Age-related muscle anatomy

In a newborn, skeletal muscles are relatively well developed and make up 20-22% of the total body weight. In children aged 1-2 years, muscle mass decreases to 16.6%. At the age of 6, due to the child's high motor activity, skeletal muscle mass reaches 21.7% and continues to increase. In women, muscle mass is 33%, in men - 36% of body weight.

In a newborn, muscle fibers in bundles are loose, the thickness of the bundles is small - from 4 to 22 microns. Subsequently, muscle growth occurs unevenly depending on their functional activity. In the first years of a child's life, the muscles of the upper and lower extremities grow rapidly. In the period from 2 to 4 years, the long muscles of the back and the gluteus maximus grow intensively. The muscles that provide an upright position of the body grow intensively after 7 years, especially in adolescents aged 12-16. At the age of over 18-20 years, the diameter of muscle fibers reaches 20-90 microns. In people aged 60-70 years, the muscles partially atrophy, their strength noticeably decreases.

Fasciae in a newborn are weakly expressed, thin, loose, and easily separated from the muscles. Formation of fasciae begins in the first months of a child's life, which is interconnected with the functional activity of the muscles.

The muscles of the head, including the facial muscles, are thin and weak in a newborn. The frontal and occipital bellies of the occipitofrontal muscle are relatively well expressed, although the tendinous helmet is poorly developed and loosely connected to the periosteum of the bones of the skull roof, which favors the formation of hematomas in birth injuries. The masticatory muscles of a newborn are poorly developed. During the period of eruption of milk teeth (especially molars), they become thicker and stronger. During this period, relatively large accumulations of fatty tissue are observed between the superficial and deep layers of the temporal fascia above the zygomatic arch, between the temporal fascia and the temporal muscle, between this muscle and the periosteum. The fatty body of the cheek is formed outside the buccal muscle, which gives the face the rounded outlines characteristic of a newborn and children of the first years of life.

The muscles of the neck of a newborn are thin and differentiate gradually. They reach their final development by the age of 20-25. In newborns and children up to 2-3 years, in accordance with the higher position of the neck borders, the triangles of the neck are located somewhat higher than in an adult. The triangles of the neck occupy the position characteristic of adults after 15 years.

The plates of the cervical fascia in a newborn are very thin, there is little loose connective tissue in the interfascial spaces. Its quantity increases noticeably only by the age of 6-7 years. From 20 to 40 years, the quantity of loose connective tissue in the interfascial spaces changes little, and after 60-70 years it decreases.

Of the chest muscles, the age-related features of the diaphragm are most clearly expressed. In newborns and children under 5 years old, it is located high, which is associated with the horizontal position of the ribs.

The dome of the diaphragm in a newborn is more convex, the tendinous center occupies a relatively small area. As the lungs straighten during breathing, the convexity of the diaphragm decreases. In older people, the diaphragm is flattened. After 60-70 years, signs of atrophy are found in the muscular part of the diaphragm against the background of an increase in the size of the tendinous center.

In a newborn, the abdominal muscles are poorly developed. Poor development of muscles, aponeuroses and fascia contributes to the formation of a convex shape of the abdominal wall in children under 3-5 years of age. Muscles and aponeuroses are thin. The muscular part of the external oblique abdominal muscle is relatively shorter. The lower bundles of the internal oblique abdominal muscle are better developed than the upper ones; in boys, some of the bundles are attached to the spermatic cord. The tendinous bridges of the rectus abdominis muscle are located high and in early childhood are not always symmetrical on both sides. The superficial inguinal ring forms a funnel-shaped protrusion, more pronounced in girls. The medial leg of the aponeurosis of the external oblique abdominal muscle is better developed than the lateral one, which is reinforced by bundles of the recurved (recurrent) ligament. Interpeduncular fibers are absent in newborns. They appear only in the second year of life. The lacunar ligament is well expressed. The transverse fascia is thin, there is almost no preperitoneal accumulation of fatty tissue. The umbilical ring in a newborn is not yet formed, especially in the upper part, which is why umbilical hernias may form. Unlike adults, in newborns and children of the first years of life, the muscle belly of the forearm and lower leg muscles is significantly longer than the tendon part. On the back of the lower leg, the deep muscles are a single muscle layer. The development of the muscles of the upper limb is ahead of the development of the muscles of the lower limb. The mass of the muscles of the upper limb in relation to the mass of the entire musculature in a newborn is 27% (in an adult 28%), and the lower limb - 38% (in an adult 54%).

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