Every mass or body is composed of a multitude of small particles attracted downwards by the force of gravity. This attraction to which the particles of the body are subject produces a system of forces that are practically parallel and the result of these forces acting vertically downwards is the weight of the body. It is possible to localise a point in which one can apply a single force that is equivalent, in terms of intensity, to the weight of the body and which acts vertically upwards, so as to confer on the body a state of equilibrium in every position.
This point is called barycentre, and can be described as the point in which the entire weight of the body is concentrated. The barycentre is the exact centre of the mass of a subject, i.e. its geometric centre when the subject has an even and symmetrically distributed mass. If the mass, as in the human body, is distributed asymmetrically in relation to the horizontal plane, the barycentre will be located proportionately closer to the larger and heavier area.
Furthermore, the centre of gravity of two segments is always on the line that joins the centre of gravity of these segments, i.e. in a point located in an intermediate position with respect to the centres of gravity of the two segments, but proportionately closer to the centre of gravity of the heavier segment. In an upright posture, if one extends the vertical line, from the centre of gravity to the contact area, it will be in the centre of the contact area (an almost trapezoidal polygon, constituted by the lateral profile of the feet and by the two lines constituting the front and rear part of the feet), ± 3 cm in front of the ankle.
The line of gravity therefore passes along the sagittal plane about halfway between the tibiotarsal and metatarsal-phalangeal joints, and along the frontal plane, in the well distributed support between the two feet. Around the line of gravity the body is hypothetically in a position of equilibrium, implying a uniform distribution of body weight and a stable position of each joint.