Biomedical Engineering Reference
In-Depth Information
The presented approaches aim to illustrate just different operating principles
for plantar pressure measurements, since this type of data can be obtained from
commercial available solutions as described before.
2.2.2
Measuring Plantar Shear Forces
In an attempt to achieve tridimensional characterization of local plantar surface
force distribution, allowing mapping the three plantar force components, as achieved
with pressure insoles for the vertical component, several authors developed different
approaches.
In 1980, Tappin et al
.
presented a centre-tapped magneto resistor in a bridge
configuration with a magnet placed centrally above it, to measure shear forces under
the toe and metatarsals heads. In this solution, any lateral movement of the magnet
will unbalance the bridge, thus generating a signal proportional to the magnet
movement which, in turn, is proportional to the shear force responsible for that
movement [
43
]. With this solution they measure these forces in barefoot subjects as
well as a variety of footwear to test their effect in modifying and redistributing shear
forces.
One symptomatic aspect related to the intrinsic and major difficulty of measuring
plantar shear forces is the fact that, more than a decade after this work, in 1992, other
authors using also magneto-resistor devices, developed different approaches for the
same purpose [
24
,
29
]. When looking for different approaches in plantar shear force
measurement, one can find the solution proposed and validated by Lebar et al., using
optoelectronic devices consisting on a photodiode and a surface mounted LED of
660 nm emission wavelength, both mounted on a shell mainly composed by an upper
and lower component where the light emitter-receiver set is located [
26
]. The sensor
has a diameter of 15 mm by 3.8 mm thick, weighting 0.85 g. Also, in 1998, Davis et
al. developed what could be thought of as a small platform consisting on 4
4sensor
matrix measuring the shear component, covering a total area of 10.5
10.5 cm2
[
14
]. Each sensor is an aluminum cylinder on top of which one can find an S-shaped
load cell with strain-gages which measure the deformation suffered by the load cell
faces, as a result of the shear force due to foot movement. This may be one of the
first well succeeded attempts of designing a 3D plantar force mapping platform. The
authors recognized that their solution was not appropriate for inshoe measurements,
thus the acquired data will always represent the force between the shoe and the floor
surface.
In an overall look at the temporal development published on this subject, it
appears that two techniques are popular: optical and magneto-resistive sensor
techniques.
Indeed, in 2000, Hosein et al. presented a bi-axial magneto resistive sensor,
together with the F-scan
®
insoles for the vertical component measurement, charac-
terizing completely the three plantar vector force components in five plantar areas:
four underneath the metatarsals heads and the fifth under the calcaneous (heel) [
21
].
As for optical techniques, Koulaxouzidis et al. developed the first optical sensor
based on Fiber Bragg Grating (FBG) disposed in three directions constituting a
