A New Mechatronical Device for Determining Human ... - IFToMM

12th IFToMM World Congress, Besançon (France), June18-21, 2007
A New Mechatronical Device for Determining Human Plantar Pressure
F. Chedevergne * M. Dahan † A. Faivre
LMARC Institut de productique Institut de productique
Besançon, France Besançon, France Besançon, France
Abstract—A medically helpful mechatronical device for
natural gait analysis is proposed. This consists of an
instrumented shoe equipped with eight force sensors. The
instantaneous plantar pressures are recorded by a personal
digital assistant (PDA) belted on the patient hips. Thus, doctors
get informed of current and future health of their patients'
locomotor system. This article describes this original concept
and the first results of its use.
Keywords: gait analysis, plantar pressure,
mechatronical device, strain gauge, PDA
I. Introduction
As illustrated in [1], [2], [3], [4], [5] and [6], both
injuries and illness change our gait kinematics from its
healthy state to some painful patterns. This kinematics
leads to an unhealthy plantar pressure pattern which can
cause other injuries in the joints of the body.
Thus gait plantar pressure is an important source of
information about both current walking issues and future
injuries. It might permit doctors to detect and to prevent
from musculoskeletal dysfunctioning. Reestablishing good
dynamic plantar pressures is thought to prevent patients
from joint injuries.
Most of orthopedics controls pay only attention to the
static plantar pressures because of a lack of device for
dynamic analysis. According to [7], the pressure
distribution during standing reveals only little information
about the dynamic loads under the foot during gait.
Reference [2] explains that a pes cavus may walk like a
flat foot or a normal foot and a flat foot may walk like a
pes cavus or a normal foot.
Currently, the most used dynamic analysis devices are
piezoelectric insoles which allow individuals to walk
naturally. But those insoles are to be set inside a shoe
which constrain the foot in space and dynamic contact on
the ground.
To observe the real behavior of the foot during the gait,
barefoot pressures must be obtained. For that doctors may
use pressure plates. But in this system, the patients have to
walk carefully in order to put the foot in the centre of the
plate and thus change their kinematics.
In order to help doctors to analyse their patients' gait,
a mechatronical device is being developed for measuring
the barefoot plantar pressures during the usual gait of each
individual over numerous consecutive steps.
This article presents this new device and the first results
obtained using it.
II. Our Device
Although plantar pressure analysis is not a new
preoccupation, our device is a new concept. Based on
medical knowledge, our system aims (figure 1) to measure
the pressure under the main acting zones of the foot
during natural barefoot gait.
Fig. 1. Our plantar pressure acquisition device with one shoe,
one gauge conditioner and the PDA.
This barefoot plantar pressure acquisition system is
based on a medical shoe, which do not compress the foot
and which sole welcomes several strain gauge sensors,
and an acquisition modulus which is to be hold on the
hips. The current acquisition modulus is compounded by a
gauge conditioner and a PDA which volumes still have to
be reduced.
To analyse the results when using a piezoelectric sensors
device, doctors first divide the plantar pressure
distribution into areas of interest. Such a division is called
a mask. Most masks found in publications use 8 areas
([4], [7] and [8]).
* : E-mail: fany.chedevergne@univ-fcomte.fr
† : E-mail: marc.dahan@univ-fcomte.fr

12th IFToMM World Congress, Besançon (France), June18-21, 2007
To make it easier to analyse for the doctors, our device
only measures plantar pressures under the main acting
zones of the foot during the gait. These zones are the heel,
the external side, the metatarsal heads and the hallux. For
example, for an adult our device contains eight sensors:
two under the heel, two under the external side, three
under the metatarsal heads and one under the hallux
(figure 2).
Fig. 2. Floor-sole of an equipped shoe for adult (8 sensors).
Our device works as following. When walking, the foot
compresses the force sensors against the floor: mechanical
behavior. The compression changes the voltage passing
through the strain gauge glued of the sensor: electronic
information. The voltage collected at the exit of the sensor
is amplified before arriving to the PCMCIA acquisition
card put in the PDA which will process the data. A
software allows the PDA to show and to record the
instantaneous voltage. After the walk, the data may be
transferred to a computer and the pressure lines may be
plotted.
A. Mechanical Operation
1 hallux sensor
3 metatarsal heads sensors
2 external side sensors
2 heel sensors
For volume reason, the proof body of the sensor has
been chosen to be a dynamometric ring. A plane is made
on the ring surface to determine a "standing position"
when the ring stands on its plane.
A metal case holds the ring in the standing position. The
closing system allows the top surface of the case to slide
down to compress the ring during the gait. It also allows a
pre-strain on the ring to guaranty the permanent contact
between the ring and the top and bottom of the case.
Identically shaped holes in the soles receive the metal
cases (eight per shoe in this example).
Two aluminium plates, screwed on the top and at the
bottom of the case, take in sandwich each sensor
(figure 3).
Fig. 3. Sensor assembly: a dynamometric ring placed inside a metal case
taken in sandwich between two aluminium plates.
When walking the foot pushes down the top aluminium
plate and so the top of the case. The ring is so compressed
against the bottom of the case. The latter lays on the
bottom aluminium plate which lays on the floor. The sole
does not interfere with the force transmission. The sole
only keeps all the sensors linked together under the foot.
Each sensor is equipped with a precision strain gauge
(resistance of 350 Ohms) pasted on the external face and
placed at the equator of the ring when the ring is standing
on its plane (figure 4).
12.5
mm
9.3
mm
Dynanometric ring
Strain gauge
Ring plane
Fig. 4. Location of the strain gauge on the dynamometric ring
in its standing position.
During the highest pressure that may appear is twice the
body weight according to [1] and [9], i.e. about 2000 N
for an individual of 100 kg. Thus, to validate the elastical
behaviour of our sensor during gait compression, we
simulate the repetitive walking compressions using an
Instron electromechanical universal testing system (6025
INSTRON).
We programmed 20 successive cycles of compression.
Each cycle combined a compression for 0.4 s, a release
for 0.4 s and a break off for 0.4 s. Thus, we simulated a
1.2 s step, i.e. a "normal" speed step as revealed by [10].
Each compression runned from 0 to 2000 N and back to
0 N, because 2000 N should be the highest verticale force
during the gait as previously noticed. Thus, the

12th IFToMM World Congress, Besançon (France), June18-21, 2007
compression speed was 5000 N/s and each acquisition
lasted 24 s. The chosen acquisition frequency was 100 Hz.
We observed in figure 5 that during these 20 cycles the
sensor keeps its elastic behaviour.
with
• a: ring width
• E: ring Young modulus (elasticity)
• F: ring submitted force
• R: ring mean radius
• t: ring thickness
Thus, knowing all these parameters permit us to obtain
the force (F) exerted on the ring by (3):
2
F 4V
Eat / 3RK(
V − 2V
)(1 − 2 / Π)
(3)
=
out
in out
Fig. 5. 20 cycles of gait mean speed compression
of the sensor inserted into the shoe.
The case allows a 0.5 mm vertical compression but the
one of the ring does not exceed 0.15 mm at 2000N. Such
sensor is thus validated for individuals until 100 kg.
B. Electronic Operation
The electronic operation is to translate the ring
deformation into the force exerted on the ring.
A dual PCMCIA card expansion jacket allows the
connection of two PCMCIA cards on the PDA iPAQ HP
5500. Each PCMCIA acquisition card can record up to
8 sensors.
C. Data Acquisition
A software has been developed in C++ language for the
PDA to acquire the voltages and to record them on
request.
During acquisition, as well as during recording, the PDA
may display the value of 16 sensors on the screen.
The acquisition frequency can be chosen between 50
and 500 Hz.
Figure 7 presents the PDA screens of the software.
Each sensor uses an extensiometric bridge. This bridge
is a high sensibility Wheatston bridge in which one strain
gauge is used instead of one resistance element. The strain
gauge is said to be set in a quarter of bridge (figure 6).
Vin
Vout
Fig. 6. Gauge set in quarter of bridge.
The entry voltage in the bridge (V in ) is known and the
exit one (V out ) is measured. The variation of this tension is
due to the variation of the strain gauge resistance which
translates its stretching (ε) as written in (1).
−3 −6
Vin / Vout
= Kε × 10 /(4 + 2Kε
× 10 ) (1)
with K: strain gauge coefficient.
On the other hand, the gauge deformation during the
ring compression may be known by the ring parameters
(2) as written by [11] taken from [12].
2
ε = ( 1−
2 / Π)
× 3FR
/ Eat
(2)
Fig. 7. Screens of the acquisition software on the PDA:
the acquisition options screen (on the left) and
the acquired sensor voltages screen (on the right).
The PDA is belted on the patient hips so that individuals
may go in any direction and with any movement without
disturbance from connection wires.
The recorded data are then transmitted to the main
computer. These data are the voltage variations due to the

12th IFToMM World Congress, Besançon (France), June18-21, 2007
strain of each sensor but they are not translated into the
implied forces. Figure 8 shows the example of a healthy
adult walk: one step of one foot equipped with 8 sensors.
% of BW
Body weight (BW)
Fig. 8. Curves of the 8 sensors strain in one shoe during one step.
S1: internal heel ; S2: external heel ; S3: external rearfoot ; S4: external
middle-foot; S5: fifth metatarsal head; S6: third metatarsal head ; S7:
first metatarsal head; S8: hallux.
Thus, we are informed for each area of the foot about
the moment of appearance of the pressure, the length of
time of its increase, the moment of its maximum value, the
length of time of its decreasing and the moment of its end.
For example, the seventh sensor (S7) which is located
under the head of the first metatarsus, begins to detect
pressure at the sixth acquisition, the signal goes up until
the fifteenth acquisition and then fall down until the
eighteenth. Although this curve reaches the highest value
of voltage, it does not mean that the implied force is the
highest of all the sensors. These values aren't comparable
between the sensors because the implied force values
depend of the strain gauge coefficient of each sensor.
Knowing the latter, we can indeed obtain the implied
force using (3).
Time
Fig. 9. Plantar pressure distribution [13] and
global vertical force curve and values (expressed in % of body weight)
during the gait of a healthy individual.
To identify the usual gait pattern of one patient, the
latter is asked to walk for 10 meters on a line for 3 times.
The five middle step of each walk are recorded. The mean
of these 15 steps is calculated and then plotted like in
figure 10.
III. Medical process and Data Treatment Allowed by
our Device
Our new device will permit to compare the patient
plantar pressures to this healthy pattern. The foot-roll is
part of a healthy gait plantar pressure pattern as well as
the “camel back” shape of the vertical ground reaction
force curve. These two parameters are well shown, in
figure 9, by the evolution of the plantar pressure
distribution and of the global vertical force during the
gait.
Fig. 10. Mean curves of the implied force calculated for each of the
8 sensors of one shoe of one healthy patient (70 kg) during one step.
(Mean upon 15 steps)

12th IFToMM World Congress, Besançon (France), June18-21, 2007
Figure 10 clearly shows the foot-roll during one stance
phase of gait.
At the beginning of the stance phase (until the moment
T1), the force of contact between the heel and the ground
increases rapidly. It's the heel strike. The sensors S1 and
S2 under the heel reach their highest value (respectively
540 N and 430 N for this patient). During this phase, the
other sensors are unloaded.
During the "mid-stance" (T2), the force curves under
heel and metatarsal heads show a medium level (300 N for
this patient). The foot is indeed flat on the floor and the
pressure is spread all over the foot.
Then the force increases under the forefoot until a peak
at (T3) which occurs when the walker propels himself on
the tip toe. The force under the first metatarsal head is up
to 650 N for this patient.
At T4, the forces are zero. The toe off has occured and
the foot has entered in oscillation.
We also notice that the duration of this healthy stance
phase is about one second.
The healthy pattern should also includes the feet
symmetric and repetitive pressures, i.e.:
• same pressure under same area at the same relative
phase of the stance period under both feet. That's to
guaranty symmetry.
• same pressure under same area at the same relative
phase of the stance period under one same foot through
several consecutive steps. That's to guaranty repetitivity.
• touch of the foot beginning by the exclusive heel contact
phase, following by a full foot period and finishing by an
lonely forefoot phase.
With our device, we could indeed also study the
following variables suggested by [14]:
• Velocity of the centre of pressure
• Duration of the stance phase and contact times of the
areas of interest.
• Size of the contact area at a certain time point and
maximum size.
• Local peak and mean pressures underneath areas of
interest.
• Local peak and mean forces underneath areas of interest.
• Time until local pressure peak is reached.
• Time until local force peak is reached.
• Maximum loading rate of areas of interest.
• Time to maximum loading rate of areas of interest.
• Pressure-time integrals in areas of interest.
• Force-time integrals in areas of interest.
• Ratios such as one local pressure divided by another (or
the sum of some local pressures divided by the sum of
others) or relative pressure-time integrals dividing a
pressure-time integral by the sum of all pressure-time
integrals, etc.
• Differences concerning the same variable between two
different locations.
Thus, we can identify the wrong plantar pressure
patterns due to the numerous injuries and deceases
affecting the musculoskeletal system.
IV. Conclusion and Perspectives
This article has presented a new mechatronical device
which measures plantar pressures during the gait so that
the doctors may detect or prevent gait injuries and
deceases.
When walking, the foot of the patient compresses the
dynamometric rings inserted into the sole of the shoe. This
deformation is translated by a strain gauge into a voltage
variation. This information is recorded by a personal
digital assistant (PDA) belt on the patient hips. And
finally, the data is plotted on a main computer to show the
plantar pressure evolution under the main acting zones of
the foot.
As written by [15], our sensor is not a very thickness
sensor, but it has just the dimension of a standard shoe
sole. It offers a great longevity, linearity and precision.
Instead of many systems of instrumented shoes, the fact
that the sensor is integrated in the sole did not marked
prominence under the shoe; also the walker does not
observed alteration of gait or discomfort. Therefore, the
shape of the soft sole, divided in eight rigid areas, does
not perturb the natural roll of the foot on the ground.
The preparation of the sole to receive each sensor (hole
pre pierced) and the possibility of easy installation and
manipulation, authorizes the use of the same sensor for
different shoe sizes and for several areas of the sole.
Compared to a force plate, our instrumented shoes
system permits a recording of plantar dynamic along a
great number of steps; and the use of these shoes cancels
the walker anxiety relating to foot placement frequently
observed in a walk on a limited single force plate.
Instead of the plantar pressure insole which takes
pressure with the artefact of the shoes, our instrumented
shoes authorize the recording of plantar pressure and force
acting directly at the foot-ground interface. Because the
sole of the shoe does not absorb the force in order to
allow doctors to analyse the barefoot walking pressures of
their patients. Such information may help to detected
present locomotion injuries or illness and to predict the
future ones. Moreover, our product gives only locally
identified pressure to make the analysis easier. And, last
but not least, our sensors have longer life expectancy.
Regarding the dynamometric treadmill [16], our
instrumented shoes are cheaper, portable and permit a
natural walking.
This instrumented shoe is a robust tool to assess the
instantaneous vertical forces and plantar pressures exerted

12th IFToMM World Congress, Besançon (France), June18-21, 2007
during gait over a great number of steps. It offers an
objective, low cost documentation of healthy and
pathological subject gait.
The tests realised on the gait of healthy subjects and the
data obtained encourage us to continue the validation of
this tool.
Although this mechatronical device is medically helpful,
some optimisation is still required.
The sensor sensitivity could be better. We will decrease
the ring width so that a same pressure will be translated by
a bigger voltage variation.
We also want to develop wireless communication
between the PDA and the main computer so that the data
may be transfered instantaneously and the pressure line
may appear instantaneously too.
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