(2020) 7:10
Shiraishi et al. Robomech J
https://doi.org/10.1186/s40648-020-00158-1

RESEARCH ARTICLE

Open Access

Pneumatically‑driven stretching machine
for ankle dorsiflexion: safety concepts
and effectiveness test involving healthy young
subjects
Yuma Shiraishi1†, Shogo Okamoto1*† , Naomi Yamada1,2†, Koki Inoue1†, Yasuhiro Akiyama1 and Yoji Yamada1

Abstract
Stretching is a major treatment for restoring the ankle range of motion while decreasing the ankle stiffness. An automatic stretching machine can provide long-term continual stretching at home. However, no commercial stretchingmachines are currently available, and therefore, no safety guidelines exist for them. In this study, we prototyped astretching machine using a chassis from a commercial foot exerciser for preventing disuse syndrome, and then proposed three safety concepts as general requirements for stretching machines. The concepts were determined basedon risk assessments, which included (1) preventing excessive force from being applied to the foot, (2) responding to
the ankle stiffness and the range of motion of each different individual, and (3) preventing unintended operations by
the user. The above concepts allowed us to develop a stretching machine taking into account safety measures. Further, the stretching effect of the prototype was tested on sixteen healthy young subjects (22.3 ± 3.6 years). We foundthat the passive resistance of the subjects at specific dorsiflexion angles decreased significantly after stretching theankle using the prototyped machine.
Keywords: Ankle contractures, Stretching machine, Safety concepts
Introduction
Ankle contracture is a common after effect of diseases
and injuries such as ankle fracture, stroke, or cerebral
palsy. It is caused by the stiffened soft tissues comprisingmuscles and tendons around the joint, which reduces theankle range of motion (ROM). Stretching is performed asphysical therapy to improve the ROM and decrease theankle stiffness. Long-term stretching is essential for healing ankle contracture because it may continue to develop
even after discharge from the hospital, especially forstroke patients [1]. However, it is difficult for patients toreceive continual stretching by physical therapists after*Correspondence: okamoto‑shogo@mech.nagoya‑u.ac.jp†
Yuma Shiraishi and Shogo Okamoto contributed equally to this work†
Naomi Yamada and Koki Inoue contributed equally to this work
1
Graduate School of Engineering, Nagoya University, Nagoya, Japan
Full list of author information is available at the end of the article

discharge from a hospital. Also, there are restrictions onrehabilitation opportunities because of medical insurance systems. Several devices for non-therapeutic purposes are currently available for in-home stretching. For
example, a tilt board is used for stretching ankle plantar
flexors. Though users are required to just stand on thetilt board, it is difficult for some elderly people to use
because of balance-stability requirements. Automatedstretching machines can help to solve such difficulties.
There have been few reports on ankle stretchingmachines. Zhang et al., Zhou et al. and Toda et al. developed machines for dorsiflexing the foot for stretching
using an electro-magnetic motor or linear actuator [2–7].
Yamada et al. developed a machine for three-dimensionally controlling the foot using wire-driven mechanisms
to stretch the muscles related to equinovarus [8, 9].
This machine moves the foot by pulling two wires with© The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing,
adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) andthe source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material
in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material
is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds
the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Shiraishi et al. Robomech J(2020) 7:10

McKibben type pneumatic actuators, which are independently attached to the inside and outside of the foot plate.
Other ankle rehabilitation machines can be classified
into two groups; relying on whether they utilize voluntary ankle movement or not.
First, the machines that do not involve user’s voluntary motions typically move the foot within the ankle
ROM to prevent ankle contractures, disuse syndromeor thrombus. Saga et al. [10] and Cordo et al. [11] developed machines for ankle dorsiflexion or/and plantar flexion using a pneumatic actuator or an electro-magnetic
motor. Others aimed to increase the blood flow of thelower limb [12–14]. For example, Sasanuma et al. [13]developed a sock-type ankle exercise machine driven
by pneumatics that moves the ankle with the tip of thefoot in the dorsi/plantar flexion direction. Note thatthese abovementioned studies were not designed to treat
contractures.
Second, machines involving voluntary movement
apply resistive or assistive force during the voluntaryfoot movement to improve muscle strength or/and
motor function. Burdea et al. developed a Stewart-platform type ankle exercise machine driven by double acting type pneumatic actuators [15–18]. The patient fixes
the foot to this machine and voluntarily moves the footin the dorsi/plantar flexion direction as well as the in/eversion direction. Another ankle rehabilitation robot
covered various exercises including voluntary movement
training [19]. Moreover, training machines were designed
to strengthen the tibialis anterior [20] or to encourage
patients’ active participation [21].
Only a few studies clearly mentioned safety measuresfor preventing pain in the foot or the ankle. Toda et al. [6]and Sasanuma et al. [13] implemented fuse-like mechanisms for preventing excessive force from being applied
to the foot. For these machines, the force transmission
mechanisms encompass mechanical elements that breakat certain force thresholds. In another study, passive
degree of freedom was introduced to prevent misalignment of the ankle rotation axis and the machine axis [12].
However, these measures were not against hazards identified in risk assessment.
Despite the above studies, as far as we have studied,commercial stretching machines designed following
safety guidelines are yet to be developed and no safety
standards for ankle stretching machines exist. Additionally, no previous studies on ankle stretching machines
have proposed any safety concepts or measures for suchmachines. However, the key for developing practicalstretching machines is to reduce the risks of foot pain
owing to the application of an excessive load to the foot,as well as being able to use the machine safely. In general,
the safety is defined as a state with no unacceptable risks,

Page 2 of 10

which is realized by repetitive risk assessment and risk
reduction procedures [22].
In this study, we propose safety concepts tailored toankle stretching machines based on the risk assessmentof a commercial foot exercise machine. Then, we redesign the machine as a stretching machine for ankle dorsiflexion following proposed safety concepts. Further,
this paper reports the safety design of the prototype
machine, and the effectiveness of the prototype stretching machine is experimentally tested on sixteen healthy
young subjects. The experimental verification is essential to confirm that the designed prototype achieves thestretching effect. The experiments were conducted withthe approval of the Institutional Review Board of the
School of Engineering, Nagoya University (#18-2).

Redesigning of the stretching machine
Overview of commercially available foot exerciser

The stretching machine prototype is based on a commercial foot exerciser for the plantar and dorsal flexion of theankle (Relegs, LAP Co., Ltd., Japan). Relegs machine does
not have a stretching function and is used to prevent disuse syndrome, by repetitively moving a foot that cannot
be voluntarily because of a stroke or injury.
Relegs is driven by a pneumatic actuator (bellows). As
shown in Fig. 1, the bellows inflates in the longitudinal
direction when pressurized. This deformation of the bellows rotates the machine’s moving part around the rotation axis near the ankle, which, in turn, dorsiflexes thefoot. Similarly, the bellows contracts when the internalpressure of the bellows decreases, which moves the footin the plantar flexion direction.
Some foot joint exercise systems are driven by an electro-magnetic motor that enables highly responsive, precise control of a machine. However, an electro-magnetic
motor brings the risk that the foot can get stuck in theemergency stop mode owing to the friction generated
in the reduction gear. Of course, this drawback can beovercome using various mechanisms. In the case of apneumatic actuator, load on the foot is eliminated by theFig. 1 Operational principles. Left: Initial stage. Right: Pressurization
causes bellows to extend longitudinally, leading the machine’s
moving part to rotate around the rotation center (Adapted from [23])Shiraishi et al. Robomech J(2020) 7:10

decrease in the internal pressure after the air in the bellows has been exhausted. In other words, the machine isinherently safe, as the abovementioned risk can be readily
reduced by designing the system such that it automatically exhausts the air during emergency stops.
Prototype stretching machine

While Relegs machine can move the foot within the ankle
ROM, it does not have a stretching function. Therefore,
the Relegs system was redesigned as follows.
Primarily, a stretching function was implemented. Following manual stretching by a physical therapist, the footis dorsiflexed till the end of the ankle ROM. Subsequently,
the therapist applies an additional load on the foot andmaintains the foot in this posture [24]. To simulate thisprocess, the machine was controlled such that it dorsiflexed the foot slowly and subsequently maintained thisdorsiflexed posture. In the case where the ankle ROM of
the user is restricted owing to diseases or other reasons,a mechanical limiter is used to mechanically restrict theangle of dorsiflexion. The aluminum legs were installedat the front bottom of the machine, as shown in Fig. 2,
to change the entire machine’s angle and keep the user’sknee extended.
Stretching requires a pushing force in the dorsiflexion direction to extend the plantar flexor muscles effectively. The output of the Relegs system is not enough forthis purpose. Thus, an air compressor (CP-12Si, Minato
Electrical Co., Ltd., Japan), which can output a pressure
greater than 100 kPa, was used for the driving system.
The pneumatic force applied to the bellows was continuously controlled using an electro-pneumatic regulator(ITV1030-312S, SMC Co., Ltd., Japan), which itself wascontrolled by a microcomputer (mbed LPC1768, ARM
Holdings PLC., UK).

Fig. 2 Prototype of stretching machine for ankle dorsiflexion. The
foot is dorsiflexed slowly and maintained in dorsiflexed posture(Adapted from [23])Page 3 of 10

The user operates the stretching machine using athree-position enabling switch (A4EG-BM2B041,
OMRON Co., Ltd., Japan), which is in line with theconcept of enabling devices in the field of machinesafety [25, 26]. The machine is operated continuously
only when the user clearly indicates an intent to use
the machine. The risk of foot pain during the machine’s
operation is reduced through manual operation using
the enabling switch. Thus, patients who would obviously find it difficult to control our machine and cannot
use it are those with paralyzed upper body parts or sensory impairments.

Safety concepts and safety measures for ankle
stretching machines
Risk assessments

An expert on machine safety, a physical therapist, and thedevelopers of the prototype conducted risk assessments
over our prototype machine and took safety measures
to reduce the unacceptable and conditionally acceptablerisks, following the procedures in [22]. This process was
repeated thrice until there were no more unacceptablerisks. Some of the 46 risks are listed in Table 1. The listed
risks were once judged as unacceptable or conditionallyacceptable and are described as follows: (a) An excessive
torque is applied to the ankle, causing foot pain when themachine is working. (b) The foot is moved more than thenatural ROM of the ankle when the machine is working.
(c) The user tries to reach a controller placed away fromthe body and falls, resulting in pain in the fixed ankle.
(d) The foot is fixed in a non-relaxed posture owing tomachine malfunction. (e) The stop button does not work
correctly. (f ) The user’s fingers get pinched between thegaps of the machine when the machine is working. (g)The entire machine slides on the floor and moves during use. In this case, the relative position of the user andthe stretching machine changes, and an excessive load isapplied to the foot or leg.
The risks judged as acceptable during the risk assessment process include abrasion of the skin on the footnear the belt used for fixing the foot. According to ISO
13482 [27], the risk of scratches on the body area where
the user comes in contact with the machine must be
taken into account. Mao et al. studied methods for verifying such risks for a physical assistant robot [28, 29].
Further, studies on stretching have reported that many
adverse events, such as numbness and blistering, could
possibly occur [30]. Cordo et al. [11] developed a footjoint exercise apparatus and tested it on 20 subjects, who
performed 30 min of day training using the machine athome for 6 months (i.e., for more than 2000 h). During
this period, only one event of skin abrasion was reported.

5

2

1

(b) The foot is moved more than
the natural ankle range of
motion when the machine isworking(c) The user tries to reach acontroller placed away from thebody and falls, causing pain in
the fixed ankle(d) The foot is fixed in a nonrelaxed posture owing tomachine malfunction

0.5

LO

Final risk level

Install an
emergency
stop button
to block the
input voltageof the airregulator

Install a function to movethe foot backto a relaxed
postureUse a remotecontrollerthat can be
operated by
hand

0.1

0.1

0.1

Use a mechani- 0.1
cal limiterto restrict
the range of
motion of
the machine’s
moving part

Mechanically
restrict the
maximum
pneumatic
force (ISO13482)

Solution

5

5

5

5

5

FE

0.5

0.5

0.5

0.5

0.5

DPH

0.25

0.25

0.25

0.25

1.25

HR

d
Hazard rate (HR), which is given by the product of LO, FE, and DPH. If HR is smaller than 1, the hazard is fully acceptable. If HR is smaller than 10, the hazard is judged as conditionally acceptable. Otherwise, the hazard isconsidered unacceptable

Degree of possible harm. 0.1: Scratch/bruise. 0.5: Laceration/mild ill-effect. 1: Break minor bone or minor illness (temporary). 2: Break major bone or minor illness (permanent). 4: Loss of one limb/major illness(temporary). 8: Loss of two limbs/major illness (permanent). 15: FatalitycFrequency of exposure to the hazard. 0.1: Rarely. 0.2: Annually. 1: Monthly. 1.5: Weekly. 2.5: Daily. 4: Hourly. 5: Constantly

1.25

2.5

5

12.5

12.5

HR

d

Likelihood of occurrence. 0: Impossible. 0.1: Almost impossible. 0.5: Highly unlikely. 1: Unlikely. 2: Possible. 5: Even chance. 8: Probable. 10: Likely. 15: Certain

0.5

0.5

0.5

0.5

0.5

DPH

c

b

5

5

5

5

5

FE

b

a

(e) The stop button does not work 0.5
correctly

5

LO

a

Initial risk level(a) Excessive torque is applied
to the ankle, causing foot pain
when the machine is working

Hazards

Table 1 Partial results of risk assessment

Shiraishi et al. Robomech J(2020) 7:10
Page 4 of 10

Shiraishi et al. Robomech J(2020) 7:10

In our study, as mentioned later, no participants experience skin abrasion after using the machine for more than
50 h.
Safety concepts for stretching machines

Most of the abovementioned unacceptable risks should
be handled in accordance with existing safety standards.
For example, gaps in the mechanically moving parts
should be treated by following ISO 13854 [31]. Here,
we summarize the risks and countermeasures related
to ankle stretching machines. We propose the following
three safety concepts based on the risks identified during
the risk assessment process. (1) Prevent excessive loads
from being applied to the ankle. (2) Deal with the individual differences in the ankle ROM and stiffness of theusers. (3) Prevent involuntary operations of the machine.
By implementing suitable safety measures based on theseconcepts, the risk of foot pain is reduced. Because there
are no existing safety standards for stretching machines,
the risk reduction features for these machines must be
based on referable safety measures.
Regarding Concept 1, as per the standard for personalcare robots, ISO 13482 [27], the safety is stated regarding the force control when a human and the robot arein physical contact. Stretching machines, which are in
constant contact with a human, require that the application of excessive force should be prevented in order toreduce the risk of foot pain. In other words, an automatic
stretching machine must be able to control or restrict theforce applied on the user.
Regarding Concept 2, an automatic stretching machinemust respond to the individual differences between users
through software, hardware, or both. This is because theconditions under which foot pain may be induced would
depend on the symptoms of the individual patients. The
machine should allow to tailor the stretching function foreach user.
In the case of Concept 3, we refer to the standard forindustrial cooperative robots, ISO/TS 15066 [26]. This
standard determines the requirements for man–machine
cooperative operations. For example, the cooperative
operation of a robot is approved when a hand-guided
device is used and the user can operate or stop themachine’s operation in a single action. That is, by pushing or releasing a button. A hand-guided device (i.e., anenabling switch) can be used for stretching machines inorder to prevent unintended operations.
Implementation of safety measures

Based on the risk assessments and the resulting safetyconcepts, the following safety measures were implemented in the stretching machine.

Page 5 of 10

Risks (a and b) when an excessive load is applied to thefoot, causing the foot to move more than natural ROMof the ankle, could result in injuries to the ankle and themuscles. Therefore, these risks must be primarily prevented. To ensure this, the pneumatic force, and the
ROM of the machine’s moving part, were restricted asfollows. The maximum dorsiflexion angle (i.e., the ankleROM) was restricted using a mechanical limiter for preventing excessive dorsiflexion. The maximum pneumaticforce was restricted using two regulators. One of the regulators used was an air regulator (JR-08, Astroproducts,
Japan), and it constrained the output of the compressor.
The other was an electro-pneumatic regulator for electrically constraining the upper limit of the pneumaticforce and controlling the pneumatics during stretching.
The predetermined upper limit for the force applied to
the foot was maintained even under the condition where
either of the regulators malfunctioned.
Though foot pain can be prevented basically by theabovementioned safety measures, it would be essentialthat the setting of the stretching intensity is left to theuser to further reduce the risk of foot pain with regard
to individual differences in the ankle ROM and stiffness. This approach is debatable because it is not in linewith machine safety. However, in the clinic, stretchingintensity is determined based on the pain claimed by
the patient, in part to prevent pain [8]. In other words,
the condition is similar to that of manual stretching. In
our study, we implemented the method as mentioned in"Tasks" section.
To reduce risk c of any foot pain caused by the user’s
involuntary operation of the machine, a three-position
enabling switch commonly used in the industry was
employed. When experiencing foot pain, the user must
be able to immediately stop the operation of the machineby him/herself. To ensure this, the user was given thecontrol of the machine via the above-mentioned switch.
When the switch is not gripped (Position 1), the machineis in OFF state, that is, the non-operating state. When theswitch is gripped (Position 2), the machine is in ON state,that is, the operating state. When the switch is gripped
more strongly (Position 3), the machine again goes into
OFF state, and this state does not change until the switch
is completely released (Position 1). To maintain ON state,a continuous gripping force is necessary.
When the user releases the switch or grips it strongly,
the air regulator is released to automatically reduce thepneumatic force applied to the bellows, and then thefoot is moved back to a relaxed posture. Therefore, either
stretching ceases or the posture is no longer one where
the user experiences foot pain. This is a safety measure
for risk d. An emergency stop button was also installed
for the cases where the switch does not work correctly;

Shiraishi et al. Robomech J(2020) 7:10

this is a safety measure for risk e. When this button ispressed, the input voltage to the electro-pneumatic regulator is cut off and the internal pressure of the bellows
decreases.
For risk f, wherein the fingers are pinched between
the gaps of the machine, the relevant parts were covered
as per ISO 13854 [31]. For risk g, wherein the machineslides during operation, a rubber material was placed on
the bottom of the machine as well as on the tips of its
legs. Thus, friction with the floor was improved, and theslippage of the machine was reduced.

Experimental verification
Objectives

Experiments were performed on healthy young participants to verify the stretching effect of the developed
stretching machine with the safety measures implemented. In general, stretching is suitable even for healthypeople [32, 33].
Several studies showed reductions in the maximum
voluntary contraction force of the plantar flexors (MVC)[34, 35] or the passive resistance [36–38] after stretching.
We employed these two indices to evaluate the effectiveness of the machine. In this study, we adopted the passive
resistance instead of the ankle ROM, which is often used
as an index, because the results of related studies suggested that they covary [39, 40]. Additionally, Sun et al.
showed that they were correlated with each other forstroke patients [41].
Participants

Sixteen healthy young people (22.3 ± 3.6 years old)participated in the experiments after providing written informed consent. None of the participants had
injuries to their ankles or lower legs. Further, they did

Page 6 of 10

not perform self-stretching before the experiments. It
was confirmed in advance that their calf muscles were
expected to be stretched effectively by the output of themachine and the ROM of the machine moving part.
Tasks

The participants were indicated to sit on a chair with
their knee fully extended for effectively stretching thegastrocnemius, which is the main muscle involvedin ankle plantar flexion. Kennedy and Cresswell [42]reported that the more the knee is extended, the more
the muscle is stretched when the foot is dorsiflexed. The
knee angle was adjusted by changing the position andheight of the chair relative to the stretching machine.
Further, the machine’s rotation axis was aligned with theankle by placing a few plates below the heel.
Before starting the stretching process, the upper limitof the pneumatic force for dorsiflexion (i.e. stretchingintensity) was set in the following manner. The participant operated the machine using the enabling switch.
While the switch was gripped, the foot was dorsiflexed.
The participant then released the switch when feeling thecalf muscles stretched completely compared with his/herexperience of self-stretching. The pneumatic force was
set to the upper limit immediately before the switch was
released. The machine’s operational speed reduced as thefoot was dorsiflexed, with the rate of movement dropping
below 2°/s.
For safety purposes, the machine’s operation was suspended every 5 min, and stretching was intermittently
performed for a total of 25 min. The upper limit of thepneumatic force was set every 5 min when the machine
operation was suspended, as it was assumed that theankle stiffness would change during stretching. Photographs of the machine in use are shown in Fig. 3.

Fig. 3 Scenes of stretching. Left: In the initial stage, the foot is relaxed. Middle: In the middle stage, the foot is being dorsiflexed slowly. Right: In the
final stage, the foot is maintained in the dorsiflexed postureShiraishi et al. Robomech J(2020) 7:10

Page 7 of 10

Measurements of maximum voluntary contraction force
and passive resistance

The MVC and the passive resistance were measured
immediately before and after stretching. Each measurement took approximately 2 min. The measurements
started within 2 min after the whole stretching.
The MVC was measured using a hand-held dynamometer (MT-100, Sakai Medical Co., Ltd., Japan). This device
has the functionality of a pull and push sensor, and thepull sensor function was used to measure the MVC. The
belt attached to the dynamometer was fixed at the peroneal head under the knee and the thenar, and the pulling force was measured by voluntarily pulling the belt in
the plantar flexion direction. During the measurements,
the participant kept their foot at a dorsiflexion angle of
0° with their knee fully extended. Note that the measured MVC was not the moment but the pulling force ofthe belt which depends on the length of the individual’s
lower leg and the size of their foot. The MVC was measured seven times before and after stretching. Muscle
weakness due to fatigue did not affect the measurements
because the mean values of first–third and fifth–seventh measurements had no significant differences (t-test,
pre-stretching: t(94) = 0.99, p = 0.33, post-stretching:t(94) = 0.37, p = 0.71).
The passive resistance was measured using the pushsensor function of the dynamometer and a joint angle
meter (TTM-KJ, Sakai Medical Co., Ltd., Japan). The
evaluator pushed at the thenar and hypothenar of thebottom of the participant’s foot using the dynamometer and moved the foot up to prescribed angles. The
external force required to move the relaxed foot to
dorsiflexion angles of 0°, 5° and 10° in the sitting position was recorded. The participants were instructed torelax their feet, and not to intentionally apply the force
to the dynamometer. If the experimenter judged or theparticipants declared that they were not relaxed, themeasurement was reperformed. The passive resistanceat each angle was measured five times before and after
stretching.
The standard deviations of the MVC and the passive
resistance measurements were up to 15% and 14% of theirmeans, respectively.

Results
Figure 4 shows an example of the torque generated by themachine during stretching. The torque was the maximum
when the pressure reached the set upper limit. Though
the internal pressure of the bellows was controlled to beconstant, the inclination of the machine’s moving part
became larger as stretching progressed (lower graph inFig. 4), and the torque slowly decreased as the volume

Fig. 4 Example of applied torque and angle of machine’s moving
part during 5 min of stretching. Torque was calculated from appliedpressure and angle of machine’s moving partFig. 5 Maximum voluntary contraction (MVC). Means and standard
errors of 16 participantsof the bellows increased. During steady state operation,
the internal pressure of the bellows was kept at the value
determined by each participant and was in the range of
60–140 kPa for all the participants. Accordingly, themaximum torque was 20–35 Nm.
Figure 5 shows the results of the MVC. The mean values normalized with respect to the MVC of each participant during pre-stretching and the standard errors are
given in the figure. The MVC of 11 participants out of
the total of 16 decreased after stretching by an averageof 6.1%. However, the mean values of all the participantsof pre- and post-stretching did not exhibit any significant
differences (t(15) = 1.4 , p = 0.17).

Shiraishi et al. Robomech J(2020) 7:10

Figure 6 shows the results of the passive resistance.
The results of all 16 participants are shown except for
one whose passive resistance at a dorsiflexion angle of
10° could not be measured. The mean values normalizedwith respect to the resistance at a dorsiflexion angle of
0° before stretching and the standard errors are given in
the figure. After stretching, the passive resistance at 0°, 5°,
and 10° decreased by 8.1%, 16.9%, and 15.8%, respectively.
A two-way analysis of variance was performed using thepre- and post-stretching results and the measurement
angles as factors. We found that the passive resistancesignificantly decreased after stretching ( F (1, 84) = 20.8,
p < 0.001) and had no interaction.

Discussion
Several previous studies have demonstrated that the passive resistance decreases after static stretching as theexperimental results of our study confirmed. For example, Odeen and Knutsson [36] showed that the passive
resistance decreased by 32% on average after stretching calf muscles of stroke patients for 30 min. Also, it
decreased by 18–19% [37, 38] and 8.3% [43] after stretching the hamstring muscle for a total duration of 360 s and450 s, respectively.
On the other hand, a few studies reported that thepassive resistance did not significantly decrease after
stretching the hamstring muscle or the plantar flexors
for the total duration of 90 s and 120 s, respectively [44,45], where the total duration of stretching was relatively
short.
A few studies have investigated the effect of static
stretching on the decrease in the muscle strength. Fowles
et al. reported that the MVC decreased by 28% immediately after the stretching of the plantar flexor muscles. Furthermore, Marek et al. reported that the MVC

Fig. 6 Passive resistance. Passive resistance decreased significantly
after stretching ( p < 0.001). Error bars represent standard errors

Page 8 of 10

decreased immediately after static or proprioceptive
neuromuscular facilitative stretching of the thigh muscle. The reason for the decrease in the muscle strength
after static stretching has been discussed from several
viewpoints [34, 35]. However, the actual mechanism
remains unknown. Though it was predicted that the
MVC would decrease in the present study after stretching for 5 min × 5 [46, 47], significant differences were notobserved after the stretching process. Nonetheless, it wasalso reported that MVC does not always decrease after
static stretching of the ankle, even when the joint stiffness reduces [48, 49].
We proposed three safety concepts for designing automatic ankle stretching machines. Based on the results of
the risk assessments of our prototype, three scenarios
were judged as unacceptable risks. These were the application of an excessive stretching force on the user’s footand ankle, any unexpected operation of the machine,
and individual differences in the joint deformation andstiffness. These scenarios can be considered common
ones with respect to the safety requirements of stretching machines in general. Safety measures specific to
machines should be implemented based on existing
safety standards.
In the experiments performed, which involved young,healthy subjects, including the preliminary experiments,
the prototype stretching machine was used for more than
50 h. During this period, there were no instances of skinabrasion or foot pain during stretching. Further, there
was only one incident wherein the skin was temporarily
inflamed through direct contact with the belt fixing thefoot. This was subsequently prevented by using a longsock. Further, while there were two instances when a partof the foot sole became slightly numb, the numbness did
not remain after the experiment was over.
In the future, we have to verify the effect and safety ofthe machine’s stretching through clinical trials involving
actual patients. In addition, the safety concepts established in the present study have to be verified for ankle
stretching machine other than the one we used.

Conclusions
Thus far, no commercial ankle stretching machine exists,and safety concepts of such machines have not been
established. In this study, we prototyped a stretchingmachine for ankle dorsiflexion, and we proposed threesafety concepts as the general requirements for automatic ankle stretching machines. Based on the concepts,
the safety measures were implemented to the prototype machine. Further, the effectiveness was verified for
healthy young participants to confirm that the machine
achieved the stretching effect. The machine was used tostretch the plantar flexors, and it was observed that theShiraishi et al. Robomech J(2020) 7:10

passive resistance significantly decreased after stretching.
The results indicated that the stretching machine was
effective for healthy young people.
Acknowledgements
We acknowledge technical support from LAP Co., Ltd.
Authors’ contributions
YS and SO are of equal contributions including experiments, analysis, andwriting. NY and KI largely contributed to the risk assessment and experimental
design. AY and YY helped organize the study and paper. All authors read andapproved the final manuscript.
Funding
Part of this study was supported by MEXT KAKENHI and Japan Society for the
Promotion of Science (17K13108) and Aichi Medical College.
Availability of data and materials
The readers can require the corresponding author to access the experimental
data by email.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Graduate School of Engineering, Nagoya University, Nagoya, Japan. 2 Aichi
Medical College, Kiyosu, Japan.
Received: 12 October 2019 Accepted: 12 February 2020

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