Clog-integrated plantar visualization system for evaluating activity
during walking
Yingjie Jin, Miho Shogenji, and Tetsuyou Watanabe, Member, IEEE

Abstract—This study involves the development of a novel
sensing system for the planta while walking on any surface andin any condition. In the study, the main aim is the visualizationof the planta. A plantar image is reflected at the reflecting
surface and captured by a camera embedded in a clog. The
obtained visual information differs from the pressure and IMU
data obtained from conventional sensors as it provides detailed
information on the plantar surface. This is used to investigate therelationship between the activity in the toe area and stumbling
experiences. The results indicate that high toe activity is
associated with the group with no stumbling experiences and low
toe activity is associated with the group with stumbling
experiences.
I.

INTRODUCTION

The Cabinet Office of the Government of Japan [1]reported that falls are the fourth main reason for elderly
individuals above 65 years of age to require support fromcaregivers. The demand for fall risk assessment to prevent
falls increases along with aging. Several previous studies
focused on this issue [2]–[8]. In this context, this study
examines planter observation. A planter is a popular place fora self-health check in oriental medicine, and it is expected toreveal a significant amount of health-related information.
However, a limited number of observation methods exist to
obtain quantitative data. Plantar pressure is an extremely
popular method [9] that is used to observe parts of the footthat are used by individuals while walking. However, thisinvolves indirect observation and corresponds to a limited
amount of obtained information. Examples of unobservable
information by conventional indirect measurements include
accurate contact area, skin deformation, and skin color. It
should be noted that it is difficult to use pressure sensors todetect a contact area with low loads. Additionally, it is alsodifficult to identify the exact contact area using pressure
sensors. Thus, the aim of this study involves visualization anddirect observation of the planta. The main contributions
include the following:
Wearable plantar visualization system: This study
presents a novel plantar visualization system for direct
observation of the planta. A transparent sole, a refraction
mirror film, and a camera are embedded in a clog (shoe). The
plantar image is reflected in the film and captured by thecamera. The image reflection mechanism provides the thin
Y. Jin is with the Graduated school of Natural science and Technology,
Kanazawa University, Kakuma-machi, Kanazawa, 9201192 Japan (e-mail:kin-e.k@stu.kanazawa-u.ac.jp).
M. Shogenji is with the Faculty of Health Sciences, Institute of Medical,
Pharmaceutical and Health Sciences, Kanazawa University, 5-11-80
Kodatsuno, Kanazawa, 9200942 Japansole with a thickness of 26 mm. When the clog is wornbarefoot, the visualized planta is observable during any
activity and while walking on any surface. The viewable areaof the embedded camera is restricted to the planta, whichmeans that privacy of the wearers is protected, and thephysical and psychological burdens of the wearers are
reduced. Thus, efficient and reliable data collection is
expected. Passive joints are embedded at the camera holder
and around tarsometatarsal joints such that individuals canwalk normally while wearing the developed sensor clog. Sizeadjustable bands are also installed to deal with different sizes
and foot shapes. A comfortable sensor clog with a plantar
observation function is then realized.
Extraction of toe area activity: Visualization of the planta
provides accurate information with respect to the contact area
when compared to that of the insole pressure sensors. This
information is utilized to investigate the relationship between
the activity of the toe area and stumbling experience as a firststep to evaluate the effectiveness of the proposed sensing
system. The difference between the contact areas at the footstrike and kicking phases is examined. The results indicatedthat the difference is associated with the extent to which thefoot fingers rotate while walking, that is, the activity of thetoe area. The experimental investigation reveals that theparticipants with a small difference possess experience of
stumbling and vice versa.
The rest of this paper is organized as follows. The
subsequent subsection describes related literature. This isfollowed by a discussion of the developed clog-integrated
plantar visualization system. The experimental investigationis then presented in conjunction with the procedure to derive
the activity of the toe area. Finally, the conclusion is discussed.
A. Related work
As discussed above, several studies focus on walking
evaluation or gait analysis [2]–[8]. However, this section
describes studies that focus on shoe-integrated walking
measurement systems, which are closely related to the context
of the present study [9]–[12]. Measurement systems aremainly classified into two types.
The first type corresponds to pressure sensor-based
systems. There is an insole inside every shoe, and pressure
T. Watanabe is with the Faculty of Mechanical Engineering, Institute of
Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa,
9201192 Japan (e-mail: te-watanabe@ieee.org).

profiles during walking can be obtained by embeddingpressure sensors into the insole. There are several
commercially available pressure sensors that are utilized toinvestigate the relationship between plantar pressure andwalking speed [13] [14], ages [15], and foot type [16].
Majumder et al. [17] predicted falls by using sensory datafrom a smart phone and four pressure sensors on a shoe.
Ayena et al. also [18] assessed the risk of falls at home with asystem similar to that in the study by Majumder et al. [17]. Yu
et al. [19] developed a shoe system that monitored walking
with shoe-integrated force sensors. Crea et al. [20] developed
a pressure-sensitive foot insole based on optoelectronic
technology.
The second type of measurement systems corresponds to
IMU (Inertial Measurement Unit) sensor-based systems [21].
Hung and Suh [22] reduced the position error of shoes
estimated by IMU sensors by utilizing additional IR camera
information. Do and Suh [23] developed a walking
measurement system that utilized inertial sensors and acamera with floor markers to estimate step length and footangle. An optical sensor was utilized although the targets did
not correspond to a plantar observation. Foxlin [24] tracked
pedestrians with shoe-mounted IMU sensors. Ojeda andBorenstein proposed a personal dead-reckoning navigation
system with a shoe-mounted IMU sensor. Sim et al. [25]proposed an algorithm to detect falls with acceleration sensors
on a shoe. Rampp et al. [26] estimated a stride parameter thatutilized IMU sensors on a shoe. Mariani et al. [27] assessedthe 3D spatial parameters of the gait with IMU sensors on a
shoe.
There are also systems that involve both pressure and IMU
sensors. Scheoers et al. [28] proposed an instrumented shoewith force and IMU sensors. Bamberg et al. [29] proposed awireless wearable sensor system that included an
accelerometer, a gyroscope, and a ground force reaction
sensor that were embedded in a shoe. Bebek et al. [30]developed a personal navigation system with pressure andIMU sensors. Chen et al. [31] detected an abnormality withthe shoes that included four force and IMU sensors. Hegde etal. [32] developed a shoe termed as the SmartStep shoe inwhich pressure and acceleration sensors were embedded.
Kawsar et al. [33] presented an activity detection system thatutilized pressure data from a shoe and smartphone data.
However, to the best of the authors’ knowledge, moststudies have not examined the development of a sensor shoethat utilizes visual sensors for plantar observation. The small
space of a shoe sole makes it difficult to integrate visualsensors. Nevertheless, visual sensors provide the abovementioned information that cannot be obtained with pressure
and IMU sensors. This study focuses on this issue.

4.
5.

The walking motion is not disturbed
The size can be adjusted according to the shape andsize of the footThe aim of this study includes developing an observation
system that provides visual information related to the planta
while walking. In order to realize the observation at any placeand during any activity, it is necessary that the sensing systemshould be embedded in a clog and then worn. The visible fieldmust be restricted to the planta to protect the privacy ofwearers. Additionally, the system should not disturb thewalking motion to observe plantar information. There areindividual differences in the sizes and shapes of a foot, andthus size adjustable functions are preferable.

Figure 1. Clog-integrated plantar visualization system

Figure 2. Underpart of the plantar observation system(a) Side view of the sensing system

II. CLOG-INTEGRATED PLANTAR VISUALIZATION SYSTEM
A. Design requirements
1. The planta is visually observable while walking
2. The system can be embedded in a clog and then worn.
3. The visible field is restricted to the planta.

(b) Bottom view of the sensing system
Figure 3. A schematic illustration of the sensing system

Figure 4. Definition of phases in a walking cycle and an example of an image obtained from the developed sensing system

B. Overview of the system
The sensing system shown in Fig. 1 is developed byconsidering the design requirements. A clog (Crocs)
constitutes the base and constructs the upper area. Fig. 2
shows a schematic view of the underpart area and the mainpart for the sensing system that mainly consists of a reflecting
surface and a camera. The image of the planta is reflected at
the reflecting surface and is captured by the camera fixed at
the holder.
C. Sensing system
Fig. 3 shows a schematic illustration of the sensing system.
The camera is the fundamental sensor to obtain plantar visual
information, and its position is important to observe the planta.
The space under the insole is very low, and it is difficult to set
a camera at the bottom of the clog and to directly capture theplanta. For this setting, either a camera with an extremely
wide field of view and extremely short focal length or
multiple cameras are required. The utilization of only a single
low cost camera is preferable from the viewpoints of low costand system simplicity. The refracting surface is then set at theinsole of the clog, and the reflected plantar image is capturedby the camera at an external position (Fig. 3). The significant
features of the developed system allow the observation of footfinger motions and acquisition of plantar surface information.
The heel area of the foot is usually utilized only at the footstrike phase and not at the push-off and swing phase (See Fig.
4 for the definition of the phases). Therefore, the primaryobservation target corresponds to the toe area of the planta,and especially the thumb that is necessary for the push-off
motion. Hence, the camera (holder) position and the reflecting
surface angle were determined in order to satisfy thefollowing:1) The image for the toe area and especially thumb is clear,
2) The image for the heel area is obtained at the foot-strikephase3) The visible area is restricted to the plantar area.
An endoscopic camera (GIWOX; Resolution: 640 480,
Focal length: 6 cm infinite, Angle of view: 60°) with a short
focal length is utilized to obtain a wide field of view. A

reflecting film (with a thickness of 0.6 mm) is utilized suchthat the thickness of the reflecting surface is minimized andthe thickness of the clog sole is close to the original thickness.
Fig. 4 shows an example of the sequential images observed ina cycle of walking. It is observed that the toe area is visible atall phases while the heel area is visible at the foot-strike phase(The dark area in the figure corresponds to the heel area).
D. Walking facilitation system
Several walking facilitation systems are embedded andinclude the following:
1) Passive joints for rotating tarsometatarsal joints
The foot bends during walking and therefore elastic
materials are utilized for the sole part of commerciallyavailable shoes. The developed system is solid to obtain stablerefracted images. Passive joints are installed around thetarsometatarsal joints (Fig. 5) to minimize the stress on thesolid part while walking and to allow natural foot bending.
The maximum allowable joints corresponded to 40°. It should
be noted that the heel area is not visible at the push-off phaseowing to the passive joints.
2) Passive joint at the camera holder
The position of the camera holder is external to the clog,
and it can disturb the walking motion during the push-off
phase. A passive joint is installed at the root of the holder (Fig.
6) to minimize the effect. The camera holder then rotatesaccording to the push-off motion at the push-off phase eventhough the heel area is not observable at the phase.

Figure 5. Passive joints to allow foot-bending around tarsometatarsal
jointsFigure 6. Passive joints at the camera holder

stumbling experiences and the activity of the toe area during
walking, using the developed sensing system.

Figure 7. Structure of the clog-integrated plantar visualization system; a:
Silicone plate, b: Acrylic plate, c: Cast coated paper (reflection film), d:
Density Fiber board, e: Frame created by a 3D printer, f: Wood plate, and g:
Camera holder

A. Participants
The participants are 8 young healthy individuals (7 men
and 1 woman, age 22.4 ± 2.5 years, weight 62.1 ± 11.2 kg,
height 166.8 ± 10.1 cm, foot size 24-27 cm) without any
diseases, impediments, and medical history involving
walking problems. The procedure and purpose of theexperiments are approved by Medical Ethics Committee ofKanazawa University.

Figure 8. Structure of a camera holder

B. Procedure
The participants walked straight along a 6.5 m long flatroad wearing the developed plantar sensing system on theright foot that includes a clog with a sole that has the same
thickness as that of the sensing system on the left foot suchthat the difference in the sole thickness did not affect walking.
The experiment is performed 3 times for each participant. We
questioned the participants about their stumbling experience,and based on the experience, we separated the participants
into two groups, which are, the group with no stumbling
experience (5 individuals) and the group with stumbling
experiences (3 individuals). The activities of the toe area arecompared across the two groups.

3) Adjustment of size and shape
Velcro (Fig. 1) is installed to allow the developed clog to
fit according to the size and shape of the foot of a wearer.
E. Structure
The structure for all the parts of the system is shown in Fig.
7. It is assumed that the developed clog is worn on a bare foot.
The silicone plate provides comfort and a good fit. The
silicone and acrylic plates are transparent to enablevisualization of the planta. The materials for the other parts
are selected to support the weight of a wearer while
minimizing the total weight of the clog. The connecting parts
are manufactured by a 3D printer (uPrint SE).
Fig. 8 shows the structure of the camera holder. The
direction of the fixed camera can be changed to fit the shapeand size of the foot of a wearer. The main specifications of
the developed sensor clog are shown in Table 1.
TABLE I.

MAIN SPECIFICATIONS OF THE DEVELOPED SENSOR CLOG

Allowable foot size
Allowable foot breadth
Allowable instep heightThe size of the main part
Visiblity of the toe area
Visiblity of the heel area

25

27

27 cm
11 cm
5 15 cm
11.7 2.4 Sole thickness cm
AlwaysWithout the push-off phaseIII. EXPERIMENTAL EVALUATION
Experiments are conducted to examine the effectiveness ofthe developed clog-integrated plantar visualization system. A
main feature of the sensing system involves the capacity toobserve the accurate contact area of the planta. The activity ofa certain area can be estimated from the variation in its contact
area. Based on this, we investigated the relationship between

C. Extraction of the activity of toe area
First, the toe area of the captured image is extracted at eachframe. The activity of the toe area (that is the extent to which
the participants moved their toe) is then evaluated.
The toe area is fixed with respect to the camera frame, andthe toe area is then evaluated from the obtained image. Next,
the contact area in the toe area is derived. The skin color of
the bare foot provides information about contact and loading.
The skin color turns white if the contact load is increased, andthis is due to blocking of the blood flow inside capillary
vessels. The whitened area then corresponds to the contact
area. The contact area is extracted based on this concept. The
color map of the image is transformed from RGB to HSV, andthe extracted area that satisfies the following conditions
corresponds to the whitened area:
0.055<H<0.167, 0.05<S<0.75, 0.4<V
Then, banalization is adapted to ensure that the extractedarea has a value of 1, and the value of the other area
corresponds to 0. This is followed by removing the small area
to eliminate the noise area. The overall procedure is shown in
Fig. 9. Fig. 10 shows a representative result of the extracted
contact area in the toe area. It is observed that the extraction
method worked well.
The activity of the toe area is evaluated based on the
difference between the two peak contact areas in the toe area.
It should be noted that the recognition of the walking phase
was not needed for the evaluation, and the walking phases
shown in Fig. 10 and the other figures were then manuallyannotated. Fig. 11 shows an example of the time-series dataof the contact area in the toe area for the two groups. The firstpeak (in Fig. 11) occurs while pushing off prior to taking off,

Figure 10. Representative result of the extracted contact area in the toe area

(1)

denotes the contact area at the first peak,denotes the contact area at the second peak, anddenotes the minimum contact area.

where

Algorism 1: Extraction of the contact area at the toe area
Given

Image captured by the camera at each frame

1

Pick up the toe area from the captured image

2

A RGB image is transformed into a HSV image.

3

Extracted area satisfies 0.055<H<0.167, 0.05<S<0.75, 0.4<V.

4Banalization; Extracted area: 1, The other area: 05Remove small area

6
Count the area with a value corresponding to 1
End
Figure 9. Procedure to derive the contact area in the toe area

D. Comparison of groups with and without stumbling
experiences
This involved comparing the results for the groups with andwithout stumbling experiences. As shown in Fig. 11, theDPCA is high for the group with no stumbling experience andlow for the group with stumbling experiences. The

IV. CONCLUSION
This study presents a novel plantar sensing system. The
planta is visualized and measured by a camera embedded in aclog, and the measurement can be conducted at any place andany condition. The obtained visual information is different
from conventional pressure and IMU sensory data as it
Contact Areas [pixels]

DTCA

normalized DPCA for each participant is evaluated, and themean and standard deviation values are calculated for eachgroup. The results are shown in Fig. 12. There is a statisticallysignificant difference between the two groups. This indicatesthat the activity of foot fingers is high for the group with no
stumbling experience and low for the group with stumbling
experiences. The results validated the effectiveness of thedeveloped clog-integrated plantar visualization system withrespect to the walking evaluation for stumbling risk.

Difference of two Peak Contact Areas(DPCA):26.4%

14000012000010000080000

Reference: 100%

60000
40000
20000

Push‐off

0
0

0.2

0.4

Foot strike
0.6

0.8

1

1.2

1.4

Time [sec](a) Group with no stumbling experience
Contact Areas [pixels]

while the second peak occurs at the foot-strike phase (asshown in Fig. 10). When the results for the two groups arecompared, the values of the contact areas at the second peak
are similar to each other. In contrast, the value at the first peakis large for the group with no stumbling experience and small
for the group with stumbling experiences. It is assumed thatthe former group largely moves/rotates their foot fingers forpushing off, and the value is subsequently high, and vice versa.
The difference in the two peaks corresponding to the activityof the foot fingertips or toe area is then examined. Thisdifference is termed as DPCA (Difference of two PeakContact Areas). There are individual differences with respect
to foot size and lightening. The DPCA is then normalized by
the variation in the width of the contact area (Fig. 11) tocompare the value for each group. The normalized DPCA
(NDPCA) is given as follows:

140000120000

DPCA

10000080000

Reference

60000
40000
20000

Push‐off

0
0

0.2

0.4

Foot strike
0.6

0.8

1

1.2

1.4

Time [sec](b) Group with stumbling experiences
Figure 11. Time-series data of the contact area in the toe area

Normalized Difference of
two Peak Contact Areas [‐]0.6

P<0.05

0.5
0.4
0.3
0.2
0.1
0

Group with 1 Group withno stumbling stumbling
experience
experiences

Figure 12. Mean value of the normalized Difference of two Peak Contact
Areas for the groups with and without stumbling experiences

provides detailed information on the plantar surface. Thus, it
is possible to accurately observe features such as the contact
area, skin deformation, and skin color. Therefore, the study
investigates the planta for the groups walking straight withand without stumbling experiences. The results reveal that theactivity of the toe area is high for the group with no stumbling
experience and low for the group with stumbling experiences.
The developed sensing system provides a significant amountof information about the plantar surface. Future research will
involve the investigations with more participants and withmore symmetric gender distribution, the development ofanalysis methodologies such as automatic walking phase, andthe application to health care such as fall risk assessment.
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