but long-lasting muscle actions.
Slow fibers | Fast fibers | Intermediate fibers | |
Atp production | Oxidative phosphorylation (aerobic) | Glycolysis (anaerobic lactate) phosphocreatine (anaerobic alactacid) | Oxidative phosphorylation (aerobic) Glycolysis (anaerobic lactate) |
| Oxidative enzymes | Abundant | poor | Intermediate characteristics |
| Glycolytic enzymes | meager | Abundant | |
Color (myoglobin) | Intense red | Clear | |
Mitochondria | Numerous | meager | |
| Energy substrates | Mainly lipids | Mostly carbohydrates | |
Fiber diameter | Small with many capillaries | Great with a few capillaries | |
Features motor neurone | Small axon and body mobile phone, low speed of conduction e discharge frequency | Great axon and body mobile phone, high speed of management and frequency of download | |
Speed of fatigue | slow | rapid | |
Feature | Maintain activities tonic for long periods | They maintain an activity explosive and powerful for few moments |
| Type I fibers (red or slow) | Type IIa fibers (Intermediate) | Type IIx fibers (intermediate white) | Type IIb fibers (white or fast) | |
| Time of contraction | Slow | Moderately Fast | Fast | Very fast |
| Size of motor neurons | Small | Average | Great | Great |
| Fatigue resistance | high | Fairly high | Intermedia | Low |
| Type of activity for which they are responsible | Aerobics | Prolonged Anaerobic | Anaerobic in the short term | Anaerobic in the short term |
| Maximum duration of use | hours | <30 minutes | <5 minutes | <1 minute |
| Power Produced | Low | Average | high | Very High |
| Mitochondria density | high | high | Average | Low |
| Capillary density | high | Intermedia | Low | Low |
| Oxidative capacity | high | high | Intermedia | Low |
| Glycolytic capacity | Low | high | high | high |
| Main storage fuel | Triglycerides | Phosphocreatine, glycogen | Phosphocreatine, glycogen | Phosphocreatine, glycogen |
The conduction of the nervous stimulus is not as rapid as in the previous case, but much more continuous and stable over time. The motoneurons that innervate the red fibers are in fact smaller than those that transmit the nerve impulse to the fast fibers. While the former discharge continuously at low frequencies, the latter discharge repeatedly with high frequency salvos.
In the marathon runners, road cyclists and other athletes engaged in endurance sports, a clear predominance of slow fibers is observed: a feature partly of genetic origin and partly due to the process of adapting the intermediate fibers.
DID YOU KNOW THAT: the strength developed by a muscle fiber depends on its length at the beginning of the contraction. It must have an optimal value, outside which (muscle retracted or excessively lengthened) the strength performance is reduced.
The white muscles, rich in type IIb fibers (but also type IIa), are called FASIC MUSCLES, because they are capable of rapid and short contractions.
The red muscles, where type I fibers prevail, are called MUSCOLI TONICI, due to their ability to remain in contraction for a long time.
CONTINUE: muscle fibers and training »
Hatfield and Poliquin tests for muscle fiber composition »
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