2019 19th International Conference on Control, Automation and Systems (ICCAS 2019)
Oct. 15∼18, 2019; ICC Jeju, Jeju, Korea

Validation of trip-induction system on a treadmill by comparison with awalking lane
Hazuki Miyata1 , Akiyama Yasuhiro2∗ , Yoji Yamada3 , and Shogo Okamoto4
Department of Mechanical Systems Engineering, Nagoya University,
Nagoya, Japan (miyata.hazuki@b.mbox.nagoya-u.ac.jp) ∗ Corresponding author2
Department of Mechanical Systems Engineering, Nagoya University,
Nagoya, Japan (akiyama-yasuhiro@mech.nagoya-u.ac.jp)
3
Department of Mechanical Systems Engineering, Nagoya University,
Nagoya, Japan (yoji.yamada@mae.nagoya-u.ac.jp)
4
Department of Mechanical Systems Engineering, Nagoya University,
Nagoya, Japan (okamoto-shogo@mech.nagoya-u.ac.jp)
1

Abstract: Trip-related falls, which account for a signiﬁcant portion of falls by the elderly, often result in serious injury.
A method to evaluate fall risk accurately will be increasingly required in the aging society. For this purpose, it is necessary
to analyze the natural reaction motion against tripping. We therefore developed a device and method to induce tripping at
an arbitrary time during walking on a treadmill. To evaluate the validity of that method, another experiment that induced
tripping on a walking lane was conducted. The reaction motions in the two environments were then compared. In thisstudy, we focused on the difference in the preferred walking speed between the ground and the treadmill. The results
suggest that the joint patterns during the reaction motion become similar when the subjective velocities, not the physicalvelocities, become the same for the subject, whereas physical parameters such as the trajectory of the center of mass
become similar when the physical velocities match. These results help to understand the reaction motion against tripping
and to reproduce the natural reaction motion on the treadmill.
Keywords: Fall, Recovery motion, Trip, Treadmill

1. INTRODUCTION

different from natural fall motions unless anticipation is
sufﬁciently prevented [2].

A fall is one of the main events that decrease the quality of life and daily living activities of the elderly. A typical cause of a fall is tripping during walking. Therefore,
estimating and preventing the risk of trips and falls is socially important. For this purpose, the measurement andanalysis of the natural fall avoidance motion are essential.
Many researchers have attempted to understand themechanism of falling by reproducing the fall experimentally. A typical method for causing tripping during walking uses an obstacle that is located or suddenly appears
just in front of the swing leg [1]. As a result of this experiment, two fall recovery strategies, ʠ elevating ʡ andʠ lowering, ʡhave been observed. The elevating strategy
was observed mainly when the subject was tripped during
the early swing phase. When this strategy was used, thesubject stepped over the obstacle using the tripped leg. In
contrast, the lowering strategy was observed mainly when
the trip occurred during the middle or late swing phase.
The tripped foot was landed quickly in front of the obstacle and the other leg was stepped forward to prevent theforward fall.
The advantage of the gait and trip experiment conducted on the ground is that the experimental environment is physically the same as the actual situation. However, it is difﬁcult to prevent subjects from expecting thetripping owing to the limited length of walking lane andsize of recording space. Therefore, the responses may beThis paper was supported by JSPS KAKENHI
26750121.

978-89-93215-18-2/19/$31.00 ⓒICROSThe treadmill is an option to solve the problem ofsuch anticipation because the timing of the tripping canbe controlled easily and randomly. However, treadmill
walking differs from walking in an actual environment.
For this reason, the difference between treadmill walking and ground walking has been examined by many researchers from various perspectives. With regard to timeand distance factors, the rate of treadmill walking was often reported to be higher than that of ground walking. At
the same time, stride length and stride time were shorter[3-6]. On the other hand, regarding the double support
phase, results were not consistent among previous studies. As for the kinematic parameters, the joint angle patterns of the hip, knee, and ankle became similar between
treadmill and ground. Although the angles were statistically different, they differed by at most 2◦ [7]. The
ground reaction force (GRF) patterns also became similar
under the two walking conditions. However, a differencein the peak value of the GRF between the walking conditions was reported. According to Reiley et al., the GRF
peak value in three directions decreased at the beginningand end of the stance phase [7]. White et al. reported
that the GRF peak value in only the vertical direction decreased in the middle and late stance phases [6].
Differences in subjective perception were also reported. Sabetian et al. displayed a background image infront of the subject and moved this image in pace with thewalking speed during treadmill walking to ease the con-

Grant Number

156

aluminum cylinder, and a linear slider. The tripping plate
was ﬁxed to the cylinder such that the height was 15 cm
from the surface of the treadmill belt. The cylinder was
ﬁxed to the linear slider, which moved in a front-to-backdirection. The speed of the linear slider was set to thesame speed as the treadmill belt.
A laser range ﬁnder (LRF) (UST-20LX, Hokuyo Automatic Co., Ltd.) was installed in front of the treadmill
to measure the distance between the tripping plate and thesubject’s leg. Furthermore, pressure sensors (FSR400,
Interlink Electronics Inc.) were attached to the heel andtoe of the insoles of the subject’s shoes to detect the timing of the gait. The heel contact of the left foot was usedas the reference of the gait cycle.
When 30 to 60 s had passed since the subject started
walking on the treadmill and the gait had become stable,
a trip occurred at a random time. However, the timingof the trip was controlled such that the tripping plate collided with the toe of the subject during the preset gait
phase using an algorithm that estimated the timing of collision from the signal of the foot switch and the LRF.
The reaction motion against the tripping was measured
using an optical motion capture system (OptiTrack, NaturalPoint, Inc.).
Furthermore, to prevent the subject from anticipating
the obstacle collisions, the subject wore goggles whose
lower half was covered. We also prevented the ﬂuctuation of gait by placing a mark at the center of the walking direction. For safety, the subject was supported by asafety harness to prevent contact with the surroundings.
Furthermore, the subject wore protectors on his extremities.

Fig. 1 Experimental overview; Based on the walking cycle detected by the foot switch and the Laser Range
Finder, the linear slider with the obstacle moves toward the lower limb of the subject at the set timing

tradiction of visual information [8]. Treadmills were also
used to simulate falls. Sessoms et al. simulated trippingby rapidly accelerating the treadmill belt [9]. Schillings
et al. reproduced tripping physically by placing an obstacle on the belt. However, the similarity between thefall motions simulated on the treadmill and those on the
ground requires further discussion because the measured
parameters to date are limited [10].
In this study, we developed an experimental systemthat could induce tripping during walking on a treadmill.
The reaction motions against tripping on the treadmill
and ground condition were then compared. The purposeof this experiment was to evaluate how the physical andsubjective speeds affect the reaction motion against tripping. The study also contributed to reproducing the natural fall motion when the experiment involves a treadmill.

2.3 Protocol
In this study, the reaction motions against tripping on
the treadmill and on the ground lane at different walking
speeds were compared. Three conditions were tested foreach subject. In the ﬁrst experiment, the tripping trial
was conducted on the treadmill, whose speed was adjusted to a comfortable speed for the subject. The second experiment was conducted on the ground lane at the
self-selected speed of the subject. The third experiment
was also performed on the ground lane; however, the gaittiming of the subject was controlled to match the one obtained in the ﬁrst experiment using a metronome or similar device.
By comparing the reaction motion on the treadmill andground lane at the same subjective speed (ﬁrst and second
condition) and the same physical speed (ﬁrst and third
condition), it becomes clear which is the important factor to reproduce the fall avoidance motion on the groundusing the treadmill. In the ﬁrst experiment on the treadmill, the subject’s comfortable walking speed was acquired during a pilot trial for approximately 5 min. In
the second and third experiments on the ground lane, theself-selected speed was determined in the same manner.
In addition to the use of the metronome to stabilize andcontrol the gait timing on the ground lane, targets of step
position were placed at the height of the subject’s face.

2. METHODS
2.1 Subjects
Two healthy males aged 19 and 24 participated in theexperiment. Their heights were 171 and 173 cm and theirweights were 60 and 65 kg, respectively. The experiments were performed with the approval of the Institutional Review Board of Nagoya University. Written informed consent was provided by each subject.
2.2 Apparatus
We developed an experimental system that induced
tripping at arbitrary times during walking on a treadmill.
An overview of this system is shown in Fig. 1. A speedcommand signal that was input to the treadmill (Ohtake
Root Co., Ltd.) controlled the speed of the belt locatedunder the right and left foot. At the same time, the obstacle used to trip the subject was moved toward the subject
using a tripping machine (e-Valley Co., Ltd.).
The tripping machine consisted of a tripping plate, an157

Table 1 Normal gait parameters
Condition
Treadmill

Stride length [m]Gait cycle [s]Walking speed [m/s]1.40 ± 0.09

1.19 ʶ 0.08

0.97

1.04 ± 0.04

1.30 ± 0.08

1.35 ± 0.09

Stride length [m]Gait cycle [s]Walking speed [m/s]1.44 ± 0.03

1.15 ± 0.04

0.97

Subject A Experiment 1
Experiment 2

Overground1.40 ʶ 0.05

Experiment 3

Overground1.43 ± 0.09

Condition
Subject B Experiment 1
Experiment 2
Experiment 3

Treadmill
Overground1.41 ± 0.05

1.05 ± 0.02

1.43 ± 0.07
1.28 ± 0.08
Overground
Notes : Values are mean ± SD

Moreover, to suppress the subject’s adaptation to the trip
perturbation, the trials in which the right and left feetwere tripped were randomly conducted. Finally, dummy
trials in which the subject was not tripped were randomly
inserted in the ground experiments.
In this study, control of the gait phase at the trip moment was important. Therefore, for the ﬁrst subject, tripping occurred during the early swing phase (65% of thesubject’s gait cycle) or the middle swing phase (80% of
the gait cycle). For the second subject, the trip timing
was set at the early swing phase to focus on the elevating strategy, whose reaction motion can be represented
by few parameters for simpler analysis.

1.06 ± 0.05

1.36 ± 0.05
1.00 ± 0.05

applied, the speed of the ground walking was 1.39 times(subject A) and 1.38 times (subject B) greater than thatof the treadmill walking. In experiment 3, for which theself-selected speed of experiment 1 on the treadmill wasapplied to the ground, the walking speed was successfully
reproduced.
Table 2 shows the number of trials. In experiment 1 on
the treadmill, subject A selected the elevating strategy in
two trials that aimed to trip during the early swing phase(65% of the walking cycle); the actual collision occurred
at 72.5±3.5%. In contrast, in two trials that aimed totrip during the middle swing phase (80% of the walking
cycle), the actual collision timing was 83.5±2.9%, andthe lowering strategy was selected. However, in experiments 2 and 3 on the ground lane with subject A, the lowering strategy was selected signiﬁcantly less often evenwhen the trip occurred during the middle swing phase.
In addition, in experiment 1 with subject B, in which thesubject’s self-selected speed was applied to the treadmill,the elevating strategy was selected against the collision at
70.4±4% of the cycle.

2.4 Data processing
All acquired data were processed using a 6 Hz Butterworth ﬁlter. The coordinates of each marker in threedimensional space were projected on the sagittal plane.
The hip joint angle was deﬁned as the angle between theline connecting the anterior superior iliac spine (ASIS)
and posterior superior iliac spine (PSIS) and the line connecting their midpoint and the knee. The knee joint angle
was deﬁned as the angle formed by the line connecting
the greater trochanter and knee and the line connecting
the knee and heel. In addition, the inclination of the upper body was deﬁned as the angle formed by the line connecting the acromion midpoint and the midpoint between
the ASIS and PSIS with the vertical line.
Heel contact was detected based on the time serieschange of the vertical position of the heel marker. Determination of stance phase and swing phase was performed
with the timing of heel contact as the boundary.
The center of mass (COM) velocity was calculated using the Software for Interactive Musculoskeletal Modeling (SIMM, Musculographics Inc.)

3.2 Time and distance parameters
The recovery motion was deﬁned as the motion between the tripping and the landing of the tripped leg. The
time interval between them was deﬁned as the recovery
time. The stride length of this recovery motion was determined as the footstep distance of the tripped leg between
the tripped foot leaving the surface and the foot landingon the surface. This distance was measured using the position of the heel marker attached to the tripped leg in thetravelling direction. These parameters are shown in Table
3. The stride length and step time differed slightly among
the three experiments in the case of subject A. In contrast,
for subject B the parameters of experiment 1, which was
performed on the treadmill, were signiﬁcantly larger thanthose of the ground experiments.

3. RESULTS
3.1 Recovery motions

3.3 Joint angles and trunk inclination

Table 1 shows the parameters of normal walking thatwere obtained from dummy trials on the ground lane orfrom the gait before the trip on the treadmill. Comparing
experiments 1 and 2 in which the self-selected speed wasFig. 2 shows the transition of the knee, hip joint angleof the tripped leg, and trunk inclination angle during recovery motions of subject B whose tripping phases wereclose. The dashed lines in this ﬁgure indicate the trip

158

Table 2 Trial detail
Condition

Elevating strategy

Lowering strategy

Dummy trial

Missed data

Total

Subject A Experiment 1
Experiment 2

Treadmill

2

2

5

1

10

Overground

12

2

5

4

23

Experiment 3

Overground

11

0

10

2

23

Condition

Elevating strategy

Dummy trial

Missed data

Total

Subject B Experiment 1
Experiment 2

Treadmill

8

10

4

22

Overground

8

10

4

22

Experiment 3

Overground

10

10

2

22

(a) Experiment 1: Treadmill(b) Experiment 2: Overground (fast)(c) Experiment 3: Overground (slow)

Fig. 2 The transition of the knee, hip joint angle of the tripped leg, and trunk inclination angle during recovery motionsof subject B whose tripping phases were close; The dashed lines in this ﬁgure indicate the trip timing and recovery
step timing, respectively. The results of the experiment 1 and 2 were similar for the maximum knee joint angle.
3.4 COM velocity
Fig. 3 shows the horizontal and vertical velocities of
the COM at the time of landing of the recovery leg, whichshould be the tripped leg in the elevating strategy and the
opposite leg in the lowering strategy. The ﬁlled area ofthe bar represents the belt speed of the treadmill. According to Fig. 3 (a), which shows the COM vertical velocity for subject A, the values of experiments 1 and 3, forwhich the subject’s self-selected speed on the treadmill
was used, were approximately equal in the case of the elevating strategy. In contrast, the COM vertical velocity
became larger in experiment 2, where the subject walked
on the ground with the self-selected speed of the lane.
However, Fig. 3 (d), which represents subject B, showsthat the COM vertical velocity became high in experiment 1. This difference may be caused by the differencein the treadmill experience between subjects.

timing and recovery step timing, respectively. Throughexperiments 1 to 3, the knee and hip were signiﬁcantly
ﬂexed after tripping and the trunk inclined forward. Compared with the ground experiment results shown in Figs.
2 (b) and (c), the ﬂexion angle of the knee joint of
the treadmill experiment shown in Fig. 2 (a) increased
sharply during the recovery motion and the knee ﬂexion
started late.
According to Fig. 2, the maximum knee ﬂexion angle
became similar between experiments 1 and 2 whereas it
decreased by approximately 20◦ in experiment 3. Therefore, from the viewpoint of the reproducibility of the joint
pattern of reaction motion against tripping on the treadmill, matching the subjective speed seems more important. In contrast, there was no signiﬁcant difference between the three experiments in terms of hip joint angle
and maximum upper body angle.

4. DISCUSSION

Furthermore, for subject A the hip and knee angles andtrunk inclination became similar between experiments 1
and 2 in which the subject’s self-selected speed was usedon the treadmill and ground lane. The maximum angleof these parameters decreased slightly in experiment 3
although the trend of joint patterns was similar.

During the recovery motion, the maximum knee angle
became similar between experiments 1 and 2, which wereconducted on the treadmill and ground lane with the subject’s self-selected speed for each condition. This was acommon trend for both subjects. On the other hand, with

159

Table 3 Recovery motion parameters
Subject A

Condition
TreadmillExperiment2

OvergroundExperiment3

Overground

Recovery time [s]1.46 ± 0.11

0.68 ± 0.04

1.54 ± 0.10

0.97 ± 0.05

1.36 ± 0.14

0.74 ± 0.10

1.45 ± 0.05

0.65 ± 0.05

1.40 ± 0.08
0.70 ± 0.04
Notes : Values are mean ± SD

1
0.5

1Elevating
2 3 4 5 6
7 8 9
Lowering(a) Horizontal COM
velocity: Subject A

-0.2
-0.4
-0.6
-0.8
-1

1.45 ± 0.15

1.5
1
0.5
0

1 2 3 4 5 Lowering
6 7 8 9Elevating(b) Vertical COM
velocity: Subject A

0.71 ± 0.06

02

Velocity [m/s]1.50Stride length [m]0Velocity [m/s]

Velocity [m/s]2

Recovery time [s]Velocity [m/s]Experiment1

Subject B

Stride length [m]

1

2
3
4
Elevating5(c) Horizontal COM
velocity: Subject B

-0.2
-0.4
-0.6
-0.8
-1

1

2
3
4
Elevating5(d) Vertical COM
velocity: Subject B

Fig. 3 COM velocity at recovery step; The horizontal and vertical center of gravity velocities at the timing of recovery
step for two subjects are shown. The bar graphs represent Experiment 1 on the treadmill 1 and Experiment 2 and 3
on the ground, respectively, from the left. In (a-c), the results of experiments 1 and 3 were similar for the elevating
strategy. On the other hand, there was a different tendency in (d).
of some treadmill trials in the normal gait motion suggested that subjects perhaps anticipated the timing of thetrip. Sufﬁciently preventing this anticipation of trippingis necessary.

regard to the COM velocity at the time of the recovery
step, experiments 1 and 3, which used the same speed on
both treadmill and ground lane, had similar results. The
result for experiment 2, which used a greater speed thanthe other experiment on the ground lane, became large asshown in Figs. 3 (a), (b), and (c). However, as shownin Fig. 3 (d), for subject B the COM vertical velocity
with the treadmill in experiment 1 became larger than thevelocity with the ground.

5. CONCLUSIONS
Although a treadmill is useful for gait analysis, the gait
motion on the treadmill differs from that on the ground tosome extent. Therefore, in this study, a series of tripping
experiments on a treadmill and a walking lane was performed to compare and analyze the difference between
them to reproduce the reaction motion on the ground using the treadmill.
We developed an experimental system that induced
tripping on a treadmill. The timing of the tripping wascontrolled using an algorithm that monitored and estimated the gait timing of the subject. This study focusedon the difference in subjective and physical speeds on thereaction motion. Therefore, the tripping experiment was
conducted on the treadmill and ground lane at different
speeds.
The results suggest that the COM velocity becomessimilar between the treadmill and ground when the physical speeds are the same rather than the subjective speeds.
On the other hand, the joint pattern tends to become similar when the subjective speeds are matched. However,differences between individuals, especially the experience of treadmill walking, should be considered for further analysis.

Considering the similarity of the maximum ﬂexion angle of the knee joint, from the viewpoint of joint pattern, the leg and trunk motions of the reaction motion
probably become similar when the subjective speed ismatched between the treadmill and ground walking. On
the other hand, when the physical speed of both treadmill
and ground walking are matched, the COM velocity at thetime of the recovery step on the sagittal plane probably
becomes the same. However, unlike the COM horizontal
velocity, the vertical velocity differs between subjects.
Adaptation to the treadmill experiment is suggested asa cause of the difference of the vertical COM velocity
between subjects. Before this experiment, subject A had
experience in treadmills, whereas subject B did not havesuch experience. Therefore, the experience of the treadmill experiment probably affected the reaction motion.
Therefore, when considering a treadmill as an alternative method for reproducing the fall reaction motion on
the ground, individual differences between the subjects
should be considered.
In addition, the difference in the hip joint pattern

160

REFERENCES[1] Janice J. Eng, David A. Winter, and AE Patla.
Strategies for recovery from a trip in early and lateswing during human walking. Experimental brain
research, 102:339–49, 02 1994.
[2] Ting-Yun Wang, Tanvi Bhatt, Feng Yang, and YiChung Pai. Adaptive control reduces trip-induced
forward gait instability among young adults. Journal of Biomechanics, 45(7):1169 – 1175, 2012.
[3] Jaclyn R Watt, Jason R Franz, Keith Jackson, Jay
Dicharry, Patrick O Riley, and D Casey Kerrigan.
A three-dimensional kinematic and kinetic comparison of overground and treadmill walking in healthyelderly subjects. Clinical biomechanics, 25(5):444–
449, 2010.
[4] M P Murray, G B Spurr, S B Sepic, G M Gardner, and LA Mollinger. Treadmill vs. ﬂoor walking:Kinematics, electromyogram, and heart rate. Journal of applied physiology (Bethesda, Md. : 1985),
59:87–91, 08 1985.
[5] F. Alton, L. Baldey, S. Caplan, and M.C. Morrissey.
A kinematic comparison of overground and treadmill walking. Clinical Biomechanics, 13(6):434–
440, 1998.
[6] White Scott C, Yack H. John, Tucker carole A., and
Lin Hsin-Ying. Comparison of vertical ground reaction forces during overground and treadmill walking. Medicine & Science in Sports & Exercise,
30:1537–1542, 1998.
[7] Patrick O. Riley, Gabriele Paolini, Ugo Della Croce,
Kate W. Paylo, and D. Casey Kerrigan. A kinematicand kinetic comparison of overground and treadmill walking in healthy subjects. Gait & Posture,26(1):17–24, 2007.
[8] P. Sabetian and J. M. Hollerbach. A 3 wire bodyweight support system for a large treadmill. In 2017
IEEE International Conference on Robotics and Automation (ICRA), pages 498–503, 5 2017.
[9] Pinata H. Sessoms, Marilynn Wyatt, Mark Grabiner, John-David Collins, Trevor Kingsbury,
Nancy Thesing, and Kenton Kaufman. Method for
evoking a trip-like response using a treadmill-based
perturbation during locomotion. Journal of Biomechanics, 47(1):277–280, 2014.
[10] A.M. Schillings, B.M.H. Van Wezel, and J. Duysens. Mechanically induced stumbling during human treadmill walking. Journal of Neuroscience
Methods, 67(1):11–17, 1996.

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