2019 IEEE 1st Global Conference on Life Sciences and Technologies (LifeTech 2019)

Pneumatic–driven ankle stretching machine
Yuma SHIRAISHI∗ , Shogo OKAMOTO∗ , Naomi YAMADA∗† , Koki INOUE∗ ,Yasuhiro AKIYAMA∗ and Yoji YAMADA∗∗ Graduate School of Engineering, Nagoya University, Nagoya, Japan† Aichi Medical College, Kiyosu, Japan

Abstract—Paralysis after a stroke typically causes equinovarus,
for which ankle stretching is a major treatment to facilitate therecovery of the ankle’s range of motion. To provide patientswith long–term stretching at home after discharge from thehospital, we have been developing a pneumatic–driven stretchingmachine that stretches the ankle plantar flexor muscles that cause
equinovarus. Thus far, commercial products for this purpose
have not been available. In the present study, we conducted
experiments on healthy young people (22.3±3.6 years of age) inorder to test the effectiveness of our prototype. The participants’
ankle stiffness, which is an index used to assess the joint
contracture, was significantly decreased (F (1, 84)=20.8，p<0.001)
by their use of our prototype machine.
Index Terms—equinovarus, stretching machine, test of effectiveness

I. I NTRODUCTION
Paralysis after a stroke typically causes equinovarus, athree–dimensional foot deformity, by increasing muscle hyper
tone of the ankle’s plantar flexor muscles. This deformity
causes a deterioration in the walking ability of the patient, andincreases their risk of falling. Physical therapists treating anequinovarus foot primarily stretch affected muscles to recover
the ankle’s range of motion (ROM). Long–term extended
stretching is essential since the muscle hypertonia continues
after the patient is discharged from a hospital. Therefore, astretching machine for home–use offers particular value topatients post–discharge.
A number of studies on the topic of ankle stretchingmachines have been reported in the literature. Waldman etal. developed a machine for ankle dorsiflexion (DF) using anelectromagnetic motor and demonstrated the effects on stroke
patients’ feet [1]. Yamada et al. prototyped a three-dimensional
stretching machine by adopting a wire–driven mechanism [2],[3].
Toda et al. developed a machine for ankle DF [4]. Hommaand Usuba developed a machine to move a foot in plantar
flexion (PF) and DF direction for improvement of blood
circulation [5]. Their machine also offers a degree of freedomof passive motion, allowing both inversion and eversion soas to facilitate physical therapy for a wider range of footdeformities than just those requiring PF and DF. Despite thesestudies, and the established value of the service that could bePart of this study was supported by a MEXT KAKENHI grant (17K13108)
and Aichi Medical College. We also acknowledge technical support from LAPCo., Ltd.

978-1-7281-0543-7/18/$31.00 ©2019 IEEEFig. 1. Prototype of stretching machine for ankle dorsiflexion (DF).

provided with an ankle stretching machine, no such machines
are currently commercially available.
In the present study, we develop a prototype ankle stretchingmachine for treating equinovarus. Further, we verify the effectof the machine’s stretching on the feet of healthy young
people by investigating ankle stiffness. The experiments wereconducted with the approval of the Institutional Review Boardof the School of Engineering, Nagoya University (#18–2).
II. P ROTOTYPE STRETCHING MACHINE
The prototype stretching machine shown in Fig. 1 was basedon the commercial foot exerciser (Relegs, LAP Co., Ltd.,Japan) that provides repeated PF and DF for the preventionof disuse syndrome. This movement is controlled by applying
pressure to the bellows, which expands in the longitudinal
direction, and then relieving this pressure. The expansion ofthe bellows pushes and rotates the machine’s moving partaround the rotation center located to the outer–facing side ofthe ankle (Fig. 2). However, the Relegs is not intended forstretching equinovarus feet. To do this effectively, it needs tobe remodeled, because it lacks the functionality to facilitate
extended stretching of tense ankle plantar flexor muscles.
Therefore, the present study remodeled the Relegs for use withequinovarus feet.
The protorype was driven and controlled using an aircompressor (CP–12Si, Minato Electrical Co., Ltd., Japan）
and an electro–magnetic regulator (ITV1030–312S, SMC Co.,Ltd., Japan). The regulator controlled pressurization to thebellows and thereby the resultant DF of the foot. Details ofthe machine’s stretching operation are provided in the next
section.

10Fig. 2. Operational principles. Left: Initial stage. Right: Pressurization causes
bellows to extend longitudinally, leading machine’s moving part to rotate
around rotation center.

III. E XPERIMENTS : T EST OF EFFECTIVENESS
Healthy young people (14 men and 2 women, 22.3±3.6
years of age) provided informed consent and participated inthe experiments to test the machine’s effectiveness.
Stretching was performed as follows. The participants sat
on a chair and were subjected to the stretching operation ofthe machine 5 times in a row on a particular foot, with the footrelaxed and with the knee in an extension position. The footwas first dorsiflexed and then maintained in the dorsiflexed
posture, following the manual stretching procedure used byphysical therapists [6]. The stretching duration was 5 minutes(25 minutes in total). The machine’s operational speed slowed
as the foot was dorsiflexed, the rate of movement becoming
smaller than 2◦ /s near the end of the ankle’s ROM. For safety
purposes, the machine was operated using a 3–positioned
enabling switch (A4EG–BM2B041, OMRON Co., Ltd., Japan)
and the operation continued only while the participants gripped
the switch properly.
To respond to individual differences in the ankle’s ROMand stiffness, the upper limit of pressure for ankle DF wasset by the individual participants according to the following
procedures. While the switch was gripped, the foot wasdorsiflexed slowly. When a participant felt their calf muscle
stretching, the switch was released and the pressure at thatpoint was adopted as the upper limit for their specific flexion
range. The upper limit was determined at the beginning of
every 5 minutes period of stretching.
Ankle stiffness before and after stretching was measured
using a hand–held dynamometer (MT–100, Sakai Medical Co.,Ltd., Japan). The dynamometer was pushed against the thenarof participant’s sole on the foot involved in the stretchingexperiments. The relaxed foot was passively moved up to DF
angles of 0, 5, and 10◦ , and the passive resistance force ateach angle was measured. Resistance was measured 5 times
before and after stretching, respectively, and the mean values
were calculated for each individual.
IV. R ESULTS
The result of the ankle stiffness investigation at DF 10◦ isshown in Fig. 3. Passive resistance was normalized with eachparticipant’s value at DF angle 0◦ before stretching. Ankle
stiffness at DF angle 0, 5 and 10◦ decreased by 8.1%, 22.3%
and 26.8%, respectively. A two–way ANOVA with pre– andpost–stretching and the measurement angles (0, 5, and 10◦ ) being factors showed that ankle stiffness significantly decreased

Fig. 3. Ankle stiffness (passive resistance) at dorsiflexion angle 10 degrees.
Ankle stiffness of post–stretching decreased by 26.8%. The error bars represent standard errors.

after stretching (F (1, 84)=21, p<0.001) and interaction was
not observed.
V. D ISCUSSION AND CONCLUSIONS
A commercial pneumatic–driven foot exerciser was remodeled to develop a prototype ankle stretching machine for thetreatment of equinovarus. To verify the effectiveness of theprototype, experiments were conducted on the feet of healthy
young volunteers and ankle stiffness before and after stretching
was investigated. The previous studies about static stretchingreported that ankle stiffness decreased after stretching [7], [8];our research supported this finding, as, ankle stiffness wasdecreased significantly. This result indicates that the stretching
performed by the prototype machine is indeed effective inimproving the ankle’s ROM.
R EFERENCES[1] G. Waldman, C. Y. Yang, Y. Ren, L. Liu, X. Guo, R. L. Harvey, E. J.
Roth, and L. Q. Zhang, “Effects of robot-guided passive stretching andactive movement training of ankle and mobility impairments in stroke,”
NeuroRehabilitation, vol. 32, no. 3, pp. 625–634, 2013.
[2] N. Yamada, S. Okamoto, Y. Akiyama, K. Isogai, and Y. Yamada, “Anklestretching rehabilitation machine for equinovarus: Design and evaluationfrom clinical aspects,” Proceedings of IEEE International Conference onSystems, Man, and Cybernetics, pp. 1687–1692, 2017.
[3] T. Kimura, S. Okamoto, N. Yamada, Y. Akiyama, K. Isogai, and Y. Yamada, “Ankle stretching rehabilitation machine for equinovarus: Automation of eversion and flexion control,” Proceedings of IEEE International
Conference on Systems, Man, and Cybernetics, pp. 2696–2700, 2017.
[4] H. Toda, T. Matsumoto, R. Tanizaki, and T. Imaeda, “Ankle joint pushingmechanism by stabilization of ankle position using a brace structure,”
Journal of Advanced Mechanical Design, Systems, and Manufacturing,
vol. 10, no. 1, p. JAMDSM0013, 2016.
[5] K. Homma and M. Usuba, “Effects of passive motion using mechatronic system of improvement of peripheral circulation,” Proceedings ofIEEE/SICE International Symposium on System Integration, pp. 543–548,
2011.
[6] N. Yamada, S. Okamoto, Y. Akiyama, K. Isogai, K. Kawakami, andY. Yamada, “Commonality and individuality in manual stretching of physical therapists for equinovarus,” Proceedings of IEEE/SICE International
Symposium on System Integration, pp. 495–460, 2014.
[7] T. Mizuno, M. Matsumoto, and Y. Umemura, “Viscoelasticity of themuscle–tendon unnit is returned more rapidly than range of motion afterstretching,” Scandinavian Journal of Medicine and Science in Sports,
vol. 23, pp. 23–30, 2013.
[8] C. I. Morse, O. R. Seynnes, C. N. Maganaris, and D. A. Jones, “The acuteeffect of stretching on the passive stiffness of the human gastrocnemius
muscle tendon unit,” Journal of Physiology, vol. 10, no. 1, pp. 97–106,
2008.

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