Human Anatomy - H. I. Koliadenko 2009

Myology (The Study of Muscles)
General Information
Muscle Development

In lower animals (such as amphioxus and fish), the body wall consists of striated, metamerically arranged Muscles, though in fish these muscles are subdivided into parietal and visceral components.

Parietal muscles develop from somites, whereas visceral muscles originate from the visceral arches. In terrestrial vertebrates, the musculature differentiates into cervical, thoracic, abdominal, and caudal regions. Due to more complex bodily Functions and limb specialization in mammals and humans, segmentation largely disappears, persisting only partially in the rib cage, spine, and abdomen.

The visceral Muscles of the mandibular and hyoid arches evolve into the Muscles of Mastication and facial expression, as well as the suprahyoid Neck Muscles. The pharyngeal and laryngeal muscles develop from the first branchial arch, while the sternocleidomastoid and trapezius muscles are formed from the third and fourth branchial arches. All muscles of visceral origin are innervated by the fibers of cranial nerve pairs V, VII, IX, X, and XI.

The Muscles of the Trunk and limbs develop from myotomes; the ventral part of the myotome gives rise to the ventral musculature, including the intercostal muscles of the chest, as well as the rectus and oblique Abdominal muscles. The deep back muscles develop from the dorsal part of the myotome. These muscles are innervated by Spinal Nerves, which correspondingly divide into ventral and dorsal branches. The muscles of the trunk and limbs are classified into intrinsic (autochthonous) muscles and immigrant muscles. Intrinsic muscles attach with both ends to the trunk, whereas immigrant muscles either originate on the trunk or limbs and subsequently migrate to the limbs or trunk. For instance, the following muscles originate on the trunk and migrate with one end to the upper limb: trapezius, rhomboids, levator scapulae, sternocleidomastoid, and serratus anterior, having initially formed on the trunk. Conversely, muscles such as the latissimus dorsi, pectoralis Major and minor, fascia lata, and quadratus lumborum initially develop on the limbs and later attach one tendinous end to the trunk.

Muscle mass increases unevenly and depends on the Functional Characteristics of specific Organs and body systems. For example, During the first months of an infant's life, the abdominal muscles develop most rapidly, followed by the muscles of mastication. At 10–12 months, as the child actively crawls and begins to walk, the mass of the back and limb muscles increases. Flexor muscles develop more actively and rapidly, whereas limb extensors lag slightly behind in development. In the early years of life, dynamic muscles develop earlier than static ones. Consequently, children's movements at this age are swift, rapid, but short-lived, which is associated with rapid fatigue.

Throughout human life, muscles undergo both quantitative and qualitative changes. A child is born with a muscle mass that does not exceed 24% of total body weight. Their muscle fibers are insufficiently developed, and the nuclei within the sarcoplasm occupy a central position and a larger relative area compared to adult muscles, making such muscles prone to rapid fatigue. Children's muscles grow slowly, increasing in mass by only 4–5% during the first 6–7 years of life. Rapid muscle mass growth occurs in early school-age children (7–12 years). During this period, muscle mass increases due to the thickening of fibers, The chemical composition of the sarcoplasm changes, and the nuclei shift closer to the sarcolemma; functionally, the muscles become more resilient and less fatiguable. From the age of 13 to 15–16 years, active growth of the long bones occurs. The limbs lengthen significantly, causing adolescents during this phase to appear tall and awkward, with long, slender muscles that lag behind skeletal growth. Between 16 and 18 years of age, muscle mass increases actively, reaching 35–40% of body weight by age 18. Regular physical education and sports can increase muscle mass to 45–50%. Muscle mass remains stable up to the age of 50, after which the involution of The Musculoskeletal System begins; muscle mass gradually declines, dropping to 30% of body weight in old age.