Proceedings of the 2020 IEEE/SICE
International Symposium on System Integration
Honolulu, Hawaii, USA, January 12-15, 2020

Encouragement of Squat-Lifting: Feasibility Study of a Highly Usable
Passive Knee Assistive Device
Chen-Yu Cheng1,2 , Shogo Okamoto2 , Pengcheng Li2 , Yasuhiro Akiyama2 , Chongyang Qiu2 , and Yoji Yamada2

Abstract— Low back pain (LBP) is a serious occupational
disease. A main contributing factor to LBP is a burden on the
lumbar region, which can occur when lifting heavy objects. To
reduce the risk of LBP, squat-lifting, which uses knee torque, isrecommended as a preferred alternative to stoop-lifting, whichuses hip and back muscles. However, people tend to avoid
squat-lifting due to its relatively low metabolic efﬁciency, andthe greater force it places on the knee and rectus femoris.
Many lifting assistive devices have already been commercialized
to cope with LBP. However, the majority are designed tosupport back or hip muscles, not to encourage workers to
perform squat-lifting. Therefore, we fabricated a prototype knee
assistive device that is easy-to-wear and light-weight, using
shape-memory-alloy (SMA) wires that are inherently passive
elements. We then tested the effectiveness of the device bymeasuring electromyography (EMG) levels in the rectus femoris
during squat. At knee bending positions, the device reduced
EMG levels in the rectus femoris by at least 15% on average.

I. INTRODUCTION
Multiple original studies [1], [2], [3] and systematic reviews [4], [5] agree that manual lifting is a major cause ofdeveloping low back pain (LBP). When lifting heavy objects,
two predominant methods are stoop-lifting and squat-lifting,as shown in Fig. 1. Squat-lifting relies on knee torque forlifting, while keeping the torques from the back low. Stooplifting is characterized by an inclined trunk and almost fully
extended knees. In one deﬁnition, the postural index, whichis the ratio between knee ﬂexion from normal standing
and the sum of ankle, hip, and lumbar vertebral ﬂexions
from normal standing, is approximately 0.80 for squatlifting and 0.11 for stoop-lifting [6]. Squat-lifting reduces
the maximum voluntary contraction in the erector spinae
compared to stoop-lifting [7], and the shear forces applied
to lumbar for stoop-lifting are estimated to be 180% greaterthan squat-lifting [8]. Furthermore, stoop-lifting is reported
to have caused approximately 75% more stresses on discs
and ligaments than squat-lifting [9]. Consequently, the governmental ministry in Japan recommends squat-lifting tolower the risks of LBP [10]. However, squat-lifting is 23%
to 34% less metabolically (in terms of work per oxygen
consumption) efﬁcient than stoop-lifting [11]. The maximum*This work was in part supported by MEXT Kakenhi (19K21584).
1 Chen-Yu Cheng is with the Department of Mechanical Engineering,
University of Michigan, 2350 Hayward, Ann Arbor, MI 48105, USA

cchenyu@umich.edu
2 Chen-Yu Cheng, Shogo Okamoto, Pengcheng Li, Yasuhiro Akiyama,
and Yoji Yamada are with the Department of Mechanical Systems Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, JAPANokamoto-shogo@nagoya-u.jp

978-1-7281-6667-4/20/$31.00 ©2020 IEEE

Fig. 1: Schematic diagrams of squat-lifting (left) and stooplifting (right).

oxygen consumption during squat-lifting is 14.3% more thanthat during stoop-lifting [12], and the maximum heart rateduring squat-lifting is 6.5% more than that during stooplifting [12]. Furthermore, squat-lifting increases maximum
muscle contraction in the rectus femoris [7]. As a result,although squat-lifting can be effective in preventing LBP
because it exerts less shear stress on lumbar discs, it is more
fatiguing and thus, less frequently used.
To prevent or manage LBP, several wearable assistive
devices have been developed and commercialized in recent
years. The majority of these devices provide external support
to the lumbar spinae when lifting objects. Depending on how
external forces are applied, there are two types of assistive
devices: active and passive. Active devices use actuators suchas electromagnetic, electrohydraulic, and electropneumatic togenerate the external forces; passive devices use intrinsicallypassive elements to store energy during negative works, andthen release the stored energy during positive works.
Hybrid Assistive Limb (Cyberdyne, Ibaraki, Japan) [13],[14] and Mk2b [15], [16], a revised version of Robo-Mate[17], are examples of active assistive devices with twoelectromagnetic motors on the hip joints of the wearer. Bothdevices provide assistive torque to wearers’ hip and trunk tosupport the lifting motion. In contrast, the Laevo (Laveo,The
Netherlands) [18] is a passive assistive device that uses
pads contacting wearers’ chest, back, and upper legs, anda circular tube with spring-like characteristics that connects
these pads to generate upward forces to the chest; the reactive
forces are supported by the wearers’ legs. A garment-likedevice [19], designed by Vanderbilt University, USA, is504

another passive device that uses elastic bands to provide
extension momentum to the lumbar. Additional devices havebeen either commercialized or in development.
These assistive devices have trade-offs between effectiveness and usability. The active ones produce more effective
support, but some require external energy sources and areheavier. The wearability is not high because these devices use
rigid links and cuffs and allow limited degrees of freedom
during use. The passive devices, on the contrary, are lightweight and are more practical to use when wearers need tomove around frequently and in circumstances where external
power sources are not available.
Assistive devices can also be categorized by supported
area. Assistive devices for lifting are focused on protecting
the lower back and hips. Other assistive devices provide
support to the knees or ankles and can be utilized forwalking assistance [20], [21], [22], [23], [24]. However,giving support to the lower back and hip joints does notchange the wearers’ behavior from stoop-lifting to squatlifting. Consequently, we focused on a knee assistive device
to encourage laborers to conduct squat-lifting to lower therisk of LBP.
To expand the use of squat-lifting, this study aims todevelop a knee assistive device with high usability by using
shape-memory-alloy (SMA) wires as passive elements. This
device takes advantage of the superelastic property of SMA
to provide external forces to the rectus femoris, encouraging
workers to perform squat-lifting by compensating for the partof the necessary knee torque. In this paper, we describe thedesigning process and policies of the device and evaluate theeffectiveness of the device experimentally by measuring theelectromyography (EMG) of the rectus femoris.
II. KNEE ASSISTIVE DEVICE WITH
SHAPE-MEMORY-ALLOY WIRESThe knee assistive device consists of two main parts: acloth knee brace and SMA wires. The knee brace functions
as a splint on which to install the SMA wires, while the
SMA wires provide the external forces to support rectus
femoris. The ends and the middle part of the wires, which
are the vertices during bending, are ﬁxed to the knee brace.
This design positions the bending center of each wire at therotational center of the knee on the sagittal plane. The deviceis shown in Fig. 2.

Fig. 2: Knee assistive device built from a commercial knee
supporter with shape-memory-alloy wires.

B. Shape-Memory-Alloy Wires
The SMA wires are made of nickel-titanium (Ni-Ti) alloy(Yoshimi Inc., Japan). Supportive forces are generated by
the superelasticity property of SMA, which is more accurately described as pseudoelasticity. As shown in Fig. 3,
four wires were joined together with a thermoplastic resin.
Each strand of wire was put in a pocket at each sideof the knee brace, thus there were a total of eight SMA
wires per knee. The maximum knee torque for sit-to-standmovement is 0.38 Nm/kg for normal people [25]. Thus, fora 70-kg person, it requires 26.6 Nm to perform sit-to-standmovements. The number, length, and diameter of the SMA
wires were selected to produce 10% of the required torque.
This value (10%) was chosen so that the device would reducethe intensity of muscle activity required during the sit-tostand motion without the wearer feeling a resistive force
during the other activities, which would be perceived as a
nuisance. The choice was subjective because, to the best of
our knowledge, such a minimum resistive force that does
not subjectively or physically disturb the motion of wearers
has yet to be determined. Therefore, no current literature ordata is available to determine an optimal percentage for thelevel of support provided by passive knee assistive devices.
Furthermore, our device was designed to generate 10% of
the necessary torques when the wearer is not lifting any
additional weight. It should be noted that the support ratio ofthe device will be smaller than 10% while lifting additional
weight. The performance tests we used to determine thevalues of the length, diameter, and the number of wires will
be described in the next subsection.

A. Knee Brace
A commercial hinged knee brace (54557, Mueller Sports
Medicine Company, Germany) was chosen to be part of theassistive device. The hinges of the knee brace, which areused to prevent the medial-lateral motion of the knee toprotect damaged knees, were removed. Therefore, the brace
was used as a soft chassis on which to attach the SMA wires
to provide supportive force while bending.
The brace is designed to be easy-to-wear by using two
Velcro strips, which allow the wearer to put on and off thedevice without needing to remove his or her shoes.

C. Determination of the Length and Diameter of SMA Wires
We conducted experiments to determine the optimal lengthand diameter of SMA wires in terms of the forces generated
by the wires. A push-pull scale (FB-100N, Imada Co. Ltd.,
Japan) was used to measure the restoring force when the wire
was bent at set angles. One end of the SMA wire was ﬁxed tothe push-pull scale, and an external force was applied to theother end to bend the wire to a certain angle. The push-pullscale was aligned with the direction of the restoring force of
the wire. The maximum knee angle of a sit-to-stand motion505

Fig. 3: The left panel is a single strand of the SMA wire.
A thermoplastic resin was applied on the two ends of four
wires to bind them together. The right panel shows a SMA
wire under bending conditions.
is approximately 100◦ [26]. Therefore, the resultant torquewas measured at 100◦ .
To experimentally evaluate the diameter, SMA wires having diameters of 1.0, 1.2, and 1.5 mm, and a common lengthof 20 cm were compared. Four wires of the same diameter
were joined together for the measurements, and the results
are shown in Fig. 4. SMA wires of larger diameter generated
greater torques during bending. The relationship between
diameter and torque was nonlinear and quadratic.
For the length test, the diameter of the SMA wires was
ﬁxed to 1.0 mm, and the length ranged from 16 cm to 34 cm
with an interval of 2 cm. Different from the diameter test,
we only used one wire to measure the forces for this test.
As shown in Fig. 5, the shorter SMA wires exhibited higher
torque, whereas the torque is not signiﬁcantly different when
the wire length is more than 20 cm. Nonetheless, whenshorter wires are used, the clothing area that supports thewires is also smaller, and the restoring force damages thecloth material of the knee brace.
Based on these performance tests, we determined that the
optimal length, diameter, and number of wires is 22 cm,
1.8 mm, and 8, respectively. To make these determinations,inter- and extrapolation of the data were used. This combination achieves the aforementioned supportive torque, i.e. 10%of the moment.
III. EFFECTIVENESS TESTS
A. Participants
Three participants (one male, two females in 20s) agreed
to test the EMG value while squatting. The average height
and weight of the participants were 166.7±9.1 cm and 55.3±
5.9 kg, respectively. All participants were healthy with noinjuries or diseases.
B. Experiment Procedures
We measured the EMG signals of the rectus femoris underfour different conditions: 45◦ (half) or 90◦ (full)-squat withor without the knee assistive device. For each trial, theparticipants were asked to squat to the assigned angle, thenFig. 4: The relationship between diameter and torque of
SMA wires. All wires were 20 cm in length, and four wiresof identical diameter were joined together. Torques were
determined with the wires bent at 100◦ .

Fig. 5: The relationship between length and torque of SMA
wires. The diameter of each wire was 1.0 mm. Torques were
determined with the wires bent at 100◦ .

pause at the position for ﬁve seconds. A constant height chair
was used as a mark to let the participant know whether his or
her knee angle was close to the designated value. This task
mimicked a situation in which the participant was expected
to hold and lift an object by squat-lifting. The half squat issometimes used in earlier studies (e.g. [27]).
To avoid accidental injuries, the participants did not haveany actual loads in their hands and were instructed to avoid
quick movement. Each condition was repeated a total of
10 times by each participant. To prevent the muscle fatigue
from affecting the results, the four conditions were tested in
randomized order with each condition repeated ﬁve times insuccession.
C. Data Analysis
To process the EMG signals, we focused on the central
three seconds within the ﬁve-seconds-pause. During thecentral three seconds, relatively stable signals were recorded.
We calculated the mean voltage of the absolute envelopeof each trial, and then compared the difference between
the trials done with and without the knee assistive device.

506

(a) CAD images of the ﬁxation mechanism.

(a) Participant 1(b) Manufactured by a three-dimensional printer.

Fig. 7: Fixation mechanism. (a) CAD images. (b) From left
to right, the panels illustrate how to conﬁne the SMA wires
in the tube.

(b) Participant 2

by 27.7%, 26.8%, and 52.1% when squatting 45◦ ; anddecreased by 14.7%, 22.9%, and 38.4% when squatting 90◦ .
The effectiveness of the knee assistive device varies foreach participant, which may be because the weight of eachparticipant is different.

IV. DESIGNING THE SMA WIRE FIXATIONMECHANISM FOR OPTIMAL USABILITY(c) Participant 3

Fig. 6: EMG values of the rectus femoris. **: p<0.01, ***:
p<0.001.

Although we planned to exclude samples where the variation
exceeded two standard deviations, there were no outliers
among the samples. However, we did exclude some trials,
in which the EMG signals were intermittent because of falseskin-electrode contact as invalid.
D. Results
The mean EMG signals and the p-values of Welch’s ttest of the three participants are shown in Fig. 6. There
are signiﬁcant differences between the conditions with andwithout the knee assistive device for both angles. For eachof the three participants, the mean EMG signal decreased

To increase usability, a mechanism for easily installingand uninstalling the SMA wires is proposed in Fig. 7. The
mechanism consists of two parts. One part looks like acylinder with two square holes, which is used to conﬁne thehorizontal movement of the wires. The other part is a lid withtwo “ears” to conﬁne the vertical movement of the wires.
There are two convex squares on the ears that ﬁt into the twosquare holes in the cylinder. The lid can be easily removed
by pressing the ears. A detailed procedure of utilizing themechanism is shown in Fig. 7b. With this mechanism, users
can turn on and off the supportive torque without taking off
the knee assistive device. The bottom end of the SMA strand
is inserted into the pocket of the knee brace while the top is
anchored by the mechanism. This mechanism has no effecton the assistive performance; however, it contributes to theusability of the assistive device.

507

V. CONCLUSIONS
Many published reports and government agencies recommend using squat-lifting instead of stoop-lifting to reduce
the risk of low back pain. To encourage people to perform
squat-lifting, we made a prototype of a light-weight and easyto-wear passive knee assistive device. This device consists ofa commercial knee supporter and two strands of SMA wires.
One device is worn on each knee. The SMA wires do not
lose their elasticity even when they are bent at larger angles
and generate restoring forces. A ﬁxation mechanism for the
SMA wires was devised to easily turn the assistive device
on and off without taking off the knee brace. The device
was designed to support approximately 10% of the maximum
torques for healthy people weighting 70 kg to perform sit-tostand movements. Four different conditions were compared
to experimentally evaluate the effectiveness of the prototype:squats at 45◦ and 90◦ with and without the assistive device.
The EMG values of rectus femoris were collected while
three participants performed each condition in a randomizedorder. The results indicate that, with the knee assistive device,
the participants decreased the activity of the rectus femorismuscle by 14.7%–52.1%. The participants were lighter than
70 kg, so the observed effects were higher than expected. The
proposed knee assistive encourages people to change theirlifting behavior from stoop-lifting to squat-lifting. Nonetheless, further usability tests are needed to evaluate the practicalutility of the proposed device and include subjective feedback
from occupational users. Furthermore, although we did notuse any loads in this study, if the participants had lifted actual
weights, then the magnitude of reduction of the EMG signals
during use of the assistive device would have been lower.
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