Understanding Skeletal and Muscular Development for Scientific, Harmless Exercise

Skeletal Characteristics of Children and Adolescents

The human body has 206 bones. Because bones are "as tough as steel," they provide support, movement, and protection for the body. The spine is the pillar of the human torso, the central axis, and the pivot for upper and lower limb movement. It supports body weight, transmits pressure, cushions shocks, and protects the spinal cord and internal organs.

Most of the bones in the human body are ossified cartilage. Bone is composed of organic matter (protein) and inorganic matter (calcium, phosphorus). Organic matter gives bones elasticity, while inorganic matter makes them strong. The younger the child, the higher the proportion of organic matter in their bones. Before puberty, the ratio of organic to inorganic matter in bones is approximately 1:1, while in the elderly it is 2:5. Therefore, children's bones are elastic and easily bent and deformed under stress. For children with normal growth and development, correct posture in daily life has a significant impact on their bones; the posture maintained frequently will influence their growth shape. To ensure the normal development of children's bones, especially the spine, it is essential to cultivate and practice correct postures for standing, sitting, walking, reading, and writing from a young age. There is an old Chinese saying, "Stand like a pine, sit like a bell, walk like the wind." People also say, "Stand properly, sit properly, walk properly," all emphasizing correct postures for standing, sitting, and walking.

The correct posture for standing, sitting, walking, reading, and writing is as follows: When standing, the whole body should be upright, with the head straight, eyes looking straight ahead, chest slightly out, abdomen slightly tucked in, arms hanging naturally, and lower legs slightly extended backward. Viewed from the side, the ear, shoulder, and upper thigh (the prominent part of the thigh) should be in a straight line; viewed from behind, the shoulders should be level, and the shoulder blades should be symmetrical on both sides of the spine. The proper sitting posture is to keep the torso upright, head slightly raised, chest slightly out, chin perpendicular to the ground, feet flat on the ground, arms naturally bent and resting on the thighs (elbows on the table when reading or writing), shoulders level, neither drooping nor hunched, eyes looking straight ahead or about 30-35 cm away from the book. If working at a desk, the bending should be at the junction of the torso and thighs, not the waist or back. Some children and teenagers like to hunch over, tilt their heads, and lean their bodies while reading or writing. Over time, this can become a habit, affecting chest development and cardiopulmonary function, and even causing spinal curvature and deformities. Hunchback is an example. When walking, head slightly raised, chest slightly out, waist slightly engaged, upper body upright, shoulders level and symmetrical, arms swinging back and forth with slight force, gait steady, and paced with a relatively fast frequency, giving the impression of a "gust of wind." The human spine, viewed from the front, should be straight; viewed from the side, it should have four normal physiological curves: cervical flexion, thoracic flexion, lumbar flexion, and sacral flexion.

When standing against a wall, the occiput of the head, shoulder blades, buttocks, and heels should be in a straight line, with a distance of about 10 centimeters between the waist and buttocks and the wall. This is because the cervical and lumbar spine have a forward curve, while the thoracic and sacral spine have a backward curve. The backward flexion of the thoracic spine compensates for the forward protrusion of the chest, forming a curve that makes the chest protrude forward. The forward flexion of the lumbar spine and the backward flexion of the sacral spine correspond to the contraction of the abdomen, causing the buttocks to naturally protrude backward. It is precisely because of this physiological curvature of the spine that the lines of the human body objectively form, especially in women, fully displaying the beauty of curves and graceful posture. Muscle Characteristics in Children and Adolescents

From a physiological and anatomical perspective, while height and weight, as well as bones and muscles, develop synchronously during childhood growth and development, the rate of increase varies at different ages. In urban my country, children aged 7-9 grow an average of 4-5 cm taller per year, while their weight increases by only 2 kg. During the second growth spurt, between 12-15 years old for boys and 11-14 years old for girls, height increases by an average of 6.2 cm per year for boys and 5.3 cm per year for girls. The rapid weight gain period generally lags behind the rapid height growth period by 1-2 years. The increase in weight in adolescents, besides the increase in bones and body fluids, is mainly due to the increase in muscle volume and weight. For example, muscle accounts for 27.2% of body weight in an 8-year-old child and 32.6% in a 15-year-old adolescent, resulting in a weight gain of approximately 10 kg over 7 years. This weight gain is disproportionate to the rapid increase in height, leading to the pattern of "growing taller first, then bulking up."

The younger the child, the higher the proportion of organic matter and water in their muscles, and the better the elasticity of their muscle fibers. However, children's muscle fibers are thinner, their muscles are smaller, and their muscle strength is poor, especially in terms of endurance. As children grow older, the inorganic matter content in their muscles increases, and their muscle strength increases accordingly, approaching adult levels after age 18, with peak strength between 20 and 30 years of age.

The development of muscles in different parts of a child's body is also uneven. Generally, large muscle groups develop earlier, while small muscle groups develop later. During the development of upper limb muscles, flexor tone is stronger and extensor tone is weaker; during the development of lower limb muscles, extensor tone is stronger and flexor tone is weaker.

Human muscles have two main characteristics: extensibility, elasticity, and viscosity. Muscle extensibility is not proportional to the magnitude of external force; as the load increases, the muscle length actually shortens. After the external force is removed, due to the viscosity of muscle plasma, the muscle length does not immediately recover. The rate of muscle growth and development also varies at different ages. During puberty, muscles primarily grow in length to accommodate the rapid longitudinal growth of bones. Therefore, muscle fibers are thin, muscle cross-sections are small, and muscles are lightweight and weak. While strength training is not very effective during this period, it should still be done moderately. Otherwise, allowing muscles to grow freely is extremely detrimental to muscle mass. Therefore, males should begin consciously using light weights for moderate-load training from 11-12 years old, and females from 10-11 years old. This not only improves the central nervous system's ability to mobilize and control muscles, enhancing muscle function, but also lays a solid foundation for future training and bodybuilding, benefiting them throughout life.

The Function of Muscles
All human activities are accomplished using muscles as power sources, bones as levers, and joints as pivots. This is because muscles contain 20% protein, 75% water, and 5% other substances. The proteins include myosin and myofibrillarin, which are filamentous. When the motor center sends nerve impulses, myosin and myofibrillarin combine and contract to generate force.

Generally, each muscle in the human body can be divided into the muscle belly and the white tendons at both ends. Tendons are what people call "tendons" or "ligaments." Most muscles in the human body attach to a bone at one end and cross a joint to attach to another bone at the other end, making the connection between muscles and tendons, and tendons and bones, more secure. Muscles are much like rubber bands; they can stretch and contract. When a muscle stretches, it can straighten the bones at both ends of a joint; when it contracts, it can bring the bones at both ends of a joint together. This constitutes the various complex movements in daily life.

Muscles vary in location, cross-section, and size, resulting in different levels of force during contraction. Larger muscle groups generate greater force, enabling strenuous physical activity; smaller muscle groups generate less force, allowing for lighter or more delicate work. For example, the muscles of the upper limbs are delicate and numerous, making them the most flexible part of the body's movement system. The shoulder muscles, in particular, not only support the upper limbs but also enhance their flexibility through their own movement. Therefore, upper limb muscle contraction enables a wide variety of complex and delicate movements, such as supporting, hanging, throwing, pulling, and even writing. Lower limb muscles, on the other hand, are strong and robust, allowing for a wide range of high-intensity movements during contraction, such as running, jumping, straddling, standing, and kneeling.