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medial arch do not have the intended effect because the lateral arch has
been allowed to continue to move.
In the frontal plane, the arch support should extend as far as the fourth
metatarsal to fully support the tarsal structures. Recent research has
demonstrated how significant tarsal and midfoot joints are in foot motion,
often moving much more than the rearfoot joints ( Lundgren et al 2008 ,
Nester et al 2007 ). This means that in order to control foot motion, all the
rear- and midfoot joints must be supported.
Reducing foot pressures and shear forces
Pressure is related to both the amount of force applied to the plantar side
of the foot and the area over which that force is applied. Thus, to reduce
the harmful effects of pressure at a specific site (area) under the foot, the
biomechanical objective of the orthosis is either to reduce the magnitude
or rate of application of force, or to increase the area over which that
force is applied.
The total force applied to the foot is a function of body mass and
acceleration of body mass relative to the ground, and, assuming both
remain constant between strides, the same total force has to be applied
to the plantar surface of the foot, regardless of the orthosis used.
Reducing the rate at which force is applied to the foot can be achieved
using thick layers of soft cushioning materials. Thin layers of even the
most cushioning materials can be ineffective if the forces applied are high.
They will quickly compress completely ('bottom out') and become stiff
under the foot. In contrast, single thick layers of material, or multiple thin
layers, will deform under load but retain their compliance without becom-
ing stiff and bottoming out.
In addition, it is likely that use of these conforming materials has
important effects local to specific sites under the foot. For example, as a
metatarsal head compresses an insole, the material deforms and con-
forms to the shape of the metatarsal head. As it conforms (adopts the
same shape as the metatarsal head region), it will increase the contact
area, thereby reducing the peak pressure at that site. Coupled with the
reduced rate at which force is applied, this can be a very useful clinical
effect.
As well as such 'local' changes in contact area, there are obviously
possible gross changes in contact area. The use of a deep heel cup can
ensure that the medial, lateral and posterior sides of the calcaneal fat pad
bear load. The use of arch supports will increase the contact area under
the foot as soon as the anterior aspect of the plantar surface of the heel
is loaded. At the stage when the heel and forefoot are on the ground, the
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