Statics and dynamics of the human body: the center of gravity

The vertical position of the human body, its movement in space, various types of movements (walking, running, jumping) have evolved in the course of a long evolution along with the emergence of man as a species. In the process of anthropogenesis, in connection with the transition of the human ancestors to the terrestrial conditions of existence, and then to the movement on the two (lower) extremities, the anatomy of the whole organism, its separate parts, organs, including the locomotor apparatus, has changed significantly. Riding has freed the upper limb from the musculoskeletal function. The upper limb turned into an organ of labor - the arm and in the future could be improved in dexterity of movements. These changes as a result of a qualitatively new function affected the structure of all the components of the belt and the free section of the upper limb. The shoulder belt serves not only as a support for the free upper limb, it significantly increases its mobility. Due to the fact that the scapula connects to the skeleton of the trunk mainly with the help of muscles, it acquires greater freedom of movement. The shoulder blade participates in all movements that make the collarbone. In addition, the scapula can freely move independently of the clavicle. In the multiaxial spherical shoulder joint, which is surrounded almost from all sides by muscles, the anatomical features of the structure allow the movement along large arcs in all planes. Especially noticeable specialization of functions has affected the structure of the brush. Thanks to the development of long, very moving fingers (primarily the thumb), the brush has become a complex organ that performs subtle, differentiated actions.

Lower limb, assuming the entire weight of the body, has adapted exclusively to the musculoskeletal function. The vertical position of the body, the uprightness reflected on the structure and functions of the belt (pelvis) and the free section of the lower limb. Belt of the lower extremities (pelvic girdle) as a solid arched structure has adapted to transfer the gravity of the trunk, head, upper limbs to the femoral head. Established in the process of anthropogenesis, the slope of the pelvis at 45-65 ° facilitates the transfer to the free lower limbs of the body's gravity in the biomechanical conditions most favorable for the vertical position of the body. The foot has acquired a vaulted structure, which increased its ability to withstand the body's weight and act as a flexible lever when moving it. The musculature of the lower limb has developed strongly, which has adapted to the performance of static and dynamic loads. In comparison with the muscles of the upper limb, the muscles of the lower limb have a large mass.

On the lower limb muscles have extensive support surfaces and application of muscle strength. The muscles of the lower limb are larger and stronger than the upper limb. Extensors are more developed on the lower limb than flexors. This is due to the fact that extensors play a big role in keeping the body in a vertical position and during movement (walking, running).

On the arm, flexors of the shoulder, forearms and hands are concentrated on the front side, since the work done by the hands is performed in front of the trunk. Grooming movements are made by a brush, which is acted upon by a greater number of flexors than extensors. Turning muscles (pronators, insteps) in the upper limb is also greater than in the lower one. In the upper limb, they are much better developed than in the lower extremity. The mass of pronators and insteps of the hand refers to the remaining muscles of the upper limb as 1: 4,8. In the lower extremity, the ratio of the mass of the turning muscles to the rest is 1: 29.3.

Fascia, aponeurosis in the lower extremity due to a large manifestation of force under static and dynamic loads are much better developed than in the upper limb. The lower limb has additional mechanisms that help to keep the body in a vertical position and ensure its movement in space. Belt of the lower limb is almost immovably connected to the sacrum and is a natural support of the trunk. Aspiration of the pelvis to tip backwards on the head of the femurs is impeded by the strongly developed ileum-femoral ligament of the hip joint and strong muscles. In addition, the vertical of gravity of the body, which extends in front of the transverse axis of the knee joint, mechanically promotes retention of the knee joint in the extended position.

At the level of the ankle joint, when standing, the area of contact between the joint surfaces of the bones of the lower leg and the talus bone increases. This is facilitated by the fact that the medial and lateral ankles span the anterior, wider section of the talus block. In addition, the front axes of the right and left ankle joints are set to each other at an angle open to the back. The vertical of the body's weight passes anterior to the ankle joints. This leads, as it were, to the infringement of the anterior, wider segment of the talus block between the medial and lateral ankles. Joints of the upper limb (shoulder, elbow, wrist) do not have such braking mechanisms.

Deep changes in the process of anthropogenesis were bones, muscles of the trunk, especially the axial skeleton - the spine, which is the support for the head, upper limbs, chest and abdominal cavities. In connection with the upright, spinal bends were formed, powerful dorsal musculature developed. In addition, the spine is practically immovably connected in a pair of sturdy sacroiliac joint with a belt of lower limbs (with a pelvic belt), which biomechanically serves as a distributor of the gravity of the trunk on the femoral head (on the lower extremities).

Along with the anatomical factors - the features of the structure of the lower extremity, the trunk, developed in the process of anthropogenesis to maintain the body in an upright position, to ensure stable equilibrium and dynamics, special attention should be paid to the position of the center of gravity of the body.

The common center of gravity (OCT) of a person is called the point of application of the resultant forces of gravity of parts of his body. According to MF Ivanitsky, OCT is located at the level of the IV sacral vertebrae and is projected onto the anterior surface of the body above the pubic symphysis. The position of the TC in relation to the longitudinal axis of the body and the spinal column depends on the age, sex, bones of the skeleton, muscles and fat deposits. In addition, daily fluctuations in the position of the OCT are observed in connection with the shortening or lengthening of the spinal column, which arise due to uneven physical activity day and night. In older and older people, the position of the OCT also depends on the posture. In men, the OCT is located at level III of the lumbar vertebrae, V of the sacral vertebrae, in women it is 4-5 cm lower than in men, and corresponds to the level from the V lumbar spine to the I of the coccygeal vertebra. This depends, in particular, on the greater than in men, the deposits of subcutaneous fat in the pelvic region and thighs. In newborns, the OCT is at the level of the V-VI thoracic vertebrae, and then gradually (up to 16-18 years) descends and moves somewhat posteriorly.

The position of the OCT of the human body also depends on the type of physique. In individuals with a dolichomorph type of physique (in asthenics), the OCT is relatively lower than in persons of the brachymorph type of constitution (in hypersthenics).

As a result of the studies it was found that the body's OCT is usually at the level of the II sacral vertebra. The vertical line of the center of gravity passes 5 cm behind the transverse axis of the hip joints, about 2.6 cm behind the line connecting the large trochanteres, and 3 cm anterior to the transverse axis of the ankle joints. The center of gravity of the head lies slightly anterior to the transverse axis of the atlanto-occipital joints. The common center of gravity of the head and trunk is at the level of the middle of the anterior margin of the X thoracic vertebra.

To maintain a stable equilibrium of the human body on the plane, it is necessary that the perpendicular, dropped from its center of gravity, falls on the area occupied by both feet. The body is stronger because the wider the support area and the lower the center of gravity. For the vertical position of the human body, maintaining equilibrium is the main task. However, by straining the appropriate muscles, a person can hold the body in different positions (within certain limits) even when the projection of the center of gravity is moved beyond the support area (strong torso forward, sideways, etc.). At the same time, the standing and movement of the human body can not be considered stable. With relatively long legs, a person has a relatively small footprint. Since the common center of gravity of the human body is relatively high (at the level of the II sacral vertebra), and the reference area (the area of the two soles and the space between them) is insignificant, the stability of the body is very small. In a state of equilibrium, the body is held by the force of muscle contractions, which prevents it from falling. Parts of the body (head, trunk, limbs) take up position corresponding to each of them. However, if the proportion of body parts is broken (for example, stretching the arms forward, flexing the spine when standing, etc.), then the position and balance of other parts of the body change accordingly. The static and dynamic moments of the action of the musculature are in direct connection with the position of the center of gravity of the body. Since the center of gravity of the entire body is located at the level of the II sacral vertebra behind the transverse line connecting the centers of the hip joints, the strongly tentered muscles and ligaments that strengthen the hip joints resist the desire of the trunk (along with the pelvis) to reverse. This ensures the balance of the entire upper body, which is held on its legs in an upright position.

The desire of the body to fall forward when standing is due to the passage of the vertical of the center of gravity in front (3-4 cm) from the transverse axis of the ankle joints. The fall is opposed by the action of the muscles of the posterior surface of the shin. If the sheer line of the center of gravity moves further farther to the front - to the fingers, then the heel of the lower leg is shortened by the contraction of the hind muscles of the leg, the heel is lifted from the support plane, the vertical line of the center of gravity is moved forward and the toe is the support.

In addition to supporting, the lower extremities perform locomotor function, moving the body in space. For example, when walking a human body performs translational motion, alternately resting on one leg or the other. In this case, the legs alternately perform pendulum movements. When walking, one of the lower limbs at a certain moment is the support (back), the other - free (front). With each new step, the free leg becomes the supporting leg, and the supporting leg is moved forward and becomes free.

Reduction of the muscles of the lower extremity when walking noticeably strengthens the curvature of the sole of the foot, increases the curvature of its transverse and longitudinal arches. At the same time, at the same time, the body tilts forward along with the pelvis on the femoral head. If the first step is started by the right foot, then the right heel, then the middle of the sole and the fingers rise above the support plane, the right leg bends in the hip and knee joints and is carried forward. Simultaneously, the hip joint of this side and the trunk follow forward behind the free leg. This (right) leg energetically contracting the quadriceps femoris straightens at the knee joint, touches the support surface and becomes the supporting one. At this point, the other, the left leg (up to this point, the back, the supporting leg) breaks away from the support plane, moves forward, becoming the front, free leg. The right leg at this time is left behind as a supporting leg. Together with the lower limb and the body moves forward and somewhat upward. So both limbs alternately do the same movements in a strictly defined sequence, propping the body from one side and then the other and pushing it forward. However, during walking, there is no time for both feet to be torn off from the ground at the same time (support plane). The front (free) limb always has time to touch the plane of support with the heel before the back (support) leg completely separates from it. This is different from walking and jumping. At the same time, when walking, there is a moment when both feet touch the ground at the same time, the supporting one at the entire sole, and the free one with the fingers. The faster the walking, the shorter the moment of simultaneous contact of both legs to the support plane.

Tracing when walking changes the position of the center of gravity, you can note the movement of the whole body forward, up and side in the horizontal, frontal and sagittal planes. The greatest displacement occurs forward in the horizontal plane. The displacement up and down is 3-4 cm, and in the sides (lateral swings) 1-2 cm. The nature and extent of these displacements are subject to considerable fluctuations and depend on age, gender and individual characteristics. The combination of these factors determines the individuality of the gait, which can change under the influence of training. On average, the length of the usual quiet step is 66 cm and takes 0.6 s.

At acceleration of walking the step passes in run. Running differs from walking in that, with it, only the support and the contact of the support area with one or the other leg alternate.

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