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

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Ankle Stretching Rehabilitation Machine forEquinovarus:
Design and Evaluation from Clinical Aspects
Naomi Yamada

Shogo Okamoto

Yasuhiro Akiyama

Kaoru Isogai

Yoji Yamada

Aichi Medical College
Kiyosu, andNagoya University
Nagoya, Japan

Nagoya University
Nagoya, Japan

Nagoya University
Nagoya, Japan

Tokoha University
Hamamatsu, andNagoya University
Nagoya, Japan

Nagoya University
Nagoya, Japan

Abstract—Three-dimensional stretching is needed to treat
equinovarus, which deforms the patient’s foot to plantarflexion,
adduction, and inversion postures. We have prototyped a threedimensional stretching machine that the patient can use forthe treatment of equinovarus by him- or herself. By adoptinga cable-driven mechanism with two independently controllable
pneumatic actuators, the stretching machine can apply a force tothe foot along the dorsiflexion direction as well as the direction
combining abduction and eversion. In this study, we verified
the effectiveness of a prototype stretching machine for healthy
subjects. For evaluation, the muscle stiffness and maximum
voluntary contraction (MVC) of plantarflexion were compared
immediately before and after stretching and 10 min later. As aresult, the MVC decreased after stretching, which is a clinicalindex for effective stretching.

I. I NTRODUCTION
Spastic paralysis after a stroke causes an increase in muscle
tone. As a treatment for hypertonia, stretching is performed
very often. In some previous studies, static stretching offoot improved the joint range and walking speed for stroke
patients [1], [2], [3]. Stretching is major treatment in rehabilitation, and its effectiveness has also been shown for healthy
subjects [4], [5], [6], [7].
The part targeted for stretching for spastic paralysis after astroke is often the foot. This is because the foot deformation
called equinovarus tends to occur easily by increasing themuscle tone of the lower limb. The gastrocnemius, soleus, posterior tibial muscle, flexor pollicis longus muscle, and flexor
digitorum longus muscle become involved in equinovarus [8].
The gastrocnemius and soleus run parallel to axis of lower
leg, whereas the posterior tibial muscle, flexor pollicis longus
muscle, and flexor digitorum longus muscle run around thefoot in a spiral; thus, equinovarus deforms the foot threedimensionally.
Clinical stretching of equinovarus is often carried out bya physical therapist manually. The therapist stretches themuscle by applying the appropriate force while feeling theneurological and biomechanical response of the muscle during
stretching. At the same time, while confirming the reaction of
the subject, the magnitude of the applied force is adjusted
978-1-5386-1644-4/17/$31.00 ©2017 IEEE

to avoid causing pain. Furthermore, most therapists stretch
the stroke patient’s foot in a three-dimensional direction [9]In clinical practice, complicated controlled stretching is performed on equinovarus.
The above-mentioned prior studies [1], [2], [3] regarding
stretching for after-stroke patients showed the results of continuous stretching on a daily basis. It has been reported that morethan 40% of spastic paralyzed patients have spasticity after 3
months or more after the onset of a stroke [10]; therefore,
stretching of the spasticity muscle should continue at homeeven after discharge. In practice, however, it is difficult forphysical therapists to perform stretching closely in places otherthan hospitals. For this reason, a rehabilitation robot for thefoot aimed at the automation of rehabilitation is beginning to
be proposed. Saga et al. [11] proposed a foot rehabilitation
device using a pneumatic actuator that is lightweight andcapable of a large output. Waldman et al. [3], [12] developed
a compact foot stretching device and showed the effect ofstretching on stroke patients. However, these devices had only
one motion axis for dorsiflexed and plantarflexed movement.
Equinovarus caused by a stroke deforms the foot threedimensionally. Stretching for equinovarus is also performed
three-dimensionally by physical therapists; thus, the machinethat stretches the deformed foot needs to have a threedimensional control system. Therefore, we have developed athree-dimensional stretching machine for equinovarus. In thispaper, we propose a mechanism for the stretching machine andits clinical effects. Note that an automatic control method forthis stretching machine is being studied in other research [13].
II. S TRETCHING FOR EQUINOVARUS
A. Three-dimensional motion of a footAs shown in Fig. 1, the upward movement around the axis
connecting the inside and outside of the ankle is dorsiflexion,
and the downward movement around this axis is plantarflexion.
In the motion around the axis perpendicular to the foot sole,
the movement to the outside of the toe is abduction, and the
movement to the inside of the toe is adduction. Movement thatturns the foot sole outward around the anteroposterior axis is1687

Individual control

Simultaneous control

䠄Pull only outside wire)䠄Pull outside and inside wires)

Abduction & Eversion(coupled movement)

Dorsiflexion

Dorsiflexion
Plantarflexion
Inversion
Eversion
Abduction
Adduction
Fig. 1. Three-dimensional foot movement (right foot).

Fig. 3. Mechanism of double-wire control. The left figure shows the individual
control of two wires, which are connected along the inside and outsideof a footplate. When only the outside wire is pulled, the outside of thefoot plate is moved outward and upward; thus, foot abduction and eversion,
which are an anatomically coupled movement, occur. The right figure shows
the simultaneous control of wires. When the inside and outside wires are
simultaneously pulled, the front side of the foot plate is pulled upward; thus,
the foot is dorsiflexed.

Talocrural joint

The axis oftalocrural joint

III. D ESIGN OF THE STRETCHING MACHINE
A. Three-dimensional foot control by two actuated wires

Fig. 2. The axis of the talocrural joint. The line indicates the anatomical axis
mainly related to dorsi-/plantarflexion.

eversion, and the inward motion is inversion. Anatomically, theabduction movement is linked with the eversion movement andadduction is linked with inversion [14]. Dorsi-/plantarflexion
mainly occurs at the talocrural joint, and ab-/adduction ande-/inversion occur at the subtalar joint. The anatomical axis of
the talocrural joint is inclined, heading from the inside to therear and outward (Fig. 2). Since the outer surface of the talus
is slightly larger than the inner surface, as the dorsiflexion
occurs, the talus rotates outward while rotating backward. For
such anatomical features, the plantar surface rotates to theoutside with the dorsiflexion motion; that is, foot abductionand eversion necessarily occur [15].
B. Manual stretching by therapists
When physical therapists perform manual stretching, they
move the target parts along the directions in which the muscles run to induce appropriate articular movement. Therapists
stretch the deformed foot three-dimensionally because the partof the muscle involved in equinovarus is in the spiral, as
described above. Focusing on the articulation of the foot withdorsiflexion movement, the talus should rotate out, and the footshould be abducted and everted; thus, therapists correct the
adduction and inversion deformation in advance for stretching
the equinovarus foot. They perform such complex stretchingon the basis of knowledge and experience. In order to perform
effective foot stretching, it is also necessary to achieve suchcomplicated control in a stretching machine.

For foot control to dorsiflex after correcting inversion andadduction, a complicated mechanism is required in order toindependently control all three directions. Therefore, in the
prototype machine we have developed, we adopted a mechanism that performs three-dimensional control by pulling wires
along both sides of a foot plate individually or simultaneously.
As shown in Fig. 3, when only the outside wire is pulled, theouter side of the foot plate is moved outward and upward, andthe foot is abducted and everted, which are an anatomicallycoupled movement. When both side wires are pulled at thesame time, the upper side of the foot plate is moved upward,
and the foot is dorsiflexed. In this way, it is possible to movethe foot three-dimensionally by controlling the two wires.
Although an electromagnetic motor having a speed reduction mechanism is often used for the motion control of arehabilitation machine, a pneumatic actuator was used for thewire control in the prototype machine. The force required topull the wire is adjusted by changing the air pressure applied tothe pneumatic actuator, which is connected to each of the two
wires. Since the pneumatic actuator can easily exert a larger
force than the motor, it is suitable for a stretching machine
requiring a large output for correcting deformed feet.
B. Safety consideration
When using a geared motor to control the foot, it isnecessary to transmit a force through a rigid part; however,with the use of a pneumatic actuator, it is possible that thedesign reduces the rigidity of the mechanism. Therefore, thisprototype machine can maintain the flexibility of the mechanism and reduce the burden on the foot fixed to the device. For
example, an exoskeleton-type mechanism using motors have

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Lower leg support

Relaxed position

Idler

Stretched position

Wire

Foot plate
Heel holder
Pole brace

Pneumatic actuator

Fig. 4. Structure of the stretching machine.

Foot plate

Fig. 6. Stretching motion. Left: Relaxed position before beginning stretching.
Right: Foot position during stretching with the prototype machine instrumented with a double-wire mechanism.

Heel holder
Fig. 5. Foot plate and heel holder.

its own centers of rotation that are separated from those ofhuman joints, but by using a pneumatic actuator and the twowire driving mechanism, the center of rotation of the humanbody is utilized effectively. Hence, it is possible to reduce theload on the human foot joints by joint misalignment.
When a large unexpected force occurs owing to a malfunction in the machine or the like, the power supply may
be shut off for the purpose of protection or an emergency
stop. At this time, if an electromagnetic motor with reduction
gears is used, a dangerous posture is retained because of
their friction and the risk of injury remains. On the other
hand, if a pneumatic actuator is used, by shutting off thepower supply and removing the air in the actuator, the load
applied to the foot is removed, and a state of safety can beeasily achieved. The prototype machine in this study has an
emergency stop switch for removing the pressurization of thepneumatic actuator.
The amount of deformation of equinovarus depends on the
degree of disability. To adjust for individual differences thatcannot be compensated with the two-wire driving mechanism
alone, a flexible sole plate is used. Moreover, the flexibilityof the wires enables the prototype machine to accommodate
individual differences.
IV. A PPARATUS
The prototype stretching machine consisted of a foot plate,heel holder, lower leg support, and pole brace (Fig. 4). The

subject sat on a chair, and the heel on the target side wasplaced on the heel support. The foot plate was fixed using abelt to the foot of the subject, and the forefoot was supported
from the bottom by the foot plate. Since the forefoot and heelparts were supported individually, the prototype could flexibly
respond to three-dimensional foot deformation (Fig. 5). Twowires were passed one by one on the inside and outside
toward the front of the foot plate. Each end of two wires
was connected to a McKibben-type pneumatic actuator (Air
Muscle, Kanda Tsushin Kogyo Co., Ltd., Japan) via an idler.
The idler was used to adjust the direction in which thewire was pulled for each subject. The lower leg support
part consisted of cloth similar to a hammock to support thelower leg of the subject. This made it possible to stabilize
the lower leg during stretching and to reliably transmit theattraction force of the wire to the foot alone. Two idlers
and two ends of McKibben-type pneumatic actuators were
fixed to the pole brace. Two wires were drawn through theidler by pressurizing the McKibben-type actuator, and the footwas stretched (Fig. 6). Since the heel holder was cup-shaped,
the heel could move slightly back-and-forth and right-and-leftaccording to the foot movement.
An electropneumatic regulator was used for the pressurization of the McKibben-type actuator. In the prototype machine, the two electropneumatic regulators connected to theactuators were each controlled manually, and the pressure
was individually adjusted. In order to perform stretching ina way to dorsiflex after braking the foot in the abductionand eversion direction, the actuator linked to the outer wire
was first pressurized; then, the actuators on both sides werepressurized.

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V. C LINICAL EVALUATION
A. Objective
In order to verify the three-dimensional stretching function
and its clinical effects, we performed an experiment involving
a healthy subject.
It is known that even healthy joints are affected by stretching, and its effects are investigated by examinations comprising joint stiffness and voluntary muscle contraction tests. Previous studies have shown that the ankle stiffness and maximum
voluntary contraction (MVC) decrease immediately after static
stretching [4], [5], [16]. Therefore, we also measured these twoquantities before and after stretching.
B. Method

toe, and the other was fixed to the chair. The subject’s heel
was placed on the heel holder of the stretching machine. The
MVC was measured three times in one evaluation. A statisticalanalysis using a t-test was performed for the MVC.

C. Results
1) Foot control: Fig. 7 shows the results for the pressuresapplied to the two actuators and the angles of the foot postureduring stretching in each of the three experiments. In all three
trials, the foot was abducted, everted, and dorsiflexed while
only the outer wire was pulled. By additionally pulling theinner wire, the dorsiflexion angle further increased. These
results indicated that the prototype machine could perform
stretching to dorsiflex after moving the foot for abduction andeversion.

1) Procedure: A 22-year-old university student who provided informed consent participated. With the subject sitting
on a chair, the right heel was placed on the heel holder,
and the forefoot was fixed to the foot plate. At this time,
the knee joint was flexed 55◦ from the straight extended
While maintaining the maximum dorsiflexion for 1 min, forposition. In order to investigate the foot angle during the all three stretching operations, the dorsiflexion angle slightly
experiment, goniometers (SG110, SG110/A, Biometrics Co., increased regardless of the constant attractive force of theLtd., UK) were affixed to the lower leg and foot, and the actuator. This suggests that the plantar flexor muscles were
dorsi-/plantarflexion, ab-/adduction, and ex-/inversion angles sufficiently stretched, and the viscoelasticity of these muscles
were measured. The anatomical basic limb position, i.e., the had changed.
state where the toe is not twisted and faces the front and the
The abducted foot was slightly turned toward the insidefoot is at a right angle to lower leg, is defined as 0◦ around when the inner wire began to be pulled. In this study, since theall three axes.
pressurization of the two actuators was controlled manually, it
Stretching by the prototype machine was carried out for was only possible to delay the timing at which the pressure was
1 min. By applying pressure manually to the pneumatic applied. In the future, an automatic control system is required
actuators, the foot was abducted, everted, and dorsiflexed in order to adjust the amount and timing of the pressurizationfrom the relaxed position, and it was held in a posture that appropriately according to the change in the foot posture.
reached maximum dorsiflexion. The subject was instructed to2) Ankle stiffness: The results for the ankle stiffness arereport when he felt pain during stretching. The ankle stiffnessshown
in Fig. 8. The solid line shows the relationship beand MVC were measured before stretching, immediately after
tween
the
passive dorsiflexion angle and the applied force
stretching, and 10 min after stretching. One experiment infor
dorsiflexion
before stretching, the dotted line shows thecluding stretching and the measurements of two values were
results
immediately
after stretching, and the broken line shows
repeated three times with 15 min of rest in between.
theresults
10
min
after
the end of stretching. Owing to the2) Measurement of the ankle stiffness: The stiffness was
natureofthenonlinearpropertyof the muscle, a plot of theevaluated by indicating the relationship between the angle andforce
versustheangle
isgenerallya curve. From the results
the external force to dorsiflex a foot passively approaching
beforestretching,
as
the
angle
increased,
an exponentiallya certain angle. For measurement of the external force, alargerexternalforce
was
required.
On
thecontrary,
the applied
handheld dynamometer (MT-100, Sakai Medical Co., Ltd.,
force
linearly
increased
immediately
after
stretching
and after
Japan) was used. The examiner pushed the subject’s foot
◦
◦
10
min.
Further,ascomparedtobeforestretching,a largerfrom the bottom of the foot to a certain angle (0 ，5 ，◦◦◦◦
externalforce
was
required
immediately
after
and
after
10
10 ，15 ，20 ，25 ) through the handheld dynamometer. The
min
to
reach
the
same
angle.
From
these
results,
thestiffnesshandheld dynamometer always contacted with the toe baseduring measurement. The force recorded by the handheld slightly increased by stretching in this experiment, which was
unexpected for us.
dynamometer when reaching each angle was recorded.
3) Measurement of the maximum voluntary contraction:
The external force for passive dorsiflexion was manually
The isometric MVC of the plantar flexors, gastrocnemius, measured by using the handheld dynamometer, however, thisand soleus was measured, which were the target muscles measurement method is generally susceptible to error. In thisfor stretching. For the measurement, a pull sensor (MT-150, study, the examiner was the same for all experiments, but the
Sakai Medical Co., Ltd., Japan) connected to the handheld variance of the measured values became larger as the angle
dynamometer was used. During measurement, one of the belts increased; thus, manual measurement might have led to theof the sensor was placed on the bottom of the base of the inaccuracy of measurement.
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[N]
200

1st stretching
Dorsi (+) /Plantar (-)

Angle [deg]30
20Beforestretching

Abduction (+) /Adduction (-)

10

Eversion (+) /Inversion (-)

0-10Immediately
after stretching100

Air pressure [MPa]-20

10 minafter stretching

Outsideactuator

0.6
0.4

0

Insideactuator

0.2

0

5

10

15

20

25 [deg]

0
0

20

40

60

Fig. 8. Measured ankle stiffness before and after stretching.

80

Time [s][N]300

2nd stretching

Angle [deg]

30
20
10
0
-10

Dorsi (+) /Plantar (-)

250

Abduction (+) /Adduction (-)

200

Eversion (+) /Inversion (-)

150+ : p < 0.1

100

-20

Air pressure [MPa]+50

Outsideactuator

0.6
0.4

0

Insideactuator

0.2

Beforestretching

Immediately after
stretching

10 min after
stretching

0
0

20

40

60

Fig. 9. Maximum voluntary contraction of the plantar flexors.

80

Time [s]3rd stretching
Dorsi (+) /Plantar (-)

Angle [deg]30
20

Abduction (+) /Adduction (-)

10

Eversion (+) /Inversion (-)

0-10

Air pressure [MPa]-20
Outsideactuator

0.6
0.4

Insideactuator

0.2
0
0

20

40

60

80

Time [s]Fig. 7. Foot angles (upper) and air pressure (lower) during operation. Top:the results of the 1st experiment, Middle: the results of the 2nd experiment,
Bottom: the results of the 3rd experiment.

In previous research, stretching is often performed using
a torque sensor, and the relationship between the angle andthe torque measured by the torque sensor is often used toevaluate the stiffness. In the future, we will implement a
method to evaluate the stiffness in a way that is not affectedby examiners.
3) Maximum voluntary contraction : The results for the
MVC are shown in Fig. 9. The MVC just before stretchingwas 230 ± 29 N (mean ± SD); however, immediately after
stretching, it was 211 ± 23 N, and after 10 min, it was 208 ±26 N. That is, the MVC decreased after stretching. A statisticalanalysis using a t-test was performed to compare the MVC
before and immediately after stretching (p = 0.07) and before
and 10 min after stretching (p = 0.11).
It has already been shown that the MVC decreases bysustained stretching [16], and our results indicated the same
findings. This experiment only focused on the plantar flexor
MVC, but the MVC of the posterior tibialis and flexor pollicis/digitorum longus muscles, which are the targets of threedimensional stretching, should be evaluated in the next step.
Since the pull sensor of the handheld dynamometer used in thisexperiment cannot measure the muscle force at the inversion
and adduction foot positions, in order to evaluate the effectsof stretching by three-dimensional control in more detail, it

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is necessary to consider a method capable of measuring the
maximum muscle force during three-dimensional foot motion.
VI. C ONCLUSION
In order to develop an effective stretching machine for athree-dimensionally deformed foot, i.e., equinovarus, a design
for a stretching machine capable of three-dimensional control
was proposed in this study, and its effects were examined froma clinical viewpoint.
The three-dimensional foot control mechanism using two
wires actuated by pneumatic actuators is instrumented in the
prototype. This mechanism makes it possible to stretch theankle with an anatomically efficient method while reducing
the burden on the human joint. Dorsiflexion foot movement
after adduction and inversion was performed during stretchingby this prototype.
Although the stiffness after stretching slightly increased, theMVC decreased. We do not have conclusive results regarding
the stiffness because the ankle stiffness was measured in a
simplified manner. As in previous research, the plantar flexor
MVC decreased after stretching by the developed prototype
machine; thus, the efficacy of this machine was demonstrated.

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ACKNOWLEDGMENT
This study was supported by JSPS KAKENHI Grant Number JP17K13108 and a Grant-in-Aid for Individual Researchby Aichi Medical College.
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