Edited by Dr. Dario Mirra
Skeletal muscle: hints of functional anatomy
The muscle is made up of different elements that form its structure. The different functional units of the striated muscle are called sarcomeres or inocommi, real functional units of movement.
To have a clear understanding of the way in which the muscle creates movement, and having already present the biochemical, physiological and neurological function that are the basis of muscle contraction, it is necessary to have two clear concepts:
- the constitution of the protein mesh that underlies the functions of the muscle itself;
- the physical relationships that are the basis of the movement.
1 From a simplistic point of view, the proteins that make up the sarcomere can be divided into 3 categories:
- Contractile proteins: Actin and Myosin.
- Regulatory proteins: Troponin and Tropomyosin.
- Structural proteins: Titin, Nebulina, Desmin, Vinculin, etc ..
If you then observe a muscle preparation under a microscope, you can easily observe the presence of bands of different colors, which correspond to different functional areas.
So from a purely didactic point of view considering these areas, we have:
- Discs Z - They delimit the sarcomere. They are the anchor points for proteins, they are the site of injuries during muscle work, they come close to each other during contraction.
- Band A - Corresponds to the length of the myosin filament.
- Band I - Corresponds to two rows of Actin in two contiguous sarcomeres.
- Band H - Corresponds to the area between two rows of Actin in the same sarcomere.
- Line M - Divide the sarcomere into two symmetrical portions.
Spatial relationships of myofilaments in the sarcomere. A sarcomere is bounded at its ends by two Z series
2) On the other hand, below are the physical relationships that can help to better understand some peculiarities of human movement:
a) Force-Length relationship
The peak force (L0) depends on the degree of overlap of the contractile proteins. A fiber at rest has a length of about 2.5 micrometers, with the possibility for the sarcomere to reach lengths that can reach about 3.65 micrometers, as the thick filaments have a length of 1.6 micrometers, while the thin ones of 1 micrometer. The peak of strength is obtained when the protein overlap is around 2 - 2.2 micrometers.
Length-tension relationship in muscle contraction. The image shows the tension generated by a muscle based on its length before the start of the exercise / muscle contraction. We focus our attention on the active force curve (muscle contraction), leaving out the red one relating to the total force and the blue one. relative to the passive force (due to non-contractile components of the sarcomere - connectin / titin); in particular, following the trend of the curve relating to the active force we note that:
a) there is no active force since there is no contact between the myosin heads and the actin
Between a) and b): there is a linear increase in the active force due to the increase of the available binding sites of actin for the myosin heads
Between b) and c): the active force reaches its maximum peak and remains relatively stable; in this phase, in fact, all the heads of the myosin are bound to the actin
Between c) and d): the active force begins to decrease as the overlap of the actin chains reduces the binding sites available for the myosin heads
e): once the myosin collides with the Z disc there is no active force since all the myosin heads are attached to the actin; moreover, the myosin is compressed on the Z discs and acts as a spring opposing the contraction with a force proportional to the degree of compression (therefore of muscle shortening)
All this presupposes the theory of the sliding of the filaments, according to which: the tension that the muscle fiber can generate is directly proportional to the number of transverse bridges that are formed between thick filaments and thin filaments.
b) Force-Speed Relationship
c) Speed-Length relationship
If the muscle strength is proportional to the transverse diameter of the fiber, the speed depends on the number of fibers in series along the course of the fiber itself. So if we assumed a Delta L shortening and we had 1000 sarcomeres in series, the total shortening would be:
1000xDelta L / Delta t
So the longer the muscles, the more acceleration trajectories they will have.
Speed relationship - Hypertrophy
Anyone who has tried their hand at work with weights without having performed a lengthening and stretching work parallel to it could easily notice the sensation of greater rigidity during sports movements or in normal daily gestures. In fact, excessive hypertrophy increases internal viscosities and connective retraction; it is therefore deductible that muscle hypertrophy does not favor explosive-ballistic movements or in any case of speed, as it is known that the internal friction of the muscle must be minimal for allow optimal flow of contractile proteins. The greater eccentric strength of the Bodybuilders can also be deduced from this relationship, as the exasperated hypertrophy creates strong internal frictions which act as a support in the yielding movements.
Conclusions
Through the explanation of the constitution of the structural mesh and of the physical relationships that bind the muscle to movement, it was my intent with this article to give the reader a greater element to understand with a little more clarity that sports gestures, as well as everyday ones , go beyond what can be lifting a barbell or simply walking; to be better understood in their complexity, these gestures require a knowledge of anatomy, physiology, biochemistry and all complementary subjects, which make it clear how motor sciences are anything but improvisations by practitioners, and how they require multiple "knowledge" that embrace theory and practice.
