Statics and dynamics of the human body: center of gravity

The vertical position of the human body, its movement in space, various types of movements (walking, running, jumping) developed in the process of long evolution together with the formation of man as a species. In the process of anthropogenesis, in connection with the transition of human ancestors to terrestrial conditions of existence, and then to movement on two (lower) limbs, the anatomy of the entire organism, its individual parts, organs, including the musculoskeletal system, changed significantly. Bipedalism freed the upper limb from the musculoskeletal function. The upper limb turned into an organ of labor - a hand and could further improve in dexterity of movements. These changes as a result of a qualitatively new function were reflected in the structure of all the components of the girdle and the free section of the upper limb. The shoulder girdle serves not only as a support for the free upper limb, it significantly increases its mobility. Due to the fact that the scapula is connected to the skeleton of the body mainly with the help of muscles, it acquires greater freedom of movement. The scapula participates in all movements made by the clavicle. In addition, the scapula can move freely independently of the clavicle. In the multi-axial ball-and-socket shoulder joint, which is surrounded by muscles on almost all sides, the anatomical features of the structure allow movements along large arcs in all planes. The specialization of functions is especially noticeable in the structure of the hand. Thanks to the development of long, very mobile fingers (primarily the thumb), the hand has become a complex organ that performs fine, differentiated actions.

The lower limb, having taken on the entire weight of the body, adapted exclusively to the musculoskeletal function. The vertical position of the body, upright posture reflected in the structure and functions of the girdle (pelvis) and the free section of the lower limb. The girdle of the lower limbs (pelvic girdle) as a strong arched structure adapted to the transfer of the weight of the trunk, head, upper limbs to the heads of the femur. The tilt of the pelvis by 45-65° established in the process of anthropogenesis contributes to the transfer of the weight of the body to the free lower limbs in the most favorable biomechanical conditions for the vertical position of the body. The foot acquired an arched structure, which increased its ability to withstand the weight of the body and act as a flexible lever when moving it. The muscles of the lower limb developed strongly, which adapted to performing static and dynamic loads. Compared to the muscles of the upper limb, the muscles of the lower limb have a greater mass.

On the lower limb, the muscles have extensive support surfaces and application of muscle force. The muscles of the lower limb are larger and stronger than those of the upper limb. On the lower limb, the extensors are more developed than the flexors. This is due to the fact that the extensors play a major role in keeping the body upright and in movement (walking, running).

In the arm, the flexors of the shoulder, forearm and hand are concentrated on the front side, since the work performed by the hands is done in front of the body. Grasping movements are performed by the hand, which is affected by a greater number of flexors than extensors. The upper limb also has more turning muscles (pronators, supinators) than the lower. They are much better developed in the upper limb than in the lower. The mass of the pronators and supinators of the arm relates to the rest of the muscles of the upper limb as 1:4.8. In the lower limb, the mass ratio of the turning muscles to the rest is 1:29.3.

The fascia and aponeuroses of the lower limb are much better developed than those of the upper limb due to the greater manifestation of force under static and dynamic loads. The lower limb has additional mechanisms that help to hold the body in a vertical position and ensure its movement in space. The girdle of the lower limb is almost immobilely connected to the sacrum and is a natural support for the trunk. The tendency of the pelvis to tip backwards on the heads of the femurs is prevented by the highly developed iliofemoral ligament of the hip joint and strong muscles. In addition, the vertical of the body's gravity, passing in front of the transverse axis of the knee joint, mechanically helps to hold the knee joint in an extended position.

At the ankle joint level, when standing, the area of contact between the articular surfaces of the tibia and talus increases. This is facilitated by the fact that the medial and lateral malleoli embrace the anterior, wider section of the talus block. In addition, the frontal 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 gravity passes forward in relation to the ankle joints. This leads to a kind of pinching of the anterior, wider section of the talus block between the medial and lateral malleoli. The joints of the upper limb (shoulder, elbow, wrist) do not have such braking mechanisms.

The bones and muscles of the trunk, especially the axial skeleton - the spinal column, which supports the head, upper limbs, and organs of the thoracic and abdominal cavities - underwent profound changes in the process of anthropogenesis. In connection with upright posture, curves in the spine were formed, and powerful dorsal muscles developed. In addition, the spine is practically immobile in a paired strong sacroiliac joint with the lower limb girdle (with the pelvic girdle), which in biomechanical terms serves as a distributor of the weight of the trunk to the heads of the femur (to the lower limbs).

Along with anatomical factors - the structural features of the lower limbs and torso, developed in the process of anthropogenesis to maintain the body in an upright position, ensuring stable balance and dynamics, special attention should be paid to the position of the body's center of gravity.

The general center of gravity (GC) of a person is the point of application of the resultant of all gravitational forces of the parts of his body. According to M.F. Ivanitsky, the GC 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 GC in relation to the longitudinal axis of the body and the spinal column depends on age, gender, skeletal bones, muscles and fat deposits. In addition, there are daily fluctuations in the position of the GC due to the shortening or lengthening of the spinal column, which occur due to uneven physical activity during the day and at night. In elderly and old people, the position of the GC also depends on posture. In men, the GC is located at the level of the III lumbar - V sacral vertebrae, in women - 4-5 cm lower than in men, and corresponds to the level from the V lumbar to the I coccygeal vertebra. This depends, in particular, on the greater deposition of subcutaneous fat in the pelvic and hip area than in men. In newborns, the center of gravity is at the level of the V-VI thoracic vertebrae, and then gradually (up to 16-18 years) it moves downwards and slightly backwards.

The position of the human body's CG also depends on the body type. In people with a dolichomorphic body type (asthenics), the CG is located relatively lower than in people with a brachymorphic body type (hypersthenics).

As a result of the research it was established that the human body's center of gravity is usually located at the level of the second sacral vertebra. The plumb line of the center of gravity passes 5 cm behind the transverse axis of the hip joints, approximately 2.6 cm behind the line connecting the greater trochanters, and 3 cm in front of the transverse axis of the ankle joints. The center of gravity of the head is located slightly in front of the transverse axis of the atlanto-occipital joints. The common center of gravity of the head and body is at the level of the middle of the anterior edge of the tenth thoracic vertebra.

To maintain stable equilibrium of the human body on a plane, it is necessary that the perpendicular dropped from its center of gravity falls on the area occupied by both feet. The body stands more firmly, the wider the support area and the lower the center of gravity. For the vertical position of the human body, maintaining balance is the main task. However, by straining the appropriate muscles, a person can hold the body in various positions (within certain limits) even when the projection of the center of gravity is outside the support area (strong forward tilt of the body, to the sides, etc.). At the same time, standing and moving the human body cannot be considered stable. With relatively long legs, a person has a relatively small support area. Since the overall center of gravity of the human body is located relatively high (at the level of the second sacral vertebra), and the support area (the area of 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. The body parts (head, torso, limbs) occupy a position corresponding to each of them. However, if the ratio of body parts is disturbed (for example, stretching the arms forward, bending the spine when standing, etc.), then the position and balance of other body parts change accordingly. Static and dynamic moments of muscle action are directly related to the position of the body's center of gravity. Since the center of gravity of the entire body is located at the level of the second sacral vertebra behind the transverse line connecting the centers of the hip joints, the tendency of the torso (along with the pelvis) to tip backwards is resisted by highly developed muscles and ligaments that strengthen the hip joints. This ensures the balance of the entire upper body, which is held upright on the legs.

The tendency of the body to fall forward when standing is due to the vertical line of the center of gravity passing forward (by 3-4 cm) from the transverse axis of the ankle joints. The fall is resisted by the actions of the muscles of the back of the leg. If the vertical line of the center of gravity moves even further forward - to the toes, then by contracting the back muscles of the leg the heel is raised, lifted from the plane of support, the vertical line of the center of gravity moves forward and the toes serve as support.

In addition to supporting, the lower limbs perform a locomotor function, moving the body in space. For example, when walking, the human body makes a forward movement, alternately leaning on one leg, then on the other. In this case, the legs alternately make pendulum-like movements. When walking, one of the lower limbs at a certain moment is a support (rear), the other is free (front). With each new step, the free leg becomes a support, and the support leg is brought forward and becomes free.

Contraction of the muscles of the lower limb during walking significantly increases the curvature of the sole of the foot, increases the curvature of its transverse and longitudinal arches. At the same time, at this moment, the torso slightly tilts forward together with the pelvis on the heads of the femurs. If the first step is started with the right foot, then the right heel, then the middle of the sole and the toes rise above the plane of support, the right leg bends at the hip and knee joints and is brought forward. At the same time, the hip joint of this side and the torso follow forward after the free leg. This (right) leg, with an energetic contraction of the quadriceps muscle of the thigh, straightens at the knee joint, touches the surface of support and becomes the support. At this moment, the other, left leg (until this moment the back, support leg) comes off the plane of support, is brought forward, becoming the front, free leg. At this time, the right leg remains behind as a support leg. Together with the lower limb, the body moves forward and slightly upward. Thus, both limbs alternately perform the same movements in a strictly defined sequence, supporting the body first on one side, then on the other, and pushing it forward. However, during walking, there is no moment when both legs are simultaneously torn off the ground (the plane of support). The front (free) limb always manages to touch the plane of support with its heel before the back (supporting) leg is completely separated from it. This is how walking differs from running and jumping. At the same time, when walking, there is a moment when both legs simultaneously touch the ground, with the supporting leg touching the entire sole, and the free leg touching the toes. The faster the walk, the shorter the moment of simultaneous contact of both legs with the plane of support.

Tracing the changes in the position of the center of gravity during walking, one can note the movement of the entire body forward, upward and to the 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 to the sides (lateral swings) - 1-2 cm. The nature and extent of these displacements are subject to significant 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 a normal calm step is 66 cm and takes 0.6 s.

When walking accelerates, the step turns into a run. Running differs from walking in that it involves alternate support and touching the support surface with one foot and then the other.

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