The musculoskeletal system consists of the skeleton (bones), muscles, ligaments and joints. These structures form cavities for internal organs, protect internal organs, and also provide motor acts.

The skeleton forms the structural basis of the body, determines its shape and dimensions. In the skeleton of an adult, there are more than 200 bones, which primarily perform a supporting function and are a kind of leverage in the implementation of motor acts. At the same time, bones are actively involved in metabolic processes: they accumulate mineral salts and, if necessary, supply them to the body (mainly calcium and phosphorus salts). Bones also contain hematopoietic tissue – red bone marrow.
Bone content

Bones contain approximately 60% minerals, 30% organic components (mainly ossein protein and osteoblast bone cell bodies) and 10% water. Such a combination of substances in the structure of bones provides them with significant strength (30 times stronger than brick and 2.5 times stronger than granite) and greater elasticity, elasticity and viscosity (9 times higher than the viscosity of lead). The bones are characterized by a significant margin of safety (for example, the femur can withstand a load of 1.5 tons). In children, tubular bones grow in length due to the cartilage between the ends of the bones (epiphysis) and their body (diaphysis), and in thickness – due to the surface tissue – the periosteum. Flat bones grow in all directions only due to the periosteum. At the end of the growth of the human body, cartilage in many bones is replaced by bone tissue. The development of the skeleton in men ends at 20-24 years, and in women – at 17-21 years.

Individual bones and even parts of the skeleton mature at different periods. So, until the age of 14, only the middle parts of the vertebrae are ossified, while their other parts remain cartilaginous, and only at 21-23 years old do they become completely bony. By the same period, the ossification of most other bones of the skeleton is also completed.

An important stage in the development of the human skeleton is the formation and fixation of the folds of the spine, which are divided into those that are directed with the convex side forward and are called lordosis (occur in the neck and lumbar spine) and those that are directed backward and are called kyphosis (thoracic and sacral sections of the spine). The presence of lordosis and kyphosis is a necessary phenomenon due to the upright posture of a person when standing and walking; it is also necessary to maintain the balance of the body and ensure the function of shock absorption when moving, jumping, etc.. Until the age of 5-6, the folds of the spine are little fixed, and if the child lies down, then most often these folds disappear (level out). Fixation of the folds of the spine occurs gradually: up to 7-8 years, only the cervical and thoracic flexures are formed, and at 12-14 years – lordosis of the lumbar spine and kyphosis of the sacral spine. The final fixation of lordosis and kyphosis is completed with the ossification of the vertebrae of the spine (17-20 years). In the frontal projection (when viewed from the front or from the back), a normally developed spine should be even.