2017 IEEE International Conference on Systems, Man, and Cybernetics (SMC)
Banff Center, Banff, Canada, October 5-8, 2017

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The Change of Gait Motion When Curving a
Corner Owing to the Motion Restriction Caused bya Wearable Device
Yusuke Fukui∗† , Yasuhiro Akiyama∗ , Yoji Yamada∗ , and Shogo Okamoto∗∗ Department of Mechanical Systems Engineering

Nagoya University, Aichi, Japan† Email: fukui.yuusuke@g.mbox.nagoya-u.ac.jp

Abstract—Many wearable robots are not fully capable of fitting
the motion of the wearer, owing to their limited degrees-offreedom (DOF). This limitation disturbs motions such as cornercurving, which are required in daily life. In this study, the effectof the DOF restriction on the corner curving motion was observed
and analyzed. The gait motions, when curving a round cornerwith and without the restriction of out-of-sagittal plane motion,
were compared using a wearable device which could restrict
hip adduction/abduction and rotation. The result suggested thatthe restriction not only decreased the range of motion of hip
adduction/abduction and rotation, but also changed the center ofmass (COM) trajectory. Owing to the disturbance applied during
the stepping motion of the outer leg, the COM trajectory moved
further from the corner in the trials with restriction. Thus, thedistance between COM and the base of support of the inner footbecame larger during the swing phase of the outer leg, which
possibly increased the risk of loss of balance. Furthermore, it wasalso suggested that an assistive torque, which does not consider
curving motion, possibly increases such risk.

I. I NTRODUCTION
Quality of life for the elderly is an issue being addressedby many countries. It is expected that a wearable robot
could improve the quality of life and daily living activitiesof the elderly. Recently, various types of physical assistantrobots (PARs) have been developed and their popularity is
increasing. The PARs for gait rehabilitation in hospitals havebeen developed [1], [2]. Other PARs have been developed
aiming for the usage in daily life [3], [4]. There are somestudies that focused on abilities for the PARs application [5],[6]. Effects of gait training with PARs have also been studied[7], [8]. Now, because many of these PARs are used for gait
rehabilitation in hospitals, the motion which changes walking
directions is not considered.
However, the usage of the PAR is gradually expanding to
the daily living environment, where curving motions are frequently required. Nonetheless, the PAR that restricts the outof-sagittal plane motion of the wearer at the lower limb joints,(especially adduction/abduction and rotation of the hip joint)
to efficiently apply assist torque and create the mechanismof DOF, is simply not suitable for such an environment. This
incompatibility will likely prevent the widespread usage of the
PAR as the limitation in curving motions not only decreases
the flexibility of daily living activities but also exposes thewearer to the risk of collision, falls, and other accidents.
978-1-5386-1644-4/17/$31.00 ©2017 IEEE

Therefore, for further expanding the adoption of the PAR, it
is important to develop a mechanism that enables the wearer
to curve easily, even when wearing the PAR.
Although its complexity, some studies focused on the measurement and analysis of curving motion. Hicheur et al. [9]reported that the head adjusted its orientation to the curving
direction before the walking direction started changing. They
concluded that head movement was part of the mechanism
responsible for walking toward the desired direction and alsopart of the stabilization strategy. Imai et al. [10] also studied
the interaction of the body, head, and eyes during turning a
0.5 m radius and 2.0 m radius corner. Courtine and Schieppati[11] studied the curving motion when curving a corner, whose
radius was 1.2 m. They compared the gait motion whencurving with eyes opened to when curving blindly. Courtine
and Schieppati [12] focused on the pitch and roll motion of thelower limb when the curved corner’s radius was also 1.2 m.
Yamaguchi et al. [13] researched the effect of the turning angle
on falling. They induced slipping when the walking directionchanged and concluded that the steepness of the turning
motion increased the risk of falling. It was because the distanceand relative velocity between the center of mass (COM) of thewhole body and the COM of the sliding foot increased, whichmeant that the falling moment subsequently increased. Rogers
and Mille [14] investigated lateral falls of the elderly. Theysuggested that neuromusculoskeletal impairment of disrupted
the fall arrest strategy related to lateral stability. Thus, despitesome studies analyzed curving motion, the risk of fall during
curving motion, which potentially resulted in the lateral fall,
was not studied sufficiently.
On the other hand, the study which considered the effect ofthe motion restriction on curving motion is also insufficient.
Some studies focused on the motion restriction of the knee
joint. Temel et al. [15] studied the response of the subject when
the knee motion was perturbed. However, although the motion
restriction of the hip applied by the PAR probably become
important for curving, the effect of such restriction on curving
motion have rarely been studied. In addition, developing aPAR, which is fully capable of fitting the degree-of-freedom(DOF) of human joints, is not easy from the viewpoint of cost,size, and mass, which also affects the motion stability ( [16],[17]). Thus, it is inevitable to tolerate the restriction of DOF

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Fig. 1. The straight and curving path and the ideal foot steps

in some degree when using PARs.
Thus, we focused on the effect of restriction of out-ofsagittal plane motion during the curving motion to identify
how motion strategy and risk of fall changed. The analysisof the mechanism which changed the curving motion could
suggest the priority to add the DOF of adduction/abduction
direction for natural curving motion. Furthermore, the observation of the change of COM trajectory helps to improve
the safety of PARs by suggesting the necessity of the assist
algorithm which reflects the compensation motion against
motion restriction.
II. M ETHODS
An experiment was carried out to analyze the effect on
corner curving motion caused by the restriction of out-ofsagittal plane motion, applied by a wearable device. Generally,
gait assist robots consist of the pelvis, thigh, and lower thighlinks. Although the rotational joint usually connects theselinks, the DOF of these joints is limited in the sagittal plane,in most cases; therefore, the out-of-sagittal-plane motion wasrestricted. Because many characteristics of the PAR, such as
weight, size, and assist torque, affect the motion of the wearer,in addition to the restriction of the DOF of joints, the effectof these factors should be carefully analyzed. Jan F. Veneman
et al. [18] conducted experiments with a method that they
limited DOF of hip joint and knee joint rigidly on the sagittal
plane. In this study, the focus was on the DOF effect because
the effect of that restriction was probably amplified during
the curving motion, wherein the risk of falling became higher
than that when walking straight. To identify the effect of joint
restriction, the results of the two experiments were compared.
A. Apparatus
The experiment was performed with the permission of theinstitutional review board of the Nagoya University. In thisstudy, the motion when curving a small radius corner was
focused on because the effect of restriction probably became
larger in steep corner, which was more suitable to the analyze
the risk of fall. In addition, such curving motion with small

radius is frequently required in the daily living environment(i.e. turning corridor). Therefore, a curved path with a radiusof 0.5 m was used. As mentioned above, other studies [9]–[12] adopted larger radii than 0.5 m. Fig. 1 shows the curving
path with the position of designed footsteps. The walking lane,
which was marked on the floor using tape, consisted of astraight approach line, a quarter of a circle with 0.5 m radius,
and a straight line.
The motion of the subject was recorded at 100Hz using athree-dimensional motion capture system (MAC 3D system,
Motion Analysis Corporation, U.S.). A set of critical markersof the SIMM Motion Module (SIMM, Mulsculographics Inc.,
U.S.) and additional markers (sternum, PSIS, shank) wereattached to increase the accuracy of human model fitting. The
ground reaction force was recorded to detect the timing ofgait events, such as heel contact (HC) and toe off (TO), at
100 Hz, using mobile six axis force plates (M3D, Tech Gihan
Co., Ltd., Japan); the force plates were fixed under a sole.
Fig. 2 shows the wearable device used in this study. Although this device was originally designed as the PAR, actuators were removed in order to evaluate the DOF restriction
effect independently. Because the torso link can be fixed to
the wearer tightly, using a belt, the motion of the hip joint
in the out-of-sagittal plane could be restricted. However, thisrestriction could be removed by detaching the connection
between torso and hip joint. The subject wore the wearable
device, whose abduction/adduction and rotation of the hip joint
were restricted under one condition. Under another condition,although the same device was worn, the hip joint of that device
was not connected, which meant that the wearer could movetheir hip joint freely.
The stiffness of the hip joint in the direction of abduction
was measured preliminarily. Fig. 3 shows the data of torqueand abduction angle. Because the maximum abduction or
adduction angles of the hip are approximately ± 5 degrees
during straight walking [19], the restriction was consideredenough to restrict joint motion strongly even when the wearer
walked straight.
B. Protocol
In total, three healthy young adult males, who were notauthors and did not know the intention of the experiment,
participated in our experiment. First, the subject wore wellfitted sportswear and the wearable device with reflective
markers for recognition by the motion capture system. Then,the subject had walked with the robot until he came to be able
to walk naturally. Following adaptation trials, the motion of
the subject was recorded when walking along a curved path,with or without the restriction. At the beginning of the trial,
the subject stood at a start position. In order to record the step,which was most affected by curving, the side of the leg whichstarted walking was randomly ordered in order to observe
the curving motion using a different leg. The subject was
instructed to start curving a corner at the third step. The lineof approach was adjusted to the subjects’ step length and they
were also asked to walk as naturally as possible and at their526

Fig. 2. The exoskeleton and its fixation part which restricts out-ofsagittal-plane motion

Fig. 3. The resistance torque of the exoskeleton against abduction angle

preferred pace. After all the trials under a restricted or nonrestricted condition were recorded, the torso link connectionof the robot was attached or detached in order to change therestriction condition. Then, the motion of the subject, whowalked under a different restriction condition, was recorded
following additional adaptation trials.
The trials were restricted to two DOF conditions (both withand without abduction/adduction and rotation restriction) andtwo stepping conditions (start with left or right leg). The
same restriction condition was used in the first twenty trials.
The stepping conditions were alternated. Then, another twenty
trials were conducted under another restriction condition, ina similar manner. The order of restricted and non-restrictedconditions was randomized for each subject. A total of forty
trials, which could be separated to four combinations ofdifferent conditions, were carried out for each subject.

TABLE IM AXIMUM ADDUCTION / ABDUCTION ANGLE OF HIP JOINT OF INNER FOOT

C. Data processing
As our study focused on the curving motion, analysis was
mainly carried out for the step motion at a corner section. The
motion data were smoothed using a 6 Hz Butterworth filter.
The timing of HCs and TOs were detected by ground reaction
forces with a threshold of 10 N. The angles of each joint andthe position of the COM were calculated with SIMM, basedon marker positions. The speed of the center pelvis, which
was the mean velocity of the center position between the left
and right anterior inferior iliac spine (ASIS) in a stride at thecorner section, indicated the velocity of the curving motion
and was used to validate the restriction.
III. R ESULTThe speed of the center pelvis of subject A, in non-restricted
trials was 0.79±0.03 m/s and decreased to 0.55±0.02 m/s
when the restriction was applied. The restriction also decreased
the speed of subject B from 0.93±0.04 m/s to 0.75±0.02
m/s. However, the speed of the center pelvis of the other

STEPS

SubjectA
B

direction
Abduction
Adduction
Abduction
Adduction

side
Left
Right
Left
Right

restricted
4.9± 1.4
1.2± 1.0
2.5± 0.7-2.9± 0.7

non-restricted
7.2± 1.9
3.2± 1.93.8± 1.0-0.6± 1.5

note : Value is mean ± SD.

subject did not differ between different restriction conditions.
As it seemed that the restriction affected the successive nonrestricted condition, the data of the subject, for which the trial
started from the restricted condition, were omitted.
Figs. 4 and 5 show the trajectory of a representative curving
motion, under restricted and non-restricted conditions, respectively for Subject A. The red points in the figures represent
COM positions at HC; the discontinuous red line represents the
COM trajectory. The first footprint was one step before curving
started. Then, the second to fourth footprints comprised thecurving motion. The motion between the second and third
footprints was included in the corner section. This motion
corresponded to the inner swing approximately.
Further, Figs. 6 and 7 show the curving motion trajectory,
from which the outer swing was extracted. According to Figs.
4 through 7, the COM trajectory in restricted cases usually
passed between the footprints of both legs; in non-restricted
cases, however, it got closer to the inner foot.
Then, the peak adduction/abduction angle was extracted
from both inner and outer swings to identify the effect of
the restriction on the range of motion.
Tables I and II give the peak adduction/abduction angles
during inner and outer swings. The peak angle of one side direction (adduction or abduction), whose peak angle increased,
was selected from each leg. In all cases, the peak joint angle
decreased under the restricted condition. In most cases, this527

Fig. 4. The motion trajectory of an inner swing trial in the restrictedcondition

Fig. 5. The motion trajectory of an inner swing trial in the nonrestricted condition

Fig. 6. The motion trajectory of an outer swing trial in the restrictedcondition

Fig. 7. The motion trajectory of an outer swing trial in the nonrestricted condition

TABLE II
M AXIMUM ADDUCTION / ABDUCTION ANGLE OF HIP JOINT OF OUTER
FOOT STEPS

SubjectA
B

direction
Adduction
Abduction
Adduction
Abduction

side
Left
Right
Left
Right

restricted
0.0± 0.6
1.4± 1.3
3.3± 1.0
2.4± 1.5

non-restricted
5.9± 1.2
7.9± 1.3
4.0± 0.94.5± 1.6

note : Value is mean ± SD.

meant that the range of motion decreased.
Fig. 8 shows the rotation angle of the hip joint of Subject
A, in non-restricted and restricted cases, including the innerswing. The horizontal axis represents the gait cycle, whichcorresponded to the range between the second and fourth
steps shown in Figs. 4 and 5, respectively. The vertical axis

represents the joint angle. The red line represents the mean
and SD of the rotation angle of the hip, while the blue line
represents the mean and SD of the rotation angle of the left
hip. The continuous line represents the non-restricted condition
and the discontinuous line represents the restricted cases. The
motion between 10% to 50% in Fig. 8 corresponds to the innerswing. The rotation angle of the peak values for the left andright hip were located at approximately 60%, as shown in Fig.
8. The restricted value was lower than the non-restricted value,
which was possibly due to the effect of restriction.
On the other hand, Fig. 9 shows the rotation angle during
the gait cycle, which corresponded to the range between thesecond and fourth step, as shown in Figs. 6 and 7. The motion
between 10% and 50% corresponds to the outer swing. The
peak values for the rotational angle of the left and right hip,in the restricted cases, were also lower than those in the non-528

Fig. 8. Hip rotation angle in non-restricted and restricted cases including
inner swing

Fig. 9. Hip rotation angle in non-restricted and restricted cases including
outer swing

restricted cases.

angle of the left hip, which was restricted by the device,decreased as shown in Table II.
From the viewpoint of safety, disturbance applied during
gait increases the risk of fall. Akiyama at el. [22] analyzed
a recovery motion when an assist torque was abnormallyapplied with a physical assistant robot. They found that
subjects changed the gait in response to abnormality when
they walked straight. Yamaguchi et al. [13] found that fallcaused by induced slips during turning probably resulted in
the harmful event. In our experiment, the motion restrictionchanged the COM trajectory because of the compensation
motion for the restriction torque. However, especially when theassist torque, which moved the swing leg farther, was applied
during the outer step, the assist torque probably interferedwith the short outside step, which intended to compensate forthe restriction. Such mismatch occurs when the assist robot
expected a normal gait motion despite the wearer intended tocurve. When the COM position separated from the base of
the support substantially, the fall moment increased and therisk of fall increased as well, owing to recovery failure. Thus,
it may not be appropriate to work for stepping higher during
corner curving.

IV. D ISCUSSION
Winter and Eng [20] analyzed the inner torque of hip
abduction during straight walking. They found that the hip
abduction torque was generated to control the upper body
motion in the direction of the lateral side. However, the hip
adduction torque became almost zero during walking. In our
experiment, the wearer had to exert torque to adduct/abduct the
hip joint, owing to the restriction, as shown in Fig. 3. Thus,
the range of adduction/abduction angle decreased as shownin Tables I and II. In addition, change of COM trajectory,
step length were observed as shown in Figs. 4 through 7.
Furthermore, Figs. 8 and 9 suggested the decrease of hiprotation. It seemed that they were not only related to therestriction but also related each other. Further experiment withincreased numbers of subjects will discover their walking habit
more clearly.
Among these changes caused by the restriction, the COM
trajectory was focused on in this study because it probably
stood for the risk of fall caused by the restriction. In the caseof straight walking, the COM moved close to the support leg
during the swing phase [21]. However, the COM did not moveclose to the stance leg (Second footprint) during the innerswing under both restricted and non-restricted conditions whencurving as shown in Figs. 4 and 5. On the other hand, theCOM moved close to the stance leg and sometimes exceeded
it under the non-restricted condition during the outer step asshown in Fig. 7. Although moving COM closer to the corneris effective to curve the corner, it did not happen under the
restricted case.
Owing to the restriction, it seemed that the subjects could
not move their COM close enough toward the medial directionof the body. Although the subjects were required to adducttheir inner hip to control the COM position, the adduction

V. C ONCLUSION
In this study, the effect of the out-of-sagittal plane motion
restriction at the hip joint was observed. This restriction,
which simulated the restriction of a physical assistant robot,
was applied during curving corner, which was the motion
frequently required in the daily living environment. In addition
to the decrease of the adduction/abduction and rotation of
the hip joint, the change of the COM trajectories, which was
the compensation motion for the restriction, was observed. In
particular, unlike non-restricted condition, the COM did not
move close to the support leg nor exceeded it under restrictedconditions during the outer swing. This meant that the COM
trajectory moved outside of the corner owing to the restriction.

529

By considering the decrease in the peak adduction/abduction
angle during step motion, it was inferred that the restriction of
inner leg adduction disturbed the control of the COM position
during the outer swing. From the viewpoint of safety, although
the restriction did not cause fall by itself, the restriction probably resulted in fall when the compensation motion observedin this study was disturbed. Unfortunately, the assist torque,which was applied by the wearable robot, potentially becomessuch interference when the robot expected normal gait motion.
Thus, in particular, the assist should be designed not to disturb
the stepping motion of outside leg when curving corner.
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