When standing, the healthy human foot supports on the ground at three points: the capitula of the first and fifth metatarsals, and the heel bone. Arches, similar to the load-bearing structure in architecture, span between these main points.
When the feet are placed side by side, the arches form a dome-like structure. The structure is not rigid, and the tendons and joints between the bones dampen the shocks when walking, making one's gait smooth.
Conclusions can be drawn about the statics and dynamics of the foot through the examination of the supportive surface, the pressure examination of the sole.
By looking at the sole from below a specially side-lit glass sheet with a person standing on it, a pressure point pattern is revealed. The phenomenon is caused by the fact that due to the side lighting the beams of light reach the internal surface of the glass sheet in the same or a greater angle as the critical angle of the total reflection and the air. The beams of light only pass through the transparent glass sheet at the points where its surface meets an optically denser medium than air, i.e. the part of the sole. At the same time the amount of light emerging is proportionate with the pressure on the glass sheet, thus the brightness of the points of the image precisely shows the pressure. If we create standard examination conditions and know the body weight, an absolute value for the pressure can also be established from the brightness of the recorded image.
The principle of measurement
The body weight (which is known) is spread over the surface of the sole in contact with the ground. Pressure is not even, and the computer creates isochromatic areas, i.e. of the same colour, from patches with the same intensity of light. These represent areas with the same isobar, the same pressure. The pressure values are proportionate with the colour saturation, so by measuring accurately the body weight and the isobar areas pressure values can be determined exactly.
Looking at the supportive surfaces, we can see that the pressure of the heel bone and the capitulum of the first metatarsal is stronger than that of the fifth metatarsal. This is caused by the position of the centre of gravity and how the weight is distributed. The area where pressure is greatest is the so-called weight line representing the weight of the body.
Thus the weight line of even a healthy, sound foot runs through the inner edge of the heel bone, thus increasing the load of the inner part (medial) of the foot. The force on the inner arch of the longitudinal arch is great, and even the ligament system is only able to sustain the arch to the limit of its flexibility. When standing (in a static position), the weight line is divided differently. However, when walking (dynamic position), it moves forward in a characteristic line.
The course of the weight line in a healthy foot.
The centre of the heel - the outer edge of the foot - the base of the metatarsals; here about 75 % of the force turns in on the first metatarsal and 25 % on the head of the fifth, where the foot supports on or pushes off from the ground.
The phases of walking
Analysing the image sequence recorded by the podoscope while walking, it can be stated that the static pressure point image is only comprised of mosaics of dynamic pressure point images that are created in the different phases of walking. Thus dynamic features can be deducted from the static image.
Static problems are the main cause of foot disorders, and dropped arches or changes arising in the process of them dropping cause the majority of ordinary foot problems.
Due to different exogenous and endogenous reasons (sudden heavy load, ligament weakness, overweight, improper lifestyle, hormonal changes, etc.), the ligaments supporting the arches stretch. This in turn causes the arch to drop and the weight line to change, e.g. when the longitudinal arch drops, it shifts inwards.
Diagnostic definition of an abnormal weight line
By examining the pressure point image, abnormal pressure point formations caused by static disorders can be defined. We define the course of the weight line (pressure line) by linking together the main pressure points, then compare it with the course regarded as normal. With talipes valgus and flat foot, for instance, the pressure line shifts inwards, while with a dropped transversal arch it does not proceed towards the big toe but the second toe.