Original Article

Development of a stiffness measurement system and biomechanical model of ankle joint to evaluate viscoelasticity and muscle contraction

Yutaka Tomita, RE, PhD, Genichi Tanino, RPT, Shiho Mizuno, MD, PhD, Hirofumi Maeda, MD, Hiroyuki Miyasaka, OTR, PhD, Orand Abbas, RE, PhD, Kotaro Takeda, RE, PhD, Shigeru Sonoda, MD, PhD
Jpn J Compr Rehabil Sci 5: 147-155, 2014

Objective: This research aimed to develop a system to measure the stiffness of the ankle joint for evaluating spasticity or contracture, and for quantifying the characteristics of spasticity. Functionality of the system was verified by testing it on an able-bodied individual and a hemiplegic patient. Additionally, a biomechanical model was developed to estimate the plantar flexion torque caused by viscoelasticity and muscle contraction.
Methods: An electromotor, rack and pinion, potentiometer, and torque sensor were installed on a double Klenzak ankle-foot orthosis (AFO). By rotating the electromotor, the ankle joint of the AFO moves dorsally at a fixed speed. The angle and torque of dorsiflexion were measured simultaneously. The subjects sat either in a chair or a wheelchair and wore the abovementioned AFO in the knee-extension and knee-flexion positions, while the AFO moved dorsally. Electromyograms of the tibialis anterior and gastrocnemius muscles were recorded concurrently. The contributions of elastic, viscous, and musclecontraction components to the plantar flexion torque were calculated using the system identification approach.
Results: The systemfs ability to measure dynamic characteristics, and also its accuracy, were confirmed. The plantar flexion torque was found to be larger in the knee-extension position than in the knee-flexion position in both the able-bodied person and the patient. Moreover, the patient showed larger plantar flexion torque than the able-bodied subject.
Conclusions: A system that measures ankle-joint stiffness for evaluating spasticity was developed, and sufficient functionality was verified by applying it to both an able-bodied individual and a hemiplegic patient. In addition, plantar flexion torque caused by viscoelasticity and muscle contraction was estimated.

Key words: joint stiffness, quantification, viscoelasticity, electromyogram, spasticity

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