Formation of the vertebral column and vertical posture of the human body in ontogenesis

The human spinal column successively passes through membranous, cartilaginous and bone stages of development. Its elements appear at the early stages of embryonic development. At first, the rudiments of the vertebral bodies are located far from each other, separated by layers of embryonic mesenchyme. Then the vertebral arches begin to develop, transverse and articular processes are formed, then the vertebrae differentiate almost completely, and the spinous processes are still absent.

The chord in the embryo is reduced and is preserved only in the form of a gelatinous core of the intervertebral discs. A characteristic feature of the spine at the early stage of intrauterine development is the similarity of the vertebral bodies in their shape. At the end of the second month of intrauterine development, the size of the cervical vertebrae increases sharply. An increase in the bodies of the lumbar and sacral vertebrae is not observed even in newborns due to the absence of intrauterine gravitational effects.

The longitudinal ligament is laid down in embryos on the dorsal surface of the vertebral bodies. The intervertebral disc in embryos is formed from mesenchyme. Ossification centers in the embryonic spine first appear in the lower thoracic and upper lumbar vertebrae, and are then traced in other sections.

After birth, the child immediately begins to struggle with many external influences. And the most important stimulus that will form its posture is gravity. From the moment of birth until the formation of the posture inherent in an adult, each child, according to A. Potapchuk and M. Didur (2001), goes through the following levels of movement formation:

• level A - the child, lying on his stomach, raises his head. At the same time, due to the cervical-tonic reflexes, a level is formed that ensures the body's balance and the basic threshold of muscle tension;
• level B - formation of muscle-joint connections that predetermine the development of automatism of motor cycles. This period corresponds to the stage of learning to crawl and sit; the mechanism of unilateral and then multilateral inclusion of limb muscles begins to form, which subsequently ensures the formation of an optimal stereotype of walking and standing;
• level C - is formed by the end of the first year of life and allows the child to quickly navigate in space using the existing arsenal of motor skills;
• Level D - a vertical body posture is created, in which muscular balance in a standing position is ensured with minimal muscular effort. As the levels of movement formation change, the shape of the spinal column also changes. It is known that the spinal column of a newborn, with the exception of a small sacral curvature, has almost no physiological curves. The height of the head during this period is approximately equal to the length of the body. The center of gravity of the head in infants is located directly in front of the synchondrosis between the sphenoid and occipital bones and at a relatively large distance in front of the joint between the skull and the atlas. The posterior cervical muscles are still poorly developed. Therefore, the heavy, large (in relation to the rest of the body) head hangs forward, and the newborn cannot lift it. Attempts to lift the head lead after 6-7 weeks to the formation of cervical lordosis, which is established in the following months as a result of efforts aimed at maintaining the body in balance in a sitting position. Cervical lordosis is formed by all cervical vertebrae and the two upper thoracic vertebrae, and its apex is at the level of the fifth to sixth cervical vertebrae.

At 6 months, when the child begins to sit, a curvature in the thoracic region is formed with a convexity towards the back (kyphosis). During the first year, when the child begins to stand and walk, a curvature in the lumbar region is formed, directed forward (lordosis).

Lumbar lordosis includes the XI-XII thoracic and all lumbar vertebrae, and its apex corresponds to the third-fourth lumbar vertebrae. The formation of lumbar lordosis changes the position of the pelvis and promotes the displacement of the general center of gravity (GG) of the human body posterior to the axis of the hip joint, thus preventing the body from falling in an upright position. The shape of the spinal column in a child aged 2-3 years is characterized by insufficiently expressed lumbar lordosis, which reaches its greatest development in an adult.

The sacrococcygeal curve appears in embryos. However, it begins to develop only with the first attempts at upright walking and with the appearance of lumbar lordosis. The formation of this curve is influenced by the force of gravity, transmitted to the base of the sacrum through the free section of the spine and tending to wedge the sacrum between the iliac bones, and the pull of the ligaments located between the sacrum and the ischium. These ligaments fix the lower part of the sacrum to the tubercle and spine of the ischium. The interaction of these two forces is the main factor determining the development of the sacrococcygeal curve.

As the physiological curves of the spinal column form, the shape of the intervertebral discs also changes. If the discs of a newborn are of the same height both in front and behind, then with the formation of curves their shape changes, and the cartilages in the sagittal section acquire a somewhat wedge-shaped form. In the area of lordosis, the greater height of this wedge faces forward, and the smaller one faces backward. In the area of thoracic kyphosis, on the contrary, the greater height is at the back and the smaller one at the front. In the sacral and coccygeal sections, the spinal column has a curve facing backward. The intervertebral discs of the sacral section have a temporary significance and are replaced by bone tissue at the age of 17-25, as a result of which the mobility of the sacral vertebrae relative to each other becomes impossible.

The growth of the spinal column is particularly intense in the first two years of life. Its length reaches 30-34% of the final size. Different sections of the spinal column grow unevenly. The lumbar section grows most intensively, then the sacral, cervical, thoracic, and the coccygeal section grows the least. From 1.5 to 3 years, the growth of the cervical and upper thoracic vertebrae slows down relatively. Further growth of the spinal column is observed at 7-9 years. At 10 years, the lumbar and lower thoracic vertebrae grow rapidly. An increase in the rate of growth of the spinal column is also noted during puberty.

Up to 2 years, the total length of the bony and cartilaginous portions of the spinal column increases with equal intensity; then the growth of the cartilaginous portion slows down relatively.

The vertebral bodies of a newborn are relatively wider and shorter than those of an adult. In children aged 3 to 15 years, the sizes of individual vertebrae both in height and width increase from top to bottom, from the upper thoracic to the lower lumbar. These differences (at least those related to growth in width) depend on the increase in the weight load experienced by the vertebrae located below. By the age of 6, there are independent ossification points in the upper and lower parts of the vertebrae, as well as at the ends of the spinous and transverse processes.

The overall growth of the vertebrae on average from 3-6 years proceeds with the same intensity in height and width. At 5-7 years, the increase in width of the vertebrae lags somewhat behind the increase in height, and at subsequent ages, the increase in vertebrae in all directions increases.

The process of ossification of the spinal column occurs in stages. In the 1st-2nd year, both halves of the arches merge, in the 3rd year - the arches with the vertebral bodies. At 6-9 years, independent ossification centers are formed on the upper and lower surfaces of the vertebral bodies, as well as the ends of the spinous and transverse processes. By the age of 14, the middle parts of the vertebral bodies ossify. Complete ossification of individual vertebrae ends by the age of 21-23.

As the curves of the spinal column form, the size of the thoracic and pelvic cavities increases, which in turn helps maintain an upright posture and improve the spring properties of the spine when walking and jumping.

According to many authors, the formation of the human spinal column and its vertical posture is influenced by the height of the general center of gravity of the body.

Age-related features of the location of the common center of gravity are caused by uneven changes in the sizes of biolinks, changes in the ratio of the masses of these body links during the growth period. They are also associated with characteristic features acquired in each age period, starting from the moment the child first stands and ending with old age, when, as a result of senile involution, biomechanical changes occur simultaneously with morphological ones.

According to G. Kozyrev (1947), the general center of gravity of newborns is located at the level of the V-VI thoracic vertebrae (determined in the position of maximum possible straightening of the lower limbs by bandaging). This cranial location of the general center of gravity is explained by the characteristic proportions of the body of newborns.

As the child grows, the overall center of gravity gradually decreases. Thus, in a 6-month-old child, it is located at the level of the 10th thoracic vertebra. At 9 months of age, when most children can stand independently, the overall center of gravity drops to the level of the 11th-12th thoracic vertebrae.

In biomechanical terms, the most interesting process is the transition to a vertical body position. The first standing is characterized by excessive tension of all muscles, not only those that directly hold the body in a vertical position, but also those that do not play a role in the act of standing or have only an indirect effect. This indicates insufficient differentiation of the muscles and the absence of the necessary regulation of tone. In addition, instability is also caused by the high position of the CG and the small support area, which make it difficult to maintain balance.

A 9-month-old child has a peculiar posture in the sagittal plane. It is characterized by the fact that the child's lower limbs are in a semi-bent position (the angle of flexion of the knee joint in a 9-month-old child reaches 162 °, in a one-year-old - 165 °), and the body is slightly tilted forward relative to the vertical axis (7-10 °). The semi-bent position of the lower limbs is due not to the tilt of the pelvis or the limitation of extension in the hip joints, but to the fact that the child adapts to maintaining the body in such a balance that the possibility of its unexpected violation is excluded and the safety of a fall is ensured. The emergence of a peculiar posture at this age is primarily caused by the lack of a fixed skill for standing. As such a skill is acquired, the uncertainty in the static stability of the body gradually disappears.

By the age of two, the child stands more confidently and moves the center of gravity within the support area much more freely. The height of the general center of gravity of the body is located at the level of the first lumbar vertebra. The semi-bent lower limbs gradually disappear (the angle of flexion in the knee joints reaches 170°).

The posture of a three-year-old child when standing is characterized by a vertical position of the body and a slight bending of the lower limbs (the bending angle in the knee joint is 175°). In the area of the spinal column, thoracic kyphosis and emerging lumbar lordosis are clearly visible. The horizontal plane of the general center of gravity of the body is located at the level of the second lumbar vertebra. The longitudinal axes of the feet form an angle of approximately 25-30°, as in adults.

In the posture of five-year-old children, there are no longer any signs of semi-flexion of the lower limbs (the angle in the knee joint is 180°). The horizontal plane of the general center of gravity is located at the level of the third lumbar vertebra. In subsequent years, changes in the localization of the body's CG consist mainly of its gradual decrease and more stable regulation in the sagittal plane.

As a result of the aging of the body, both anatomical, physiological and biomechanical changes occur in the musculoskeletal system.

G. Kozyrev (1947) identified three main types of posture with the most characteristic morphological and biomechanical features.

The first type of senile posture is characterized by a sharp forward shift of the center of gravity - so much so that the sagittal plane is located in front of the centers of the three main joints of the lower limbs. The support is mainly the front part of the feet, the head is tilted forward, the cervical lordosis is flattened. In the lower part of the cervical and thoracic sections, there is a sharp kyphosis. The lower limbs are not fully extended at the knee joint (the flexion angle varies from 172 to 177 °).

The second type of senile posture is characterized by a posterior shift of the center of gravity. Its sagittal plane passes behind the center of the hip joint and closes the latter passively, using the tension of the iliofemoral ligament for this. The torso is tilted backwards, the lowered abdomen is pushed forward. The spinal column has the shape of a "round back".

The third type of posture is characterized by a general sagging of the body without any forward or backward tilt of the trunk. It seems as if gravity has compressed the body along the vertical axis; as a result, the neck seems to have become shorter due to an increase in the cervical curve, the trunk has become shorter due to an increase in the thoracic kyphosis, and the lower limbs - due to flexion in the three main joints. The sagittal plane of the general center of gravity passes posteriorly from the center of the hip joint, closing it passively from behind or through the center of the knee joint. As a result, the last two joints can only be closed actively.

When examining an elderly or senile person, the first thing that attracts attention is his posture, which is often characterized by pronounced cervical, lumbar lordosis and thoracic kyphosis.

In elderly and senile people, the kyphosis of the spinal column increases, a round back gradually forms, and the cervical and lumbar lordosis also increases. Even with a normal static load, some increase in thoracic kyphosis occurs during life. With prolonged static loads (overloads) on the side of concavity, a change in the intervertebral discs occurs and a fixed curvature (age-related hyperkyphosis) develops with all the consequences. Five types of posture inherent in old age, based on the analysis of radiographs of physiological curves of the spinal column, were identified by Podrushnyak and Ostapchuk (1972):

1. unchanged, thoracic curvature angle more than 159°;
2. stooped, thoracic spine angle 159-151°;
3. kyphosis, the angle of curvature of the thoracic region is less than 151°, lumbar - 155-164°;
4. kyphotic-lordotic, the angle of curvature of the thoracic region is less than 151% of the lumbar region - less than 155°;
5. kyphotic-flattened, the angle of curvature of the thoracic region is less than 15°, lumbar - more than 164°.

The authors found that with aging, the most pronounced changes in the curvatures are in the sagittal plane of the thoracic spine, quite clearly in the cervical spine, and somewhat less so in the lumbar spine.

Up to 60 years of age, scoliosis, thoracic kyphosis, cervical and lumbar lordosis are more often detected in women. With increasing age, the number of people with unchanged posture in an upright position decreases sharply and the number of people with kyphosis increases.

Among the various changes in the structure and function of the spinal column that develop during the aging process, vertebral displacements or torsion occupy a special place, as their frequency of detection and severity increase with aging.

According to Ostapchuk (1974), torsional curvatures of the thoracic and lumbar spine are found in more than half of practically healthy people of both sexes and are detected more often with age. In most people, torsion of the spine is combined with curvature in the frontal plane and its direction is closely related to the form of scoliosis.

Torsion that develops with aging is closely related to dysfunction of the longissimus muscle. It is aggravated by a combination of torsion and lateral curvature of the spinal column. Torsion and dysfunction of the longissimus muscle develop against the background of dystrophic-destructive processes of the spinal column, increasing the negative impact on the statics and dynamics of a person with aging.

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