Classification of muscle fibers

Classification of muscle fibers
Everyone knows that each person has an individual muscle composition, that is, only he has an inherent combination of muscle cells (fibers) of different types in all skeletal muscles.
But there are several classifications of these types of fibers, and they do not always coincide. What classifications are currently accepted?
Muscle fibers are divided into:
White and red;
Fast and slow;
Glycolytic, intermediate and oxidative;
High-threshold and low-threshold.
White and red. In cross-section, the muscle fiber may have a different color. It depends on the amount of the muscle pigment myoglobin in the sarcoplasm of the muscle fiber. If the content of myoglobin in the muscle fiber is large, the fiber has a red-brown color. If myoglobin is not enough, then pale pink. In humans, almost every muscle contains white and red fibers, as well as poorly pigmented fibers. Myoglobin is used to transport oxygen inside the fiber from the surface to the mitochondria, so its amount is determined by the number of mitochondria. By increasing the number of mitochondria in the cell with special training, we increase the amount of myoglobin and change the color of the fiber.

Fast and slow. They are classified by the activity of the ATP-Aza enzyme and, accordingly, by the rate of muscle contraction. The activity of this enzyme is inherited and cannot be trained. Each fiber has its own constant activity of this enzyme. The release of energy contained in ATP is carried out by the ATP-AZE. The energy of one ATP molecule is sufficient for one turn (stroke) of myosin bridges. The bridges uncouple from the actin filament, return to their original position, connect with a new section of actin, and make a stroke. The speed of a single stroke is the same for all muscles. The energy of ATP is mainly required for the separation. The next stroke requires a new ATP molecule. In fibers with high ATP-azic activity, ATP cleavage occurs faster, and more bridge strokes occur per unit of time, meaning the muscle contracts faster.

Glycolytic, intermediate, and oxidative. Classified by oxidative potential of muscle, that is, the number of mitochondria in muscle fibers recall that mitochondria are the cellular organelles in which glucose or fat is broken down to carbon dioxide and water, resynthesise the ATP required for the resynthesis of phosphocreatine. Creatine phosphate is used for the resynthesis of myofibrillar ATP molecules, which are used for muscle contraction. Outside of the mitochondria, the muscles can also break down glucose to pyruvate with ATP resynthesis, but lactic acid is formed, which acidifies the muscle and causes it to tire.
According to this feature, muscle fibers are divided into three groups:
1. Oxidative muscle fibers. In them, the mass of mitochondria is so large that a significant increase in it during the training process does not occur.
2. Intermediate muscle fibers. In them, the mass of mitochondria is significantly reduced, and lactic acid accumulates in the muscle during work, but it is quite slow, and they tire much more slowly than glycolytic ones.
3. Glycolytic muscle fibers have a very small number of mitochondria. Therefore, they are dominated by anaerobic glycolysis with the accumulation of lactic acid, which is why they got their name. (Anaerobic glycolysis is the splitting of glucose without oxygen to lactic acid with ATP resynthesis; aerobic glycolysis, or oxidation – is the splitting of pyruvate in mitochondria with oxygen to carbon dioxide, water, and ATP resynthesis.) In non-training people, usually fast fibers are glycolytic and intermediate, and slow fibers are oxidative. However, with proper training to increase endurance, fast muscle fibers are converted from glycolytic to intermediate, and then to oxidative, and then they, without losing strength and speed of contraction, will become indefatigable.

High-threshold and low-threshold. They are classified by the level of excitability threshold of motor units. The muscle contracts under the influence of nerve impulses, which are of an electrical nature. Each motor unit (DE) includes a motor neuron, axon, and a collection of muscle fibers. The number of DE in a person remains unchanged throughout life. Motor units have their own excitability threshold. If the nerve impulses sent by the brain have a frequency below this threshold, DE is passive. If the nerve impulses have a threshold value for this FUNCTION or exceed it, the muscle fibers are activated and begin to contract. Low-threshold DE have small motor neurons, a thin axon, and hundreds of innervated slow muscle fibers. High-threshold DE have large motor neurons, a thick axon, and thousands of innervated fast muscle fibers.

As you can see, two of the presented classifications are unchanged throughout a person's life, regardless of training, and two directly depend on training. In the absence of a motor mode, for example in a coma or during a long stay in a cast, even slow muscle fibers lose their mitochondria and, accordingly, myoglobin and become white and glycolytic.
Therefore, currently in sports science, it is considered incorrect to say "training aimed at hypertrophy of fast muscle fibers" or "hyperplasia of myofibrils in slow muscle fibers", although ten years ago this was considered acceptable even in specialized scientific publications.
Now if we are talking about the training effect on the muscle fiber (MV), we only use the classification by the oxidative potential of the muscle. The classifications are the same for non-athletes and for representatives of speed-power and strength sports, where the goal is to raise the maximum weight in a single repetition.
In sports that require endurance, the classifications will not match.
For clarity, a somewhat exaggerated, although theoretically quite possible, example. Please note that all numbers are conditional, and they should not be taken literally.
Let's imagine an athlete who has the best result in the bench press 200 kg (without equipment), 180 kg he can shake 3 times, 150 kg – 10 times. The results show that the oxidative potential of the muscles is very low. The ratio of fibers, let's assume the following: 90 % – fast, 10 % – slow. According to the oxidative potential, 75% are glycolytic, 15% are intermediate, and 10 % are oxidative. The best success in increasing muscle mass an athlete achieves when working in the bench press for six repetitions. The weight of the bar is large enough to recruit 75% of the glycolytic fibers, and their oxidative potential is so low that six repetitions are enough for the necessary acidification of the muscle. But for some reason, this athlete decided to maximize his endurance and for two months, 2-3 times a day, worked daily to increase the mitochondria in glycolytic and intermediate MV.
Plus, the athlete still maintained his strength potential, performing 1-2 repetitions with a near-maximum weight every 7-10 days.
Two months is enough for the maximum saturation of the muscles with mitochondria. After two months, the athlete conducts testing. It shows that it now has 5 % glycolytic fibers, 70% intermediate and 25% oxidative. That is, the glycolytic ones became intermediate, except for 5% of the highest-threshold ones, and the intermediate ones became oxidative. The ratio of ATP-Aza activity, of course, has not changed, as well as 90% fast and 10% slow. 200 kg he squeezed on one time, myofibrils from such training did not grow, and he did not let the result fall. 180 kg he squeezed on 8 times, and 150 kg-on 25 times. A huge number of new mitochondria "ate" lactic acid, preventing the muscles from acidifying, which significantly increased their functionality.
Now our athlete to increase muscle mass work for six repetitions will give almost nothing. It uses only 5 % of the remaining glycolytic fibers in the desired mode. Now he will have to work at least 15 repetitions in the approach to achieve the necessary acidification of the muscles for the growth of muscle mass. And additionally include statodynamic exercises in training, since only they contribute to hypertrophy of oxidative muscle fibers, of which he now has 25 %, and it is no longer advisable to ignore them.

As we can see, the same person is forced to use completely different training programs to hypertrophy their fast muscle fibers after changing their oxidative potential!

That's why it is considered incorrect to talk about training effects on fiber types using the classification of ATP-Aza activity.

P.S. Do not be afraid to develop endurance. The change in the oxidative potential is reversible. I.e., if you decide to gain muscle volume in the mode of six repetitions, then in a month and a half this mode will again give its results, and the body will get rid of" extra " mitochondria. But then your stamina will drop.
Which mode to choose is up to you.
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Sunday, 19 September 2021