[035] Tension of the skeletal muscle contraction (GGB#115D01) | 基礎医学教育研究会(KIKKEN)Lab

●Muscles give power only when contracting

When thinking about the jumping force, it is called a muscle like a spring, but the muscles do not have the repelling force to push back things like a cushion spring. From the mechanism, the strength of the muscle can only give the force to shrink. In other words, it can be said that it is the ability to pull on things. That’s why it’s common to express the strength of a muscle with the word “tension”. When the muscle stretches it will be pulled from outside.
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● There is a muscle contraction which does not shrink

Activity in which the muscles generate tension is called “muscle contraction”. Normally in both Japanese and English, “contraction” is one thing with “shrinking”. As a matter of fact, the muscles do not shrink as much as they give out, but even if they do not shrink, all that the muscles generate force calls “muscle contraction”. Even if the length of the muscle is stretched in some cases! It becomes “contraction”. It is one of the troubled technical terms which makes amateurs confused.When it is necessary to point to a state of shrinking and shortening actually, there is a phrase “muscle shortening”, but how to use is not so well decided. Simply saying “shortening of muscle” also means that the length becomes shorter due to pathological atrophy, which is confusing in this.
　

Therefore, special muscle contraction that does not change while the length is fixed is called “isometric contraction”. “The scale is equal” means that the length does not change.It does not shrink even if the muscle contracts because the tension is insufficient. Even if tension is generated, it can not be shortened unless it exceeds “load”. The load is the weight when trying to raise an object placed, for example. For example, trying to lift a load of 3 kg with only a maximum of 2 kg of force will not work. In actual muscles, the muscles themselves are slightly shortened accordingly because the tendons that inherit the muscles and loads are stretched slightly even if the load does not move. So it is not strictly “isometric”.However, on understanding at the level of the textbook it is OK to ignore this much. Isometric contraction manifests as isovolumic constriction in myocardium.

If muscle contraction is forcibly pulled with a load that exceeds the tension of the muscle, the length of the muscle will be stretched even though it is “contracting.”This is called “eccentric contraction” or “lengthening contraction” (!?).
　

● Make muscle tension with the power of a number

The biceps brachii muscle and gastrocnemius muscle, etc., the mass of the skeletal muscle is actually made up of aggregates (bundles) of countless striated muscle fibers (myofiber = muscle cells), and the respective muscle fibers contract independently. Even if the adjacent muscle fibers contract, it is commonplace that certain fibers are resting irrespective of that. From olden days, to call up young people to become soldiers is said to be “recruitment”. Again, using the same expression, we say that the tension of the whole muscle is determined according to the number of recruited muscle fibers. Is this recruited muscle fiber an image called “fighting force”? Roughly speaking, when two fibers are recruited at the same time from one muscle fiber, twice the tension can be given. When three recruited at the same time, the power is tripled.　
　　　　　
When the muscle shortens, if the load is the same, the speed with which the tension of the whole muscle is stronger is shrunk faster.

● When muscle fibers shrink, are there power margins?

When the mobilized muscle fibers do not contract, it is when the load is larger than the tension generated in the entire muscle (heavy). At that time, the recruited muscle fibers generate tension to the maximum that they can withdraw in that state anyway.　The origin of the tension where muscle fibers develop is the tension generated in the filament of “myofibril”. And the magnitude of the force changes with the overlapping state of the filaments. In the state where the muscle fibers are stretched, since the degree of overlapping of the filaments is small, it is a bit bad, but still the muscle fibers work hard to the maximum in that state. Still if the load does not move, this is isometric contraction. If the recruited muscle fibers shrink, it means that the muscle’s overall tension exceeds the load before the muscle strength reaches the maximum tension. It is said that each muscle fiber has a characteristic that tension does not rise any more once it starts shortening. If you think that the energy of the contraction is distributed to move the filaments, it seems like a beginner knows it, but I do not know the point. Either way, when the muscles shorten, we have not done the maximum tension yet. Therefore, when you want to know the maximum tension of a muscle, it is measured by isometric contraction.

● Muscle thickness reflects the number of myofibrils that can be recruited

Depending on the application, there are thick and muscle fibers that make up the muscles, and in human muscles they mix together to make a large bundle. Even if the muscle fibers are thick or thin, after all, it is the “myofibril” that exists in the cells that produces the tension. Thick muscle fibers are packed with a large amount of myofibrils, and in the case of thin muscle fibers, fewer myofibrils. Therefore, when estimating the tension of the whole muscle it is easier to think about how many “myofibrils” are included in the muscle rather than how many “muscle fibers” are. Thick muscles contain numerous myofibrils as a whole, and thin muscles have fewer myofibrils. As a result, the number of myofibrils is expected to be proportional to the thickness of the muscle. In practice, we do not count the number of myofibrils, but instead of examining the relationship between the “cross-sectional area” of the muscle and the maximum tension, it has been confirmed that it is almost proportional.

● The muscles can not have all the power that it has

The tension per unit cross-sectional area is 2.5 to 3.5 kg/cm² in precise experiments carried out on a semitendinosus muscle specimen of a frog that removes unnecessary items other than muscle fibers as much as possible. Since the unit of force currently used is N (Newton), it will be about 25 to 35 N/cm² when represented by it. It is roughly the same in rodents (mouse and rat). Because it can not be taken out and measured in humans, it is about 15 N/cm² when the maximum contraction is caused in the soleus muscle by electrical stimulation from the skin of living human beings. Actual muscles may be reasonable values because there are various elements other than myofibrils that do not give connective tissue or blood vessels or force.

However, when the living human beings are given a lot of power and the maximum tension of voluntary movement is measured, it seems that this is at most about 10 N/cm². It seems that ordinary humans are around 4 to 6 N/cm².
In other words, from the construction of the muscles, it should be possible to have 15 N/cm², but it can be seen that ordinary humans can draw less than half of them. This is not to say that the measured human gut is not enough, it is believed to be due to a physiological mechanism to inadvertently exert a large force and not to destroy our bodies. This mechanism is made with the communication of the central nervous system and sometimes it is canceled. A trained athlete may be able to unlock this limit consciously.

● Strength of muscles is not a single pattern

The force of the muscle is proportional to the cross sectional area. This “cross-sectional area” is not simply the cross-sectional area of the streak cut at the thickest part (anatomical cross-sectional area). A virtual area “physiological cross-sectional area (PCSA)” (in other words, a value proportional to the number of myofibrils) which is the sum of the plane perpendicular to the strike of the muscle fibers in the muscles is important. In actual muscles, there are many cases in which the direction of the contraction of the whole muscle and the angle of the fiber striking in the muscle are out of alignment. This is a technique that increases the physiological cross-sectional area and increases the overall tension. In addition, although it was said that the maximum tension of the muscles occurs with isometric contraction, the tension generated at the approximate midpoint of the extension and contraction of the muscle fibers is the strongest among them. Actual exercise of the muscles takes different lengths by stretching, so the tension always changes accordingly. Furthermore, the physical power which the muscle put out is multiplication of the tension and the movement speed. Then, the maximum power of the muscle force will occur when shortening with approximately one third of the maximum tension load. Maximum tension is generated with isometric contraction, but load does not move, so physically speaking, “work” is not done, so power is zero (?!). Thus the performance of skeletal muscle varies greatly depending on the situation. It is a terrible nature of muscle, but it will be long, so let’s give it another opportunity for further details.

○ Referenced sites

↓Classical experiment with muscle fibers of frog
⇒・The variation in isometric tension with sarcomere length in vertebrate muscle fibres.　Gordon A. M., Huxley A. F., Julian F. J. (1966)
⇒・The sarcomere length-tension relation in skeletal muscle. Ter Keurs H. E. D. J., Iwazumi T., Pollack G. H.(1978)【pdf】
↓Experiments measuring human skeletal muscle tension (considerably carefully)
⇒・In vivo specific tension of human skeletal muscle, Constantinos N. Maganaris, et al., J Apl Physiol. (2001).
↓An old experiment measuring tension of human skeletal muscle
⇒・Effect of muscle cross-sectional area and muscle fiber composition on isometric maximum muscle force (1986)
↓Recent experiment
⇒・Effect of high strength resistance training as seen from bone mineral density, muscular strength, and cross sectional area of Masters Weight Lifting athlete, Junichi Okada et al. (2013)
↓If you would like to know what experiments on mouse muscle specimens are like, please. (Site gathering experimental techniques of bioscience)
⇒・Myo-mechanical Analysis of Isolated Skeletal Muscle (video)
⇒・Isometric and Eccentric Force Generation Assessment of Skeletal Muscles Isolated from Murine Models of Muscular Dystrophies(video)
↓A major revolution in muscle contraction studies, about the achievements and personality of Professor Reiji Natori who created skinned fiber
⇒・Memorised skinned fiber and the stairs of Natori, Yoshiki Umazume (2007)

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○ Referenced books

・肉単―ギリシャ語・ラテン語 (語源から覚える解剖学英単語集 (筋肉編))
・カラー図解 人体の正常構造と機能 全10巻縮刷版，坂井 建雄，日本医事新報社
・人体機能生理学，杉 晴夫，南江堂
・トートラ人体解剖生理学　原書8版，丸善
・イラスト解剖学，松村 讓兒，中外医学社
・柔道整復学校協会編「生理学」，南江堂
・東洋療法学校協会編「生理学」，医歯薬出版株式会社

rev.20140429，rev.20160403，rev.20170505.