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. These bundles are surrounded by connective tissue (fascia).
The muscle itself is also surrounded by fascia. About 85-90% of its volume is muscle fibers. The remainder 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 muscle fiber. In the cytoplasm (sarcoplasm) it is a large number of mitochondria. They play the role of power plants in which metabolism occurs and energy-rich substances are accumulated, as well as those that are needed to meet energy needs. As part of any muscle cell, there are 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, several tens of myofibrils are located in one cell, while the length of each can reach several centimeters. If you add 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 present in it, as well as on their transverse structure. Myofibrils, in turn, are composed of a large number of tiny sarcomeres.
Cross-striped fibers are characteristic of the muscle tissues of both women and men. However, their structure is slightly different depending on gender. Based on the results of a muscle biopsy, conclusions were drawn that the percentage of myofibrils in women’s muscle fibers is lower than in men. This applies even to high-level athletes.
By the way, the muscle mass itself is distributed unequally throughout the body in women and men. The vast majority of it in women is in the lower body. In the upper part, muscle volumes are small, and they themselves are small and often completely untrained.
Red fiber
Depending on fatigue, histochemical coloration and contractile properties, muscle fibers are divided into the following two groups: white and red. Reds are slow fibers 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-contracting. Sometimes they are called type 1 fibers.
Why red fibers got such a name
They are called red 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. Red fibers are surrounded by a large network of capillaries. They are needed to deliver a large amount of oxygen along with blood.
Slow muscle fibers are well adapted to the implementation of the aerobic system of energy formation. The strength of their contractions is relatively small. The speed with which they consume energy is sufficient to get by with aerobic metabolism only. Red fibers are perfect for non-intensive and long-term work, such as walking and light running, stayer's distance in swimming, aerobics, etc.
Muscle fiber contraction provides movements that do not require much effort. Thanks to him, the pose is also supported. These striated fibers are characteristic of muscle tissues, which are included in the work under loads ranging from 20 to 25% of the maximum possible force. They are characterized by excellent endurance. However, red fibers do not work when sprinting, lifting heavy weights, etc., since these types of loads require fairly quick consumption and energy. For this, white fibers are intended, which we will now talk about.
White fiber
They are also called fast, fast-contracting fibers of type 2. Their diameter is larger compared to red. To obtain energy, they use mainly glycolysis (that is, they have an anaerobic energy system). In fast fibers there is a smaller amount of myoglobin. That is why they are white.
ATP cleavage
Fast fibers are characterized by high activity of the ATPase enzyme. This means that the splitting of ATP occurs quickly, 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 also need a high rate of recovery of ATP molecules. And it can be provided only by 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 energy delivery from the latter to myofibrils.
Why white fibers get tired quickly
Due to glycolysis, the formation of lactate (lactic acid), which accumulates rapidly. Because of this, the white fibers get tired quickly enough, which ultimately stops the muscle from working. In red fibers, aerobic formation does not produce lactic acid. That is why they can maintain moderate tension for a long time.
Features of white fibers
White fibers are characterized by a large diameter of relatively red. In addition, they contain much more glycogen and myofibrils, but they have less mitochondria. A muscle fiber cell of this type also has creatine phosphate (CF). It is required at the initial stage of the implementation 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 with slow ones, are able to contract 2 times faster, and also develop strength 10 times greater. A person develops maximum speed and strength precisely thanks to them. If the work requires 25-30% of maximum effort and higher, this means that it is white fibers that take part in it. They are divided by the method of obtaining energy into the following 2 types.
Fast glycolytic muscle fibers
The first type is fast glycolytic fibers. The glycolysis process is used by them to produce energy. In other words, they are able to use only an anaerobic energy production 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 mass gain among bodybuilders, and they also provide maximum speed to runners and swimmers at sprint distances.
Fast oxidative glycolytic fibers
The second type is fast oxidation-glycolytic fibers. They are also called transient 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 formation (anaerobic), but are also adapted to the implementation of a fairly intense aerobic load. In other words, these fibers can develop great effort and high speed reduction. 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 efficiently. This type of fiber is involved in the work if the load is from 20 to 40% of the maximum. However, when it is about 40%, the human body immediately completely switches to the use of fast glycolytic fibers.
The ratio of fast and slow fibers in the body
Studies were conducted in the process of which the fact was established that the ratio of fast and slow fibers in the human body is determined genetically. 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.
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 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 the white fibers. They are slightly smaller in the thigh (about 50%). But in the calf 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, not slow.
Relationship of potential in sports with types of muscle fibers
We already know that the general ratio of red and white fibers in the human body is genetically incorporated. Because of this, different people have different potentials in sports. Someone is better given sports that require endurance, and others - power. If slow fibers predominate, a person is much more suitable for skiing, marathon running, long-distance swimming, etc., that is, sports in which the aerobic energy education system is mainly involved. If the body has more fast muscle fibers, then good results can be achieved in bodybuilding, short-distance running, sprinting, weightlifting, powerlifting, and other forms where explosive energy is of primary importance. And it, as you already know, can be provided only by white muscle fibers. Great sprint athletes always dominate 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 prevail, such a person will never be able to run a marathon. He will run it, but he certainly will not become a champion in this sport. Conversely, if the body has a lot more red fibers, 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.