Thin muscle fibers form each skeletal muscle. Their thickness is only about 0.05-0.11 mm, and the length reaches 15 cm. The muscle fibers of the striated muscle tissue are collected in bundles, which include 10-50 fibers each. These bundles are surrounded by 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 run 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) it contains 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 that are needed to meet energy needs. As part of anyA muscle cell has several thousand mitochondria. They occupy approximately 30-35% of its total mass.
The structure of the muscle fiber is such that a chain of mitochondria lines up along the myofibrils. These are thin threads that provide contraction and relaxation of our muscles. Usually in one cell there are several tens of myofibrils, while the length of each can reach up to several centimeters. If you add up the mass of all 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 in it, as well as on their transverse structure. In turn, myofibrils are made up of a large number of tiny sarcomeres.
Striated fibers are characteristic of the muscle tissues of both women and men. However, their structure is somewhat different depending on the sex. According to the results of a biopsy of muscle tissue, it was concluded that the percentage of myofibrils in the muscle fibers of women is lower than that of men. This applies even to high-level female athletes.
By the way, the muscle mass itself is distributed unevenly throughout the body in women and men. The vast majority of it in women is in the lower body. In the upper, the volumes of muscles are small, and they themselves are small and often completely untrained.
Red fibers
Depending on fatigue, histochemical staining and contractile properties, muscle fibers are divided into the following two groups: white and red. Reds represent slowfibers with a small diameter. In order to get energy, they use the oxidation of fatty acids and carbohydrates (such an energy production system is called aerobic). These fibers are also called slow or slow twitch. They are sometimes referred to as type 1 fibers.
Why red fibers got their name
Red they are called due to the fact that they have a red histochemical color. This is because 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 deep into 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 energy production. The red fibers are surrounded by a large network of capillaries. They are needed to deliver a large amount of oxygen along with the blood.
Slow muscle fibers are well adapted to the implementation of the aerobic system of energy generation. The strength of their contractions is relatively small. The rate at which they consume energy is sufficient to make do with aerobic metabolism alone. Red fibers are great for low-intensity, long-duration activities such as walking and jogging, distance swimming, aerobics, and more.
The contraction of the muscle fiber providesperforming movements that do not require much effort. Thanks to him, the posture is also maintained. These striated fibers are characteristic of muscle tissues, which are included in the 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 during sprint distances, heavy lifting, etc., since these types of loads involve a fairly rapid consumption and receipt of energy. This is what white fibers are for, which we'll talk about now.
White fibers
They are also called fast twitch type 2 fibers. Their diameter is larger than the red ones. To obtain energy, they use mainly glycolysis (that is, their energy production system is anaerobic). Fast fibers contain less myoglobin. That is why they are white.
ATP breakdown
Fast fibers are characterized by high activity of the ATPase enzyme. This means that the breakdown of ATP occurs quickly, while obtaining a large amount of energy, which is needed for intensive work. Since white fibers are characterized by a high rate of energy consumption, they also need a high rate of recovery of ATP molecules. And only the process of glycolysis can provide it, since, unlike oxidation, it occurs in the sarcoplasm of muscle fibers. Therefore, oxygen delivery to mitochondria is not required, as well as energy delivery from the latter to myofibrils.
Why white fibers get tired quickly
ThanksGlycolysis is the formation of lactate (lactic acid), rapidly accumulating. Because of this, the white fibers get tired quickly enough, which eventually stops the muscle from working. In red fibers during aerobic formation, lactic acid is not formed. That is why they can maintain moderate tension for a long time.
Features of white fibers
White fibers are characterized by a large diameter relative to red ones. In addition, they contain much more glycogen and myofibrils, but they have fewer mitochondria. This type of muscle fiber cell also contains creatine phosphate (CP). It is required at the initial stage of high-intensity work.
Most of all, white fibers are adapted for making powerful, fast, but short-term efforts, since they have low endurance. Fast fibers, compared to slow fibers, are able to contract 2 times faster, and also develop strength 10 times greater. It is thanks to them that a person develops maximum speed and strength. If the work requires 25-30% of the maximum effort and above, this means that it is the white fibers that take part in it. They are divided according to the method of obtaining energy into the following 2 types.
Fast glycolytic muscle fibers
The first type is fast glycolytic fibers. The process of glycolysis is used by them to obtain energy. In other words, they are able to use only the anaerobic energy system that promotes the formation of lactic acid (lactate). Accordingly, these fibers do not produce energy with the participation of oxygen, that is, aerobically. Fast glycolytic fibers are characterized by maximum contraction speed and strength. They play a major role in weight gain for bodybuilders and also provide top speed for runners and sprint swimmers.
Fast oxidative glycolytic fibers
The second type is fast oxidative-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 production (anaerobic), however, they are also adapted to the implementation of a fairly intense aerobic load. In other words, these fibers can develop high force and high contraction speed. The main source of energy is glycolysis. At the same time, if the intensity of contraction becomes low, they are able to use oxidation quite effectively. This type of fiber is activated when the load is between 20 and 40% of the maximum. However, when it is about 40%, the human body immediately switches to the use of fast glycolytic fibers.
The ratio of fast and slow fibers in the body
Studies were carried out, during which the fact 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 may predominate.
Their proportional ratio in different muscles of the body is also not the same. This is because the muscles and their groups in the body perform different functions. It is because of this that the transverse muscle fibers are quite different in composition. For example, the triceps and biceps contain approximately 70% white fibers. A little less of them in the thigh (about 50%). But in the gastrocnemius muscle of these fibers, only 16%. That is, if the functional task of a particular muscle includes more dynamic work, it will have more fast ones, not slow ones.
Connection of potential in sports with types of muscle fibers
We already know that the overall ratio of red and white fibers in the human body is genetically determined. Because of this, different people have different potential in sports activities. Some people are better at sports that require endurance, while others are better at strength sports. If slow fibers predominate, skiing, marathon running, long-distance swimming, etc., are much more suitable for a person, that is, sports that mainly involve the aerobic energy generation system. If there are more fast muscle fibers in the body, then good results can be achieved in bodybuilding, sprinting, sprint swimming, weightlifting, powerlifting, and other sports where explosive energy is of primary importance. And heras you already know, only white muscle fibers can provide. Great sprinters are always dominated by them. Their number in the muscles of the legs reaches 85%. If there is an approximately equal ratio of different types of fibers, the average distance in running and swimming is perfect for a person. However, the above does not mean at all that if fast fibers predominate, such a person will never be able to run a marathon distance. He will run it, but he will definitely not become a champion in this sport. Conversely, if there are many more red fibers in the body, the results in bodybuilding will be worse for such a person than for the average person, whose ratio of red and white fibers is approximately equal.
