The balance of the body

Fig. 1. The bearing surface of the body with free standing: S - the point corresponding projection common center of gravity. Fig. 2. Rebalancing of the body by compensating variation of activity of muscles of the legs the torso forward. The projection of the center of gravity (vertical line) returned to its former position on the bearing surface.

The balance of the body - the rest of the body in relation to some system of reference, in the particular case - the immobility of the body relative to the environment. The balance of the body is static or dynamic. In static equilibrium of the body projection common center of gravity is located within the area of support (Fig. 1). When walking, running, driving on skates , etc. dynamic equilibrium of the body is achieved by balancing, i.e. by summing the area of support under shifting the projection of the center of gravity of the body (Fig. 2).
In maintaining the equilibrium of the body involved a number of complex systems. An important role belongs to the vestibular system. Its receptor part is located in the inner ear consists of bags vestibule and three semicircular canals. When stimulation of the vestibular apparatus is irritation of sensitive hairs epithelium. Emerging impulses are transmitted to the vestibular nerve to the brain. In the process of maintaining the equilibrium of a body of the vestibular system interacts with the visual apparatus. These machines (vestibular and visual) have extensive bilateral relations with the cerebellum. The cerebellum is also very important innervation link in maintaining the equilibrium of the body. A great role is played numerous receptors in muscles, tendons, joints, and skin, and first of all the mechanisms of muscle tone, and proprioceptive reflexes. Coordination of all these mechanisms that ensure the balance of the body, are at different levels of the nervous system - in the spinal cord, the brain stem and in the bark of the big hemispheres.
The balance of the body is investigated numerous methods. Among them is the method of stabilography - registration movements projection common center of gravity in the area of support. The balance of the body is disturbed at various diseases: the defeat of the vestibular apparatus, cerebellum, lesions of the spinal cord , etc.,

The balance of the body - the stillness of it relative to the environment; in the narrow sense - preserving certain (for example, vertical position.
In accordance with the provisions of the static equilibrium of the human body by vertical post refers to an unstable type, since the common center of gravity lies above the area of support. When standing bearing area is enclosed inside surface formed by external outlines both the stop and the lines connecting them front and back of the endpoint. The balance of the body is kept up until the vertical projection), descended from a common center of gravity, does not go beyond the area of support (static equilibrium of the body). If the projection of the common centre of gravity has moved beyond the area of support, the rebalancing of the body is possible only by balancing, i.e. summing the area of support under the displacement projection common centre of gravity (dynamic equilibrium of the body). This kind of equilibrium of the body takes place in all kinds of movements - walking, running, driving on skates, Bicycle, and so on, When peaceful standing vertical through the common centre of gravity of the body (Fig. 1, bottom arrow) passes in front of the GSP ankle joints (4-5 cm) and axis knee joint (0.5-1.5 cm) and behind axis Tazo-hip joints (1-3 cm), and a vertical line through the center of gravity of the upper half of the body (Fig. 1, top arrow)located on the axis of Tazo-thigh joints, passes in front of the spine (1-2 cm in front of IV lumbar vertebra). Thus, the weight of the body creates static (tipping) moments of forces on a number of joints: the gravity of a body aimed at extension in Tazo-hip and knee joints, flexion in the ankle joints and front bending of the body, and so on
The preservation of the static equilibrium of the body is possible in case, if the total of the tipping point, created by the force of gravity and other external
forces, counteracts equal in magnitude and opposite in direction balancing (fixing) the moment created internal (muscle) forces. The more static (tipping) torque acts on the joint, the more effort should develop the muscles of the joints for fixing the position. As shown by the registration of the electrical activity of muscles, distribution muscular effort when saving vertical position corresponds biomechanical features of this body position, i.e. corresponds to the value and direction of the force of gravity acting on the joints. So, the greatest activity is detected in muscle extensor ankle joints, in which the static moment of gravity has the largest value. Together with reduction of static moments in more highly placed knee, Tazo-hip joints decreases and the level of electrical activity of muscles (Fig. 2).
Even when deliberately motionless posture of frequent mutual displacement of parts of the body relative to each other (for example, in connection with the act of breathing and other reasons), modifying static tipping points, leading to the need for continuous dynamic adaptation of appropriate balancing of muscle moments. This dynamic process is reflected in the vibrations of the body that can be registered directly (cephalography) or indirectly to move projection common center of gravity on the frame (stabilography). The complex nature of stabilograms (Fig. 3) reflects the activity of multilevel system of regulation of the vertical posture, including different parts of the nervous system.
The main working mechanism of conservation poses is spinal proprioceptive system of reflex to the tension, causing postural tone. Receptors of this reflex are located in the muscles muscle spindles, the impulses are amplified when stretching the muscles. The impulses from the muscle spindles has a reflex stimulating effect on motoneurone as your own muscles, and the muscles-synergists. In reflex postural tone can distinguish two components: static defining background activity postural muscles, and dynamic driving continuous correction slightest violations poses. According to this system of reflex to the tension includes two types of receptors - primary (dynamic) and secondary (static) the end of the muscle spindles and two types of motoneuronal - fast (physical) and slow (tonic), related to fast and slow muscle fibers. The level of system activity of reflex to the tension and its sensitivity to changes in posture is adjusted and regulated by the higher departments of a CNS in accordance with the information these departments receive from musculoskeletal, skin, vestibular, visual analyzers.

In the system motion analyzer (see Motion) for balance of the body, in addition to afferention from muscle receptors, an important role is played also afferentation from receptors of the joint-ligamentous apparatus, indicating the direction and rate of change of articular corner. Of particular importance afferentation from receptors of the joint-ligamentous apparatus of the cervical spine influencing redistribution of reflex tone of muscles of the limbs and trunk in accordance with changes in head [cervical-tonic reflexes (see Magnus-Klein reflexes)]. A similar role is played by afferentation from receptors of vestibular apparatus (see), which provides the analysis of the position and movement of the head in space and implementation in response to adequate stimuli (acceleration, change of gravity tonic reflexes, including static, including reflexes poses with labyrinth on the legs, neck and torso, and statokinetic and vegetative reactions. The imbalance of the body, which are observed in the clinic with the defeat of the cerebellum (see), seems to be connected with the value of the latter in the coordination of proprioceptive and vestibular postural reflexes.
Part of the visual analyzer in the balance of the body is connected, on the one hand, ensuring the visual orientation of body position relative to the surrounding objects, and on the other - with a total effect of light as one of the most important factors determining the level of activity of the Central nervous system. In this regard, closing his eyes leads to an increase in the amplitude of oscillations of the body, increasing also equip opaque glasses or study in a darkened room (C. S. Gurfinkel). When the subject is in the Romberg (see Romberg symptom) - with improper feet, bearing area is reduced, and stretched forth his hands to create great tipping point that shows increased requirements to the system of regulation poses. In these conditions, the closing of the eye, causing additional vibrations of the body, can result in clear breach of the balance of the body and even drop.

Fig. 2. The level of electrical activity of muscles at a convenient distance (in descending order are indicated by substantially blackened, hatching, dots and crosses): 1 - front view 2 rear view.
Fig. 3. Stabilographic registration fluctuations common centre of gravity of the body in a vertical position.