Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 1 (June 2018) 45–51___________________________________________________________________________________________________________Development of A Passively Powered Knee Exoskeleton for Squat Lifting
R.K.P.S. Ranaweera1, R.A.R.C. Gopura1, T.S.S. Jayawardena2

Bionics Laboratory - Department of Mechanical Engineering1, Department of Textile and Clothing Technology2,
University of Moratuwa, Katubedda, Moratuwa 10400, Sri Lanka
G.K.I. Mann3

Faculty of Engineering and Applied Science3, Memorial University of Newfoundland,
230 Elizabeth Ave, St. John's, NL A1B 3X9, Canada
E-mail: pubudur@uom.lk, gopurar@uom.lk, jaya@uom.lk, gmann@mun.ca
www.mrt.ac.lk

Abstract
This paper proposes a knee exoskeleton with passive-powering mechanism to provide power assistance to the knee
joint during squat lifting of objects from the ground. It is designed to capture and store 20% of the biomechanicalenergy dissipated at the biological knee joint during decent phase and return the harnessed energy in the ascent phasein a squatting cycle. The effectiveness of the proposed system was verified by evaluating performance of key musclesof knee joint using surface electromyography (sEMG) signals. Statistical data from experiments revealed a reductionof peak root-mean-square averages of sEMG signals of knee extensor muscles by 30 - 40% during squatting.
Keywords: knee exoskeleton, biomechanical energy harvesting, passive mechanisms, squatting.

resulted in formulation of bulky and heavy
electromechanical systems that are slow in operation.2
Alternatively, recent research studies indicate that
passive-dynamic powering systems are effective at
metabolic cost reduction and enhancing overall humanperformance.3 The body-powered exoskeletons areenergetically autonomous and soft lightweighttransmissions
providesuperior
biomechanical3
compliance. However, performance and endurance
augmentation through biomechanical energy harvesting
still remain an open challenge. A wearable energy
harnessing and releasing system that is capable ofrecycling the waste mechanical energy dissipated in thehuman body can help reduce metabolic energy
consumption during activities of daily living.3
Complementing limb muscles energetically will augment
the joint strength and moreover enhances endurance
limits of muscle fatigue. As a result, the exoskeleton
wearer’s productivity will improve over a period of time.

1. Introduction
Exoskeleton systems or robots are mainly developed
focusing on two different user groups, namely
differently-abled people and able-bodied people.1
According to literature, limited number of full lowerextremity exoskeletons or joint-level exoskeletons havebeen developed to support task or work related activities
by providing power-assistance.1 The application areas
are mostly limited to either locomotion assistance, tool
holding or lift assistance.1 Although functionality of
autonomous externally-powered robotic exoskeletons are
satisfactory, the wearer’s performance is notablycompromised.2 In particular, such devices struggle toreduce the human effort. The main cause is the inabilityof the system to match the complex kinematic and kinetic
requirements demanded by human biological joints. Here,limitations of the complementing technologies such as
gear drives, actuators, sensors, and power sources

Copyright © 2018, the Authors. Published by Atlantis Press.
This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
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2.1. Functional Considerations
Among different lifting techniques the most commonlyused approaches are squat lifting and stoop lifting.5 The
knee exoskeleton is envisioned to assist the wearer’s knee
joint to lift moderately heavy loads. Hence squat or semisquat lifting is selected as the technique in focus fordeveloping the proposed device, as it also promotes useof biomechanically more favorable approach.5
The device should essentially allow the wearer toambulate freely in-between lifting tasks and pose norestriction for knee flexion during swing phase of human
gait. The knee joint’s limits of ranges of motion over thesagittal plane is measured to be -100 of hyper-extension
and 1400 of flexion.4 According to gait data, 6 maximum
flexion of knee joint during swing phase is 600.
2.2. Biomechanical Considerations

Fig. 1. Technique (A) and biomechanical data (B) of squatting(adapted from references 4 and 5).

The muscle activity of thigh muscles are of interest when
performing a squatting exercise.7 Notably, the quadriceps
show greater muscle activity than the hamstrings during
the complete ascent phase.4 The focus is to relieve muscle
groups of the burden of having to concentrically andeccentrically work to control motion of the knee joint.
Biomechanical data during squatting (see Fig. 1(B))
were analyzed to identify the role of lower limb joints togenerate the mechanical power for lifting.4 The notion isto recover the potential energy dissipated as a result of
lowering the center of gravity of human body which
henceforth referred to as passive energy. Here, the hipand knee joints dissipate energy through eccentric
contraction of the main muscle groups on lower
extremity during the decent phase (2 seconds period).4
Similarly, during the ascending phase (2 seconds period),energy is generated while undergoing concentric
contraction.4 In contrast, during initial stages (0-15%
cycle) of the decent phase, the knee flexor muscles act
concentrically followed by extensor muscles acting
eccentrically for the remainder of the descent (15-53%
cycle).4 The positive power is assumed to unlock the knee
to initialize the descent. Throughout start of theascending phase (53-85% cycle) power is generated byknee extensor muscles by acting concentrically.4
However, towards end of the phase (85-100%) knee
flexor muscles started to dissipate energy gathered using
knee flexors acting eccentrically.4 The negative power
prevent undesirable level of hyperextension of knee joint.

In this context, a passively powered knee exoskeleton(PPKE) is developed to provide power assistance duringsquat lifting of objects from the ground (see Fig. 1(A)).
The bio-inspired powering system is designed to recycle
the waste biomechanical energy at the knee joint within
the squatting cycle to help lift the wearer’s own bodymass. The knee exoskeleton is especially designed topose no restrictions while walking and passively
controlled locking mechanism only engages during a
lifting task to capture and release biomechanical energy.
The design considerations for the knee exoskeleton
targeting squat lifting and walking are described in
section 2. The design details of the exoskeleton and theworking principle of passive powering mechanism ispresented in section 3. The device was evaluated bymeasuring muscle performance of knee joint using
surface muscle signals while performing semi-squats
under different test conditions. The statistical data fromexperiments are analyzed in section 4.
2. Design Considerations
Load lifting is a typical industrial activity that is energy
intensive.4 It involves moving an object from one position
to another while traversing in vertical and/or horizontal
directions. Considering the requirements for squatting aswell as for walking, design considerations in relation to
the knee joint were identified under functional andbiomechanical points of view.

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Fig. 2. CAD of PPKE on human model (A), passive-powering mechanism of PPKE (B), and exploded-view of PPKE (C).

and sagittal planes (see Fig. 2(A)). In order to avoid
collision of thigh rest and shank rest at full squat position,
shank connectors were designed with longer legs (see Fig.
2(B)). Thigh connectors were shorter in length tominimize interference at the medial side. The structure isheld firmly on the limb segments using four Velcro straps
placed through the linear slots on the connectors. Resting
links can be easily replaced to accommodate varying
limb cross-sections, if needed. The curved slots at thehinged end of the rest links provide the ability to adjust
relative angular position of rests with the connectors. The
slender acrylic structure assume the body shape over thefrontal plane if contact forces are significantly high. The
compliant structure hence reduce the possibility ofinternal joint injuries due to misalignments.

3. Design of Passively Powered Knee Exoskeleton
Design objectives were to develop a simple, lightweight
and affordable knee exoskeleton with good ergonomic
conformance and offer power assistance for squat lifting
while posing the minimal resistance for walking. Design
specifications were derived for 50th per. male population.
3.1. Material Selection
Acrylic glass, a thermoplastic polymer, is considered as
the material of choice for developing the PPKE. Acrylic
glass sheets can be easily cut in to various contours using
computer numerically controlled laser cutting machines
and formed in to complex shapes using thermoforming
methods at an affordable cost. Acrylic is also considered
to be non-toxic to the human skin and the environment
friendly. Thus making it a suitable option that has notbeen previously explored in exoskeleton development. In
order to counter the lower tensile strength of acrylic
material (approx. 70 MPa), the design of the exoskeleton
should be satisfactory. The flow of forces through theacrylic structure should be entirely compressive in nature.
Bending or shear forces have been eliminated byeffectively utilizing the symmetricity of the design.

3.3. Passive Powering Mechanism
It is assumed that industrial workers are more likely toassume a semi-squat position during lifting tasks. Hence,the design has been planned for parallel-squat operation,
where the knee bends approx. 700 to 1000. In addition, thewearer need to have freedom to ambulate at normal
walking speed6 (1.3 m/s). Consequently, the knee flexion
should be left unconstrained up to 600. The springs
should engage to capture the negative energy when theknee flexes beyond this limit. Assuming the maximum
knee angle happens up to 900 in a regular squat liftingtask, the effective angular displacement of 300 can beutilized to energize a collection of linear elastic elements.

3.2. Structural System
A computer-aided design model of the human was
prepared using scanned data. The sizing and shaping of
the structural system was then performed over the frontal

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to a stop as the lock pin 1 traverses and reaches themechanical limit over the curved slot.8 In order to allow
unconstrained flexion during walking (1.3 m/s), lock pin
1 should be inserted in position number-four (600). As aresult, during rest of the decent phase up to 900, available
passive energy is harnessed by the powering mechanism.
The spring selection is done to maximize the energy
retention. However, considering the limited tensile
strength of acrylic glass and the overall size of the device,
the maximum force applied on a single spring is limited
to 150 N. The limits were determined by analyzing thestress distribution over the critically loaded acrylic
components of the exoskeleton. In particular, the shankconnectors (see Fig. 4(A)) and pulley disks (see Fig.
4(B)) were subjected to compression loadings andtorsional loadings respectively. The finite element (FE)simulations provided an insight on the safety limits forthe same. To accommodate manufacturing and material
defects, the factor of safety (FOS) were maintained at 1.5
and above for all acrylic parts. Consequently, thestructure and powering mechanisms are capable ofwithstanding heavy loads as a consequence ofsymmetrical positioning of passive elastic actuators.
The energetic response of the proposed passive
energy recycling system can be mathematically modeled
as follows. If a bilateral system with elastic helicalsprings having 5875 N/m stiffness coefficient is used, thetotal strain energy stored in the system over a 300 knee
angle can be determined by (1),Fig. 3. Prototype of PPKE fabricated for the right limb (A) andclose-up view of passive powering mechanism (B).

The proposed system8 to achieve this function iscomprised of energy capturing/storing springs held
parallel to shank, wire cables, pulley disks, smaller return
springs and pins (see Fig. 2(C)). The fabricated prototypeof PPKE also shows the key elements of the system (SeeFig. 3(A)) with identical mechanisms bilaterally placed.
The working principal of the system8 is as follows.
The pulley disks are fitted on bearings and are free torotate about the axis of the shaft. However, by form themechanical limits are in place to prevent rotation of thepulley in opposite or counter-clockwise direction. The
clockwise rotation will be prevented by the smaller return
springs that is held between the retainer disk and thepulley disk using set of pins. The lock pin 1 can beinserted in to a pulley disk locking hole that are spaced
200 apart over a range of 00 to 1200. If the lock pin isremoved, the knee joint can be easily flexed without
notable restriction from the exoskeleton. Here the stiffer
springs act as a rigid body causing the flexible return
springs to extend. As a result the pulley disk will rotate
in a clockwise sense to allow the free length of the wire
cable unchanged. When the knee traverses back to therest position, the return spring will pull the pulley disk to
its initial position. By placing the lock pin 1 in position
number-one (00) the pulley disk gets immediately lockedwith the retainer disk (see Fig. 3(B)). Consequently,
when the knee gets flexed the stiffer springs starts to load
gradually capturing energy of decent. If the lock pin is
inserted in to position number-two (200), the knee is free
to flex up to the same limit and the pulley disk will come

E=

1
2

kx

2

(1)

Where, k – stiffness coefficient (N/m), x – spring
extension (m) and E – accumulated energy (J or Nm).

Fig. 4. Results of FE simulations: factory of safety (FOS) plot
for lateral shank connector (A), and pulley disk (B).

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4.

Evaluation of PPKE

Experiments were performed to determine the possible
benefits of using the passive energy recycling system toenhance strength and improve endurance limits of thighmuscles. According to the biomechanical studies thequadriceps muscle group, in particular the rectus femoris(RF) muscle, perform higher level of work to lift the bodyby undergoing concentric contraction during ascending
phase.4 In addition, the RF is mainly responsible to lower
the body gradually by undergoing eccentric contraction
during descending phase.4 Consequently throughout thesquatting cycle RF muscle activity is notably high.4,7Since, the proposed powering mechanism include
artificial muscles made of elastic springs that work inparallel to knee extensor muscles, the muscular workload
is assumed to be shared between them. Thus,
measurement of RF muscle activity based on surface
electromyography (sEMG) signals will provide a realistic
insight on the effectiveness of the proposed exoskeleton
to relieve muscle effort. Similarly, bicep femoris (BF)
muscle of hamstring muscle group is mainly responsible
for unlocking the knee to initiate decent phase and tocontrol the momentum of body during the ascend phasein a squatting cycle.4 Although the relative workload of
BF muscle is comparatively lower than RF muscle, effectof the system on the knee flexor muscles can beappreciated by analyzing sEMG signals of BF muscle.
Thus, sEMG signals of RF and BF muscles were used as
two measures to objectively validate PPKE performance.

Fig. 5. Prototype of PPKE fitted on male subject with sEMG
electrodes placed over muscle bellies of rectus femoris (RF) andbicep femoris (BF) muscles: front (A), rear (B), and side (C).

Fig. 6. Experimental set-up (A) and protocol (B) used forevaluation of PPKE performance.

The total energy dissipated and generated at the knee
joint during decent and ascent phases frombiomechanical studies were found to be 45 J and 50 J
respectively. The selected springs thus have the capacity
to collectively (eight springs on two PPKE units) capture
and return approximately over 20% of total mechanicalenergy required considering loses due friction. Here, themaximum spring force is evaluated to be approx. 133 N.
The overall mass of PPKE is approximately 900 g perunit. The acrylic sheets used for fabricating the prototype
is 6 mm in thickness, the shafts and pins were made ofaluminium alloys, and bearings, wire cables, springs andconnectors were made of steel. In order to minimize theabrasion at the human-machine interface and weakeningof contact during the entire range of motion a padded
cushion made of polyethylene foam was affixed on thecontact surfaces. Velcro strap located adjacent to theknee on the shank was mainly responsible formaintaining alignment of the axes of rotations of
biological and exoskeleton knee joints.

4.1. Experimental Set-Up
Wired sEMG electrodes connected to DELSYS Bagnoli
EMG system were placed on muscle bellies of RF (seeFig. 5(A)) and BF (see Fig. 5(B)) muscles to measureelectrical signals. The raw signals received by the Input
Module (see Fig. 5(C)) were amplified using the Main
EMG Amplifier and saved in the personal computer (SeeFig. 6(A)). Here, sampling frequency was set at 2000 Hz.
The raw data is then post-processed using EMGworks
software by DELSYS Inc. The root mean square (RMS)
average of sEMG signals is calculated by (2),
RMS of sEMG 

N 2
 xn Nn 1(2)

where, RMS, xn and N are RMS of channel n (n=1,2..),
raw sEMG of channel n and sample size, respectively.

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Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 1 (June 2018) 45–51___________________________________________________________________________________________________________

x 10-3 (A) Muscle activity of Rectus Femoris (RF) during Squat.
0.002
without exo
exo with assist
0.001

RMS ofsEMG (V)

4.2. Experimental Protocol
The test protocols were developed to measure relativechanges of muscle activity for both squatting as well as
for squat lifting. List of tests conducted are as follows,

RMS ofsEMG (V)004812 (BF) 16
20
Femorisduring Squat.
x 10-3 (B) Muscle activity of Bicep
Time (s)0.002
without exo
exo with assist
0.001





0
0

4

8

Time (s)

12

16

20



Fig 7. Muscle activity of rectus femoris (A) and bicep femoris(B) of right limb for a single subject during first 5 squat cycles.

RMS of sEMG (V)x 10-4
15(A)

Each of the tests were conducted for a period of 100
seconds with an ascent-descent cycle of four seconds that
ranged over 00 to 1000 knee angle (See Fig. 6(B)). A
metronome was followed to ensure consistent rate ofsquatting using visual feedback.

Muscle Activity of Right - Rectus Femoris (RF)during Squatting
without exo

12

exo with assist

9

In order to determine sample size for the statisticalanalysis, a pilot study on squatting was performed with asingle test subject. Here, RMS of sEMG signals over the
first five squatting cycles (0 – 20 seconds) were analyzed
for both RF (See Fig. 7(A)) and BF (See Fig. 7(B))
muscles. The means and standard deviations of the
maximum amplitudes for with and without exoskeleton
were determined and correlation coefficients between thepaired data were calculated. According to standardpractices, power-analysis for a paired-means test was
performed using STATA, a statistical software package
by StataCorp. Analysis outputted a sample size of 10 tomeet a statically power of 80% and p-value of 0.05.
Subsequently, an experimental study was conducted with
10 able-bodied healthy male subjects to objectively
validate the effectiveness of the PPKE performance
foreseen in pilot study. The biometric data of theparticipants are as follows: age: 27±4 years, weight: 60±9kg, and height: 170±5.5 cm.

630

RMS of sEMG (V)0
x 10-415(B)

1
2 Left - Rectus3Femoris
MuscleActivity of(RF)4Time (s)during Squatting
without exo

12

exo with assist

9
6
3
0

RMS of sEMG (V)0
x 10-4
15(C)

1 Activity of2Right - Rectus3Femoris
Muscle(RF)4Time (s)while Squat Lifting 10% Bodyweight (BW)without exo

12

exo with assist

9
6
3
0

RMS of sEMG (V)0
x 10-415
12(D)

1 Activity of2 Left - Rectus 3Femoris
Muscle(RF)4Time (s)while Squat Lifting 10% Bodyweight (BW)

4.3. Results and Discussion
The RMS of sEMG signals of each subject was
averaged over five squatting cycles, namely 1-5, 6-10,
11-15, 16-20 and 21-25 cycles. Since the amplitudes of
RMS of sEMG signals tend to increase with increasing
muscle fatigue, the five-cycle averaging approachprovided broader view of muscle activity with statisticalsignificance (p<0.05). In general, both RF and BF muscle
activity were reduced when the power assistance of theexoskeleton is provided to the wearer. However, analysisof statistical data of RF muscles is of greater importance.

without exo
exo with assist

9
6
3
0
0

1

2

squatting without exoskeleton
squatting with power assistance of exoskeleton
squat lifting 10% of bodyweight (BW) without
exoskeleton
squat lifting 10% of bodyweight (BW) with power
assistance of exoskeleton

3 Time (s) 4Fig. 8. Mean muscle activity of rectus femoris of right (A & C)
and left (B & D) limbs for a single subject averaged over first 5
squat cycles. Band represents confidence interval of std. dev.

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Max. RMS of sEMG (V)x 10-415
12

The percent reduction of Max. RMS of sEMG signals
were determined for each of the averaged five-cycle
periods for both squatting (See Fig. 10(A)) and squat
lifting 10% of BW (See Fig. 10(B)). All percent
reductions were evaluated to be statistically significant(p<0.05). According to ‘Cohen’s d’ the effect size ingeneral was also evaluated to be ‘large’ (d>0.8). Hence,
PPKE has effectively reduced peak muscle activity 3040% during the first five squatting cycles. Here, limb
preference for load bearing (right) was identified to be
the governing reason for asymmetry of the results.

Muscle Activity during Squating and Squat Lifting
Right - Rectus Femoris
Left - Rectus Femoris

96
30
without exo exo with asist without exo exo with asist
Squat lifting 10% BW
SquattingFig. 9. Mean and standard error of mean of Max. RMS ofsEMG of rectus femoris (RF) of right and left limbs over the
first 5 squat cycles of 10 subjects.

% Red. of Max. RMS sEMG

50

d=1.5

30

d=1.1

d=1.3

d=1.4

d=0.7

d=0.8

d=1.3

20
10

5.

d=2.1

40

Right-Rectus Femoris

d=0.9

0

50
40

d=1.1

30
20
10

Acknowledgements(B) Squat Lifting 10% BW: Exo with asist vs. without exo

Authors acknowledge Senate Research Council of
University of Moratuwa (Grant No. SRC/CAP/2014/08).

d=0.8

d=1.1
d=0.9
d=1.0

d=0.9

References

d=0.8

d=0.9

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3. S. H. Collins et al., “Reducing the energy cost of humanwalking using an unpowered exoskeleton,” Nature, vol.
522, no. 7555, pp. 212–215, 2015.
4. D. G. E. Robertson et al., “Lower extremity muscle
functions during full squats,” J. Appl. Biomech., vol. 24,
no. 4, pp. 333–339, 2008.
5. S. Hwang et al., “Lower extremity joint kinetics andlumbar curvature during squat and stoop lifting,” BMC
Musculoskelet. Disord., vol. 10, no. 1, p. 15, 2009.
6. B. R. Umberger and P. E. Martin, “Mechanical power andefficiency of level walking with different stride rates,” J.
Exp. Biol., vol. 210, no. 18, pp. 3255–3265, 2007.
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during kneeling and squatting,” J. Appl. Biomech., vol. 27,
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8. Pubudu
Ranaweera,
PassivelyPoweredKnee
Exoskeleton-ICAROB 2018. 2018 [Online]. Available:
https://www.youtube.com/watch?v=_ZC1cgQo8H4.
[Accessed: 24- Apr- 2018]

d=0.6

Right-Rectus Femoris

d=0.7

Left-Rectus Femoris

0
1-5
cycles

6 - 10cycles

11 - 15cycles16 - 20
cyclesConclusion

PPKE is energetically autonomous, adaptable and nimble
thus making it suitable for the industrial use. The
prototype served as a proof of concept and effectively
reduced muscle activity of rectus femoris muscles.
Statistically, the effect size was determined as ‘large’. As
future work, clinical testing with metabolic costestimation is considered to further improve the design.

d=1.6

Left-Rectus Femoris60% Red. of Max. RMS sEMG

Squatting : exo with asist vs. without exo(A)

21 - 25
cyclesFig. 10. Percent reduction of Max. RMS of sEMG of rectus
femoris of right and left limbs over the first 5 squat cycles for
10 subjects during squatting (A) and squat lifting 10% BW (B).

The mean muscle activity of RF of right and left limbs
were computed for each subject for both squatting (SeeFig. 8(A & B)) and squat lifting (See Fig. 8(C & D)).
Here, a notable reduction of muscle activity is clearlyvisible when the power assistance of the exoskeleton isprovided. The mean Max. RMS of sEMG signals of the
10 subjects provide a statistical confirmation on thepositive effect of power assistance of the exoskeleton forsquatting as well as for squat lifting (See Fig. 9).
However, the standard error of mean (SEM) bars clearly
indicate that the mean RF muscle activity between limbs
is statistically insignificant (case of overlapping bars).

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