Theoretical considerations

When standing, the healthy human foot rests on the ground at three points: the heads 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. This structure is not rigid, and the ligaments and joints between the bones dampen the shocks when walking, making your gait smooth.

Pressure analysis

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Everyone knows that, when someone presses their nose to a window pane, a distinctive pattern is visible from the other side of the glass. The pressed area is proportionate with the pressure which created it. In this case the pressure at the tip of the nose is more pronounced, while less force is exerted on the parts around it.

This phenomenon was observed by scientists long ago but it has only been applied in practice recently.

Kepler’s law

When light crosses from an optically rarer medium such as air to denser medium, the refracting angle is always smaller than the angle of incidence. When the beta angle increases, the ray can no longer leave the medium and complete reflection occurs at the boundary surface (Kepler, 1611).

For this reason the angle is called the critical angle of total internal reflection. Fibre optic cables use the same law of nature to trap light.

The Goos-Hänchen effect

It seems a contradiction that total internal reflection depends on the refractive index of a medium which the light does not even enter. More detailed studies show that in fact the light does penetrate the other medium to a minute distance of one or two wavelengths, gradually turning back towards the medium it came from, and the reflected ray shifts slightly because it did indeed travel a minute distance in the other medium. The practical application of this effect enables us to map the distribution of pressure on a body part, in this case the sole of the foot, individually and more accurately than ever before, and then to use the results.

The weight line

Looking at the areas the foot rests on, the pressure is greater at the heel bone and the head of the first metatarsal than at 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 the weight of the body exerts the most pressure is called the weight line.
This means that the weight line of even a healthy, sound foot runs along the inner edge of the heel bone, thus increasing the load of the inner part (medial) of the foot. The force on the inner arc of the longitudinal arch is great, and even the ligaments are only able to sustain the arch at the limit of their flexibility. When standing (in a static position), the weight line is divided differently, but 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.

Gait cycle

Analysing the sequence of scans recorded by the podoscope while walking, it can be stated that the static pressure map is comprised only of mosaics of the dynamic pressure maps that are created in the different phases of the gait cycle. Thus dynamic features can be deduced from the static image.

Static problems

Static problems are at the root of most foot disorders. The majority of ordinary foot problems are caused by fallen arches or the changes that occur as they fall.

Due to factors both inside and outside the body (sudden heavy load, ligament weakness, overweight, improper lifestyle, hormonal changes, etc.), the ligaments supporting the arches stretch. This in turn causes the arch to fall and the weight line to change, e.g. when the longitudinal arch drops, it shifts towards the inner side of the foot.

Diagnostic definition of an abnormal weight line

By examining the pressure map, abnormal pressure patterns caused by static disorders can be identified. The course of the weight line is defined by linking together the main pressure points and then comparing this with the course regarded as normal. With talipes valgus, a type of clubfoot, and flat feet, for instance, the pressure line shifts towards the inner side of the foot, while with a fallen transverse arch it does not proceed towards the big toe but the second toe.