Exoskeleton Simulator of Impaired Ankle:
Simulation of Spasticity and Clonus
Hiroshi Okumura∗ ⋆ , Shogo Okamoto∗ , Shun Ishikawa∗ , Kaoru Isogai∗∗ ,Naomi Yanagihara–Yamada† , Yasuhiro Akiyama∗ , and Yoji Yamada∗∗

Dept. of Mechanical Engineering and Science, Nagoya University, Nagoya, Japan∗∗
Dept. of Physical Therapy, Tokoha University, Hamamatsu, Japan†
Dept. of Physical and Occupational Therapy, Nagoya University, Nagoya, Japan

Abstract. We developed a prototype of an exoskeletal patient simulator
that allows clinical trainees to experience and learn about ankle disorders
related to hemiplegia. The exoskeleton exerts abnormal joint torques
by tendon mechanisms while realizing complex ankle movements andrealistic bone and skin features. Using this exoskeleton, we simulated
the resistances of spasticity and clonus, which are typical symptoms ofhemiplegia. We demonstrated these two types of simulated symptoms
and showed their validity.
Keywords: Patient simulator, Spasticity, Clonus, Physical therapy.

1

Introduction

Physical therapists (PTs) manually examine a diseased joint to understand its
clinical condition based on the dynamic joint resistance. Because unlicensed
PT trainees have few opportunities to treat actual patients, some researchers
have developed patient robots to simulate the symptoms of diseased joints forphysical therapy training. For example, Grow et al. and Park et al. developed
robotic simulators of spastic elbows [3][10]. Kikuchi et al. simulated the spastic
movements of the foot joint using a leg and foot robot [8][7]. In the case of robotic
patient simulators, the realism or human likelihood is typically limited because
humans have multiple degrees of freedom (DOF) in their joints in addition to theskin and bone features. In order to address such issues, Ishikawa et al. proposeda framework for exoskeletal patient simulators [5][6]. This concept is especiallyeﬀective for complex human joints such as the feet [9].
The objective of this study is to develop a simulator for spasticity and clonusof the ankle for educational purposes. These two types of symptoms are signiﬁcant and frequently developed for hemiplegia patients. The exoskeleton form andthe tendon mechanism along a human muscle allow the trainees to experiencehigh DOF ankle motion with a reduced number of actuators. PT trainees willable to learn manual examination techniques to test for clonus and spasticity
by using this simulator. Because our simulator has a tendon mechanism that⋆This work was in part supported by Naito Foundation.

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Exoskeleton Simulator of Impaired Ankle:

Cuff
Motor
PulleyWireIdler

Fig. 1. Exoskeletal ankle simulator and training scene

applies torque only in the plantar direction, it seems diﬃcult to simulate clonus,
which involves both plantar and dorsal ﬂexion movements of the ankle. However, utilizing manual force applied to the sole by a PT trainee, clonus can besimulated. We demonstrate the simulated clonus and spasticity function in thisstudy.

2

Exoskeleton Mechanism to Simulate Spasticity andAnkle Clonus

Fig. 1 shows a prototype of the exoskeletal simulator and a training scene. The
exoskeleton was constituted mainly by a shoe and cuﬀ ﬁxed to the upper calf. A
DC motor (RE35, Maxon motor, maximum continuous torque 97.2 mN·m) witha 1/23 gear head was attached to the cuﬀ. We used a servo ampliﬁer (4-Q-DCADS 50/5, Maxon motor) in the current control mode. A wire extended fromthe sole through an idler was wound by the DC motor, and its tip was attached
to a pulley (ϕ = 20 mm). The wire was set along the tibialis posterior muscle tosimulate the disorders caused by the problems of this muscle. This wire-driven
mechanism does not impede complex human ankle joint motions. One of theadvantages of exoskeletal simulator is that there is no necessity to simulate theinherent mechanical impedance of human body. For example, the elasticity of
the ankle muscle and tendon is presented by those of a wearer. The resistanceof simulated symptom is superposed with that of the wearer’s joint impedance.

3

Simulation of Spasticity

Spasticity is a disorder of the upper motor neuron, and it is characterized bya resistance proportional to the extension rate of the muscle [1]. The simulator
presents the feeling of catch which PTs feel when they dorsiﬂex a foot of patients
at higher rate than an certain value. The motor torque, τs , required to simulate

Exoskeleton Simulator of Impaired Ankle:Force sensor

3Force sensor

50403020
100-10Rotation angle ofmotor, θ [rad]86420246

Contact force, f [N]Fig. 2. Measurement setup; left: spasticity test and right: clonus test.

8Time [s]Fig. 3. Resistance force of the simulated spasticity. Force against the dorsiflexion. Angleof the DC motor with θ = 0 at full plantar flexion.

spasticity was determined by{τ0τs =τ0 + cθ̇(t)(θ̇(t) ≤ ω0 )(θ̇(t) > ω0 )(1)where c and θ̇(t) are the coeﬃcient of viscosity and angular velocity of the DCmotor (the direction of ankle dorsiﬂex is the forward direction), respectively. If
the angular velocity of the motor was less than or equal to ω0 , τs was constantat τ0 . Otherwise, τs was proportional to the angular velocity when θ̇(t) > ω0 .
This angular velocity, ω0 , represents the stretch reﬂex threshold [2]. We set eachparameter as follows: τ0 = 0.072 N·m, c = 9 × 10−3 N·m·s/rad, and ω0 =4.0 rad/s.
We measured the resistance forces of the simulated spasticity in the setup
shown in Fig. 2. A force sensor (USL06-H5-200N-C, Tec Gihan Co. Ltd, Japan.)
was installed on the fore part of the sole such that it covered the load path overthe fore foot. A PT dorsiﬂexed the ankle of the exoskeleton wearer, in a way
similar to how it is usually performed in a clinical setting. Fig. 3 shows a sampleof the measured force and θ(t). The ankle was dorsiﬂexed quickly at t = 3, andan abrupt rise in the force was observed during this phase. Such a sharp increase8252015
1050-56420024

Contact force, f [N]Exoskeleton Simulator of Impaired Ankle:

Rotation angle ofmotor, θ [rad]46Time [s]Fig. 4. Measurement results for ankle clonus (ankle was quickly dorsiflexed)in the interaction force was regarded as a typical velocity-dependent resistanceof a spastic ankle.

4Simulation of Clonus

Clonus is also caused by disorders of the upper motor neuron, and it is characterized by an involuntary rhythmic contraction of muscles [4]. To test for ankle
clonus, PTs quickly ﬂex the ankle in the dorsal direction. When clonus occurs,
PTs receive rhythmical and repetitive plantar and dorsal ﬂexion of the ankle.
The motor torque, τc , to simulate ankle clonus was determined byτ1 (θ̇(t) ≤ ωt )
τc = τ2 (θ̇(t) > ωt , nts ≤ t < (n + 1)ts , n = 0, 2, 4, · · · 20)(2)
0 (θ̇(t) > ωt , nts ≤ t < (n + 1)ts , n = 1, 3, 5, · · · 21)
whereτ1, 2 and ts are the constant resistance torques and rhythmic period of
the ankle movements, respectively. When θ̇(t) was smaller than ωt , the reﬂective
contraction did not occur and a weak constant resistance of τ1 was set. When theankle was dorsiﬂexed quickly and θ̇(t) became larger than ωt , a large resistanceof τ2 was set. τ2 was suﬃciently large to plantarﬂex the ankle joint, opposing
the PT’s manual force. This strong reﬂexive muscle contracture was then turnedon and oﬀ at a period of 2ts . As a result, rhythmic plantar and dorsal ﬂexion
movements were produced. To express the moderate disappearance of clonus, weset each parameter as follows: τ1 = 0.072 N·m, ts = 0.15 s, τ2 = 0.28 N·m for
n = 0, 2, · · · 8, 0.18 N·m for n = 10, 0.14 N·m for n = 12, and 0.12 N·m forn = 14.
We also measured the force that a PT experience on his/her hand, as shownin Fig. 2. A PT pushed the fore part of the sole manually to test for ankle
clonus. Fig. 4 shows a sample of the measured contact forces normal to thesole. The rhythmic motions were observed clearly at approximately 2 to 4 s at
3.4 Hz, which is in a typical range [4]. Although the tendon mechanism of theExoskeleton Simulator of Impaired Ankle:

5

exoskeleton could exert force in the direction of plantar ﬂexion, rhythmic clonus
was simulated using the manual force of the PT in the direction of dorsiﬂexion.

5Conclusion

In this study, we expanded the functions of the exoskeletal ankle simulator tosimulate spasticity and clonus. Because our exoskeleton was based on a tendon
mechanism, it was challenging to simulate clonus, which involves both plantarand dorsal ﬂexion movements. However, utilizing the manual force applied tothe sole by a PT trainee, we could realize the typical abnormality of clonus.
During manual examination toward each simulated symptom, resistance patterns
characteristic to clonus and spasticity were observed between the wearer’s fore
foot and the trainee’s hand or arm, suggesting the authenticity of the simulated
symptoms. PT trainees will able to learn manual examination techniques to test
for clonus and spasticity by using this simulator.

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