Muscle fibers. Types of muscle fibers

Thin muscle fibers form each skeletal muscle. Their thickness is only about 0.05-0.11 mm, and the length reaches 15 cm. Muscular fibers of the striated muscle tissue are collected in bundles containing 10-50 fibers each. These bundles are surrounded by a connective tissue (fascia).

The muscle itself is also surrounded by fascia. About 85-90% of its volume is made up of muscle fibers. The rest is the nerves and blood vessels that pass between them. At the ends, the muscle fibers of the striated muscle tissue gradually pass into the tendons. The latter are attached to the bones.

Mitochondria and myofibrils in muscles

Consider the structure of the muscle fiber. In the cytoplasm (sarcoplasm), there is a large number of mitochondria. They play the role of power plants in which metabolism occurs and energy-rich substances accumulate, as well as those needed to meet energy needs. In the composition of any muscle cell, there are several thousand mitochondria. They occupy about 30-35% of its total mass.

The structure of the muscle fiber is such that the chain of mitochondria is aligned along the myofibrils. These are fine threads, which ensure the contraction and relaxation of our muscles. Usually in one cell there are several tens of myofibrils, while the length of each can reach several centimeters. If you combine the mass of all the myofibrils that make up the muscle cell, then its percentage of the total mass will be about 50%. The thickness of the fiber, therefore, depends primarily on the number of myofibrils present in it, and also on their transverse structure. In turn, myofibrils consist of a large number of tiny sarcomeres.

Cross-striated fibers are characteristic of muscle tissues of both women and men. However, their structure is somewhat different depending on sex. According to the biopsy of muscle tissue, it was concluded that in the muscle fibers of women the percentage of myofibrils is lower than that of men. This applies even to high-level athletes.

By the way, the muscle mass itself is distributed unequally over the body in women and men. The overwhelming part of it is in the lower part of the body. In the upper volumes of muscles are small, and they are small and often completely untrained.

Red fibers

Depending on fatigue, histochemical staining and contractile properties, the muscle fibers are divided into the following two groups: white and red. Reds are slow fibers that have a small diameter. In order to get energy, they use the oxidation of fatty acids and carbohydrates (this energy generation system is called aerobic). These fibers are also called slow or slow-cutting fibers. Sometimes they are called type 1 fibers.

Why did the red fibers get that name?

They are called red because they have a red histochemical color. This is due to the fact that these fibers contain a lot of myoglobin. Myoglobin is a special pigment protein that has a red color. Its function is that it delivers oxygen down the muscle fiber from the blood capillaries.

Features of red fibers

Slow muscle fibers have many mitochondria. They carry out the oxidation process, which is necessary for obtaining energy. Red fibers are surrounded by a large network of capillaries. They are needed to deliver a large volume of oxygen along with blood.

Slow muscle fibers are well adapted to the implementation of an aerobic energy generation system. The power of their reductions is relatively small. The speed with which they consume energy is sufficient to manage only aerobic metabolism. Red fibers are great for carrying out non-intensive and long-lasting work, such as walking and easy running, stayer distances in swimming, aerobics, etc.

Reduction of the muscle fiber ensures the performance of movements that do not require much effort. Thanks to him, the posture is also maintained. These striated fibers are inherent in muscle tissues, which are involved in work at loads ranging from 20 to 25% of the maximum possible force. They are characterized by excellent endurance. However, red fibers do not work in sprint distances, heavy lifting, etc., because these types of loads assume a fairly fast flow and energy production. For this purpose white fibers are intended, which we will now talk about.

White Fibers

They are also called fast, fast-twitch type 2 fibers. Their diameter is larger in comparison with the red ones. To obtain energy, they use mainly glycolysis (that is, the system of energy production in them is anaerobic). In fast fibers there is a smaller amount of myoglobin. That's why they are white.

ATP cleavage

Fast fibers are characterized by high activity of the ATPase enzyme. This means that the cleavage of ATP occurs rapidly, and a large amount of energy is obtained, which is needed for intensive work. Since white fibers are characterized by a high rate of energy expenditure, they need a high rate of ATP-molecule recovery. And it is able to provide only the process of glycolysis, since, unlike oxidation, it occurs in the sarcoplasm of muscle fibers. Therefore, oxygen delivery to mitochondria is not required, as is the delivery of energy from the latter to myofibrils.

Why do white fibers get tired quickly?

Thanks to glycolysis, lactate (lactic acid) is formed, rapidly accumulating. Because of this, white fibers get tired quite quickly, which ultimately stops the work of the muscle. In red fibers, aerobic formation does not produce lactic acid. That is why they can maintain a moderate stress for a long time.

Features of white fibers

White fibers are characterized by a large diameter relatively red. In addition, they contain much more glycogen and myofibrils, but mitochondria are smaller in them. A cell of this type of muscle fiber also contains creatine phosphate (CF). It is required at the initial stage of high-intensity work.

Most white fibers are designed to perform powerful, fast, but short-term efforts, because they have low endurance. Fast fibers, in comparison with slow, are capable to be reduced in 2 times faster, and also to develop force, in 10 times greater. The maximum speed and strength of a person develops precisely because of them. If the job requires 25-30% of maximum effort and higher, it means that it is the white fibers that take part in it. They are divided by the method of obtaining energy for the next 2 types.

Fast glycolytic fibers of muscle tissue

The first type is fast glycolytic fibers. The process of glycolysis is used by them to generate energy. In other words, they are able to use only anaerobic energy generation system, which promotes the formation of lactic acid (lactate). Accordingly, these fibers do not produce energy with the participation of oxygen, that is, by aerobic means. The fast glycolytic fibers are characterized by the maximum rate of contraction and strength. They play a major role in the recruitment of mass athletes, bodybuilders, as well as provide runners and swimmers performing at sprint distances, the maximum speed.

Fast oxidation-glycolytic fibers

The second type is fast oxidation-glycolytic fibers. They are also called transitional or intermediate. These fibers are a kind of intermediate type between slow and fast muscle fibers. They are characterized by a powerful system of energy generation (anaerobic), but are also adapted to the implementation of a fairly intense aerobic load. In other words, these fibers can develop great efforts and a high rate of contraction. The main source of energy is glycolysis. At the same time, if the intensity of reduction becomes low, they are able to use oxidation quite efficiently. This type of fiber is used in the work, if the load is from 20 to 40% of the maximum. However, when it is about 40%, the human body immediately goes completely to using fast glycolytic fibers.

Ratio of fast and slow fibers in the body

Studies were conducted in the process of which it was established that the ratio of fast and slow fibers in the human body is genetically determined. If we talk about the average person, he has about 40-50% slow and about 50-60% fast. However, each of us is individual. In the body of a particular person, both white and red fibers can predominate.

Proportional ratio of them in different muscles of the body is also not the same. This is because the muscles and their groups in the body perform various functions. It is because of this that the transverse muscle fibers are quite different in composition. For example, in the triceps and biceps there are approximately 70% of white fibers. Slightly less in the hip (about 50%). But in gastrocnemius muscle of these fibers only 16%. That is, if the functional task of this or that muscle involves more dynamic work, it will have more fast, rather than slow ones.

Communication of potential in sport with types of muscle fibers

We already know that the overall ratio of red and white fibers in the human body is genetically laid. Because of this, different people have different potentials in sports activities. Someone is better given sports that require endurance, and someone - power. If slow fibers predominate, a person is much more suited to skiing, marathon running, long-distance swimming, etc., that is, sports that involve mainly an aerobic energy education system. If there are more fast muscle fibers in the body, then you can achieve good results in bodybuilding, short-distance running, sprint swimming, weightlifting, powerlifting, etc., where explosive energy is of prime importance. And it, as you already know, can provide only white muscle fibers. The great athletes-sprinters are always dominated by them. The number of them in the muscles of the legs reaches 85%. If an approximately equal ratio of different types of fibers is observed, the average distance in running and swimming is perfect for a man. However, what has been said above does not at all mean that if fast fibers predominate, such a person will never manage to run a marathon distance. He will run through it, but certainly will not become a champion in this sport. Conversely, if there are much more red fibers in the body, the results in bodybuilding will be worse for such a person than for the average one, the ratio of red and white fibers is approximately equal.