Center of Gravity

CENTRE OF GRAVITY of the human body does not possess a permanent anatomical position within the body, but shifts depending on changes in posture; its excursions relative to the spine can reach 20-25 cm. Experimental determination of the position of the centre of gravity of the entire body for a given posture is carried out by one of the following three methods: 1) the subject is placed on a board laid upon the sharp edge of a prism and is moved relative to the latter until (unstable) equilibrium on the prism is achieved. At this moment, the vertical plane passing through the edge of the prism also passes through the centre of gravity of the system (person + board). This (historically oldest) method is painstaking and inconvenient. 2) The subject is placed or laid on a board of a hanging swing, which is allowed to come to rest in a state of (stable) equilibrium. In this case, the centre of gravity of the system (person + board) turns out to be in the same vertical plane as the suspension axis of the swing. 3) The subject is laid on a horizontal board resting at both ends on mutually parallel sharp edges of two prisms. One of the prisms has a fixed support, while the other is connected to scales allowing the determination of the pressure it experiences. If we designate the distance between the edges of both prisms by d, the reading of the scales with an empty board by M₀, their reading with the board and a lying person by M, and the weight of the person by P, then x = d(M - M₀) / P

-- is the distance along the horizontal from the sought centre of gravity of the entire body to the vertical plane passing through the edge of the fixed prism. This method is the most perfect and accurate. Its idea is attributed to du Bois-Reymond; it was developed and improved by Scheidt and further by N. A. Bernstein. In the straightened posture, the centre of gravity of the entire human body is located close to the upper aperture of the lesser pelvis, somewhat below the promontorium of the sacrum. According to fluoroscopic observations by M. Ivanitsky, the vertical level of the centre of gravity varies from S_I to S_V. Variations in the position of the centre of gravity in the anteroposterior and lateral directions have been little studied. Its position in the vertical (head-tail) direction is usually characterized by an index, the numerator of which is the vertical distance from the soles of the feet to the centre of gravity, and the denominator is the height of the subject. According to Ivanitsky, the centre of gravity index generally lies between 0.555 and 0.565, with individual deviations reaching 0.545 and 0.580. Bernstein established in 1931 on 3 frozen corpses that the positions of the centres of gravity of long limb segments can be determined on a living person with an error of less than 1 mm using the volumetric planimetry method he proposed, which made it possible to apply du Bois-Reymond's two-support board method to the determination of the masses of body segments. A large study was carried out by this author together with collaborators on 152 subjects of both sexes and various ages, the results of which have not yet been fully processed.