Development of a Chair to Support Human Standing Motion-Seat movement mechanism using zip chain actuatorYamato Kuroda1 , Qi An1 , Hiroshi Yamakawa2 , Shingo Shimoda3 , Jun-ichiro Furukawa4 ,Jun Morimoto4,5 , Yuichi Nakamura6 , and Ryo Kurazume1
Abstract— Many elderly people suffer from declined motor
ability, and it is important to develop an assistive device to support their daily life. Some assistive device have been developed
previously to help human sit-to-stand motion, but they generally
help body extension slowly and thus the user of the device could
not utilize their own muscles. Here, we develop a chair typeassist device which lift up the seat to push the hip joint for
supporting hip rising and body extension. Our developed device
employed a zip chain actuator which realizes fast and strong
support to follow the user’s movement and provide assistance.
Evaluation experiment was performed to assess the effect ofdeveloped device on muscle activation of lower limbs during
sit-to-stand motion. Results show that the muscle activation ofknee extensor decreased significantly compared to that without
device support. However, the activation level of flexor is foundto be increased to deaccelerate the body extension.

I. INTRODUCTION
According to a survey by Department of Economic andSocial Affairs of the United States, the number of people
aged 60 and over has risen to 1.05 billion, which is 13.5
point of the total population [1]. As a result, the proportionof the elderly population with declining physical functions is
expected to increase , making assistance and nursing care aneven more important issue for society. Therefore, it is important to prevent the decline of physical functions of the elderly
by using devices that support motor functions. Sit-to-standis a frequent activity in the daily life of the elderly people,
and it is expected to reduce the burden on the caregiver andto have a great impact on society by improving sit-to-stand(STS) motion. In previous studies, various devices have beendeveloped to support STS movements [2], [3], [4]. In thesestudies, the users are asked to hold the bar by their arms
or to lean their upper body against the bar. The bar leads
the users to the desired trajectory to achieve STS motion.
As another way, we have previously developed chair typeassistive device to rise the seat to provide a start cue of STS
motion [5]. However, the previous system could not fully
follow the user’s motion, and thus it results in slowing down
1 Yamato Kuroda, Qi An, and Ryo Kurazume are with Faculty of Information Science and Electrical Engineering, Kyushu University, Fukuoka,
Japan anqi@ait.kyushu-u.ac.jp
2 Hiroshi Yamakawa is with the Department of Precision Engineering,
Graduate School of Engineering, The University of Tokyo, Tokyo, Japan
3 Shingo Shimoda is with CBS-TOYOTA Collaboration Center Intelligent
Behavior Control Unit, RIKEN, Nagoya, Japan
4 Jun-ichiro Furukawa and Jun Morimoto are with Man-Machine Collaboration Research Team, Guardian Robot Project, RIKEN, Kyoto, Japan
5 Jun Morimoto is with the Graduate School of Informatics, Kyoto
UniversityKyoto, Japan
6 Yuichi Nakamura is with the Academic Center for Computing andMedia Studies, Kyoto University, Kyoto, Japan

Forward bending

Fig. 1.

Hip rise

Full body extension

Stabilization of posture

The four phases of the standing movement

the user’s movement. Therefore, in this study, we aim todevelop a chair type assistive device which could generate
faster motion to lift up the buttocks of the users. When the
motor ability of elderly people is declined, they prefer to
employ more stabilized strategy than those who have enoughmotor ability [6]. In the stabilization strategy, the elderly
people tend to flex their trunk deeper to move center ofmass on the feet and perform extension. If people haveenough motor strength, they would bend their trunk fasterto the generate momentum and utilize it for body extension.
For development of an assistive device of STS motion, it
is important to provide a wide range of support dependingon the user’s ability. Therefore, the goal of this study is
to develop a chair type assistive device that can adjust its
supports take a little different way in which they according
to the user’s physical functions. we develop an assistive
device that can follow different movement of the users (e.g.
stabilization strategy and momentum strategy). As shown in
Figure 1, there are four states in human standing: forwardbending, hip rise, full body extension, and stabilization of
posture[8], [9]. In this study, we focus on supporting the
motion of hip rise and body extension because these motions
control when to move forward and when to extend the body,and are important states that determine movement strategies.
First, the trajectory and movement of the buttocks, which
is a part of the body that is close to the seat and good forindicating the body movements of standing, will be used
as a reference trajectory, and it will be verified whether thechair can reproduce similar body movements. Next the effectof assistive device is evaluated by muscle activation of theusers during STS motion. This study aims to develop a chairthat supports standing by reducing muscle activity during
standing.
II. D EVELOPMENT OF C HAIR T YPE A SSISTIVE D EVICE
In this study, we propose a device that assists the user’s
movement by pushing up the user’s buttocks with the seatof the chair. The chair is designed to assist the user inDistance [mm]the range from hip rise to full body extension in Figure 1.
The trajectory and speed of the support part are determinedfrom the movement of the buttocks during the actual human
movement. In this section, first we explain the measurement
experiment of buttocks movement during STS motion of
the young and elderly people. Next, the assistive device isdeveloped to realize the same trajectory as measured in theexperiment.

350

Ankle joint

Elderly
Young

250
150

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-50
-400

-300

-200

-100

0

100

Distance in front-back direction [mm](a) Trajectory of the greater trochanter

To determine the reference trajectory for supporting theuser’s buttocks in assistive devices, experiments were conducted to measure human standing movements and investigate the actual trajectory of the buttocks. Nine elderly people(66.5±6.2 years old) and eight young people (24.5±2.2 years
old) participated at the experiment. Because some elderly
also use the momentum strategy, we used young to obtaintrajectories and velocities for the momentum strategy. Opticalmotion capture system (MAC3D, Motion Analysis Corp) isused to record the marker position attached to participant’s
body. The sampling frequency of measure marker positionwas 100 Hz. To develop a chair which elevating a seatto support STS motion, the marker attached to the greater
trochanter is focused. A force plate (TF-3040, Tec Gihan Co.,
Ltd.) was placed on the seat to measure the reaction force
applied to the buttocks. The reaction force is measured in
2,000 Hz and it is processed with Butterworth second order
low-pass filter with cutoff frequency 20 Hz. The moment
when the floor reaction force in the vertical direction applied
to the buttocks became less than 5 N was considered to be thesubject’s release time of the subjects from the seat, and thedata from 1.0 s before to 2.0 s after the release is analyzed.
In the experiment, the subject was asked to cross their arms
in front of the chest and perform STS motion with their
comfortable speed.
B. Results of Greater Trochanter Movement
The trajectories of the greater trochanter averaged overall procedures in young and elderly subjects during the STS
motion are shown in Figure 2. In Fig. 2 (a), the origin in the
backward and forward direction is the horizontal positionof the ankle joint. The vertical direction shows the changeof marker position from the initial sitting position. It canbe seen that both the young and the elderly subjects firstmove their hips forward and then stand upward. Comparedwith the younger subjects, the older subjects started to standwith the greater trochanter close to the ankle and the feetclose to the body. Figure 2 (b) shows time-varying horizontal
movement of the greater trochanter. It is found that the
maximum distance and speed of movement were smaller forthe elderly than for the young, similar to the results reported
in a previous study by [10]. Figure 2 (c) shows the verticalmovement of the greater trochanter. It is shown that there
was no significant difference between the elderly and the
young in the vertical direction.

Elderly
Young

400
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100
0
-100
0.0

1.0

0.5

1.5

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Time [s](b) Forward and backward movement
Distance [mm]A. Measurement experiments

Distance [mm]

500

500

Elderly
Young

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300
200
100
0
-100
0.0

0.5

1.0

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Time [s](c) Vertical movement

Fig. 2. Experimental results of the measurement of the greater trochanter.

C. Design of Assistive Device
Based on the obtained trajectories of the greater trochanterof the elderly and young, a chair to support the STS
motion is designed. The chair was designed to move theseat in two directions, forward and backward, and vertically,
according to the movement of the hips. A ball screw anda motor (Maxon Motor, RE40148867) were installed at theleft-right seid of the chair to realize the forward-backward
movement. The chair is driven from both ends to preventthe seat from deflecting too much and to support the chaireven with a weak force. For vertical movement, a zipchain actuator (Tsubakimoto Chain ZCA35N050EL) and the
motor (Oriental Motor BXM6200M-GFS and GFS6G20)
were installed under the seat, and vertical movement was
realized by pushing up the seat from below with the ZCA.
The ZCA was used because it can operate at high enoughspeed to achieve the speed of the greater trochanter, and also
because it has high torque and high rigidity to support theweight of a person. Mechanism of a zip-chain actuator isshown in the left side of Fig. 3. By turning the shaft, thetwo zips engage each other and move in a straight line. In
particular, the use of a zip-chain actuator for pushing up in
the vertical direction enables high-speed driving to withstandthe load of the user. By placing shafts at both ends of the seatsurface, vertical movement is promoted, and by supporting
the load on the zip-chain actuator in the front-back and leftright directions, smooth vertical movement is achieved. The
developed support device is shown in Figure 3. The upper
two figures in Fig. 3 show the zip chain actuator, which

Vertical
Movement

Horizontal Movement

Movement of Zip Chain Actuator

Fig. 3.

Developed assistive devices.
Fig. 4.

engages the chain and pushes it upward when the drive unit
turns. The seat is the yellow frame shown in the figure,and the zip chain actuator is installed under the seat andconnected to the seat. Verification experiments were actuallyconducted using this device.
D. Validation Experiment
In this study, it is verified whether the greater trochanter
trajectories of the elderly and the young, which were determined in the measurement experiment of STS, could be
reproduced by the seat of this machine. An optical motion
capture system (OptiTrack, V120: Trio) is used to measure
the trajectories of the marker positions on the seat surface todetermine whether the target trajectories could be realized.
The trajectories of the elderly and the young in the forwardbackward and vertical directions measured in the former
measurement experiment were used for comparison. It is
investigated whether the target trajectories could be achievedin the two patterns for the elderly and the young, and whether
the assistive devices could make a difference between the twopatterns. In this study, a 35-year-old healthy young person
sat on the developed assistive device and was instructed not
to perform voluntary motions. It was examined whether the
motions were reproduced only by pushing up with the seatof the assistive device.
E. Results of the Reproduction
Figure 4 shows that the user of the system perform STS
motion with the assistive device. The results of reproducing
the movement of the greater trochanter are shown in Fig. 5,
where the results obtained in II-B are shown as the target
trajectory with dotted lines, and the trajectory of the seat of
the assistive device measured with the subject actually riding
on it is shown with solid lines. In the forward direction of
the movement of the greater trochanter of the young subject,the maximum distance was about 150 mm shorter than thetarget distance. In the vertical direction, the vertical direction
moved about 50 mm less in the standing position, but thetarget trajectory was almost reproduced. On the other hand,
the maximum distance in the forward direction of the elderly
was 15 mm shorter from the target distance, but the trajectory
was closer to the target compared to the support movementof the young. The vertical direction moved about 50 mm lessin the standing position as in the case of the young person,

STS motion with support by developed assistive device.

but the trajectory overlapped with the target and the speed
achieved the target value. Although the developed device
could not fully follow the trajectory of the greater trochanter,it shows that the device could lift up the user at the same
speed as they stand up by themselves.
III. E VALUATION EXPERIMENTIn this chapter, the effect of support provided by theassistive device on muscle activation during STS motion isinvestigated. Although the developed device could support
whole period of STS motion, this study focuses on assisting
the hip rise motion and beginning part of body extension.
A. Support Methods
The flow of support by a chair is shown in Fig. 6. First,
it is judged whether the user of the device performs themomentum strategy or the stabilization strategy. We hypothesized that the users who utilize the momentum strategy
bend their trunk faster than the users who performed thestabilization strategy. Therefore, flexion angle of the trunk
is measured and its angular velocity is used to judge the
STS strategy. If the angular velocity is less than 70 deg/s at
15 deg of upper body forward bending, it is judged to be astabilization strategy and the trajectory of the elderly person
is reproduced by the assistive device. If the angular velocity
is larger than 70 deg/s at the same posture, the movement
is considered to be momentum strategy and the trajectory of
the young person is reproduced. In both cases, feed-forward
control is used to generate part of the trajectory of thegreater trochanter reproduction. These threshold values were
determined by preliminary experiments. When evaluating
the effect of assistive device, it does not reproduce whole
period of the movement. The assistive device generates the
trajectory between 0.4 s before and 0.4 s after the hip rise forthe stabilization strategy and between 0.3 s before and 0.5 s
after the hip rise for the momentum transfer strategy. These
periods were determined as the approximation time when thetrunk flexion angle was 15 deg. The same trajectories as inFig. 2 is used for reference trajectory.
B. Evaluation Experiment
The method of the evaluation experiment using a chairis shown below. Two healthy young subjects(35 years old

Distance [mm]500

Seat of assistive device
Greater trochanter ofparticipant

400
300
200
100
0
0.0

2.0

1.5

1.0

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3.0

2.5

Time[s]Distance [mm](a) Forward movement in momentum transfer strategy
Seat of assistive device
Greater trochanter ofparticipant

300
200
100
0
0.0

2.0

1.5

1.0

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Time[s]Distance [mm](b) Vertical movement in momentum transfer strategy
Seat of assistive device
Greater trochanter ofparticipant

300
200

C. Results of Muscle Activation

100
0
0.0

2.0

1.5

1.0

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3.0

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Time[s]Distance [mm](c) Forward movement in stabilization strategy
Seat of assistive device
Greater trochanter ofparticipant

300
200
100
0
0.0

1.0

0.5

2.0

1.5

3.0

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Time[s](d) Vertical movement in stabilization strategy
Fig. 5.

Reproduction results of the orbit of the greater trochanter.
・

trunk

User Starts
STS Motion

Determine
Assist Strategy≧ 70 deg/s

MomentumTransfer Strategy
・

trunk

User Finishes
STS Motion< 70 deg/s

Stabilization
Strategy

Fig. 6.

To evaluate the support by assistive device on movement,
muscle activation is measured from extensor and flexor of
the hip, knee, and ankle joints by wireless surface EMG
sensors (Cometa Corp., Mini Wave Infinity). In total, sevenmuscles are measured; gluteus maximus muscle (Gmax; hipextensor), vastus lateralis (VL; knee extensor), semitendinouse muscle (SemiT; knee flexor), tibialis anterior (TA;ankle plantar-flexor ), and gastrocnemius (GAS; ankle dorxiflexor). The muscle activation is measured in 2,000 Hz and
it is band-pass filtered with fourth-order Butterworth fitlerwith cut-off frequency 40-400 Hz [11]. Then the signal isrectified and low-pass filtered with fourth-order Butterworth
filter with cut-off frequency 4 Hz [12]. The muscle activation
is normalized by the maximum activation level which was
obtained by measuring maximum voluntary contraction.

Flow of support by chairand 23 years old) intentionally performed two strategies;
ten natural STS motion (the momentum strategy) and ten
STS motion simulating the elderly (stabilization strategy)
respectively. Because it is dangerous to conduct experimentswith the elderly, young subjects were used. An optical motion
capture system (OptiTrack, V120: Trio) was used to measure
the trajectories of the marker positions on the acromion andadductor in 120 Hz. The angle between the vector formedby these two markers and horizontal direction is definedas trunk flexion. In order to adjust the timing of leaving
the seat during the STS motion, the marker attached to thegreater trochanter was used to detect hip rise timing. Whenthe marker moved 10 mm upward from the initial position,
it is determined as hip rise timing. As with the same process
in the previous chapter, 1.0 s before and 2.0 s after the hiprise is used for analysis.

The results of the evaluation experiments are shown below.
First, the results of muscle activation when the participants
performed the momentum strategy is shown in Fig. 7. In the
figure, blue lines indicate the average of muscle activation
when people stand up with support from the assistive device
and black lines indicate the average of muscle activation
when people stand up by themselves. Error bar indicates thestandard deviation at each time.
A t-test was used to determine if the maximum value wassignificantly changed. The results of average and standard
deviation of maximum muscle activation at each conditionand p-values are shown in Table I. The muscle activity ofTA and VL decreased when our developed device support
STS movement. This indicates that our device helps theparticipants move forward and it results in decrease of
muscle activation of TA. Similarly, the device rises theseat to push the buttocks so that the muscle activation ofknee extensor decreased as well. GAS increased and SemiT
decrease statistically significantly, but the amount of change
is rather small compared to TA and VL so that the effectof assistive device is considered to be small on these two
muscles. These results imply that our developed system could
reduce the burden of the participant by supporting their hip
rise and body extension.
Figure 8 shows comparison of muscle activation when theparticipants intentionally performed the stabilization strategy.
The blue and black lines show the average muscle activation
obtained from the condition with device support and without
device support respectively. Similar to the condition thatthe participants performed the momentum strategy motion,muscle activation of TA and VL decreased statistically significantly when the developed device assist the participants.
However, GAS, SemiT and Gmax increased significantly.
These muscles are attached at the back side of the human
body, and they mainly deceleration of forward body movement. This implies that the participants activate these muscle
to brake the forward movement and increase the stiffness ofthe joint to resist the seat movement. This could be attributed
to the fact that the seat of the device could not fully follow
the buttocks movement of the participants.

35

15
10
5
0
0

0.5

1

1.5

Time[s]2

2.5

Muscle activity[%]100

Muscle activity[%]Muscle activity[%]

Extension

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Gmax

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With Assist
Without Assist

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Time[s]23

2.5

EMG results during momentum strategy

Hip

Knee

Ankle
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Time[s]

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Muscle activity[%]Muscle activity[%]30

Muscle activity[%]

0

SemiT

Fig. 7.

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GAS

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Time[s]

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Gmax

VL

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Muscle activity[%]Muscle activity[%]

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0

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30

TA

Flexion

Time[s]

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15
10

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Extension

1.5

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20

VL
Muscle activity[%]Muscle activity[%]

GAS

Flexion

Hip

Knee

Ankle
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Time[s]Time[s]TA

SemiTFig. 8.

With Assist
Without Assist

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EMG results during stabilization strategy

TABLE IR ESULTS OF T- TEST FOR MAXIMUM VALUE

Momentum Transfer Strategy

GAS
TA
VL
SemiT
Gmax

Stabilization Strategy

Without Assist [%]With Assist [%]P-value

Without Assist [%]With Assist[%]P-value14.6±4.2
28.1±14.6
72.0±20.6
24.4±6.7
27.0±3.916.8±8.4
20.0±18.1
44.0±19.7
21.9±7.4
27.0±7.3

1.2E-5
1.5E-82.3E-46
2.9-E50.47

10.6±4.6
19.4±10.544.4±12.8
18.8±8.7
25.6±5.526.2±8.7
8.8±5.825.1±8.924.7±8.0
27.5±9.7

3.8E-127
2.7E-57
3.2E-95
7.7E-219.0E-4

IV. CONCLUSIONS
The assistive device of STS motion is developed in thisstudy. The device employs a zip chain actuator to realize
the fast and strong support for hip rise and body extension enough for achieve STS motion. It is found that ourdeveloped device could support the forward movement andbody extension after the hip rise despite the motion strategies of the participants (momentum transfer or stabilization
strateiges). This device is expected to support wide range ofSTS motion depneding on the motor ability of the users.
However, unintentional support might cause the increaseof muscle activation that results in deceleration of body
movement and increased joint stiffness. One of our future
directions is to enable our assistive device to adjust the user’s
movement. In the future study, it is necessary to monitor
the user’s movement by additional sensors and adjust the
movement of the device to the user.
ACKNOWLEDGMENT
This work was supported by RIKEN and Science andTechnology Hub Collaborative Research Programs RIKENKyushu Univ. Part of this work was supported by Nagamori Foundation, and JSPS KAKENHI Grant Numbers

JP18H01405, JP21H04894, and JP19H05729.
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