Original Article

Development of a method for measuring joint torque using an isokinetic machine

Motomi Igari, OTR, MS, Yutaka Tomita, RE, PhD, Hiroyuki Miyasaka, OTR, PhD, Abbas Orand, RE, PhD, Genichi Tanino, RPT, MS, Kaoru Inoue, OTR, PhD, Shigeru Sonoda, MD, PhD
Jpn J Compr Rehabil Sci 5: 141-146, 2014

Objective: To develop a method to measure knee joint resistance torque during passive motion by using isokinetic equipment (e.g., the Biodex, Cybex, and Kin-Com).
Method: We used the Biodex System 3 isokinetic equipment in the present study. To determine the resistance torque in human knee joints, we first established a method to correct for the torque due to gravity and inertia, based on the angle, angular velocity, and torque output from this equipment. Thereafter, to estimate the active torque due to muscle contraction, we measured the electromyograph (EMG) signals and fitted a potentiometer on the driving arm of the Biodex to synchronize the EMG signals with the Biodex output.
Results: At the angle 0, and denoting the torque that occurs in the lower leg due to gravity as T0, the torque due to gravity (Tg) at angle is given by Tg=T0 cos/ cos0. Denoting the moment of inertia of the lower leg as I and the angular acceleration as a, the torque caused by inertia (TI) is given by TI=I a. Letting the value of the torque that is output by the Biodex be Ttotal, the resistance torque caused by the subject (T) is given by T=Ttotal-Tg-TI. The changes in T that occur simultaneously with the EMG signals indicate the active torque that is caused by muscle contraction.
Conclusion: We measured the resistance torque that occurred during passive joint motion by correcting for the angle, gravity, and inertia, based on the angle and torque that were output by the Biodex. The joint torque, angle, and EMG signals were measured simultaneously by monitoring the output of a potentiometer mounted on the arm of the Biodex.

Key words: joint stiffness, quantification, muscle tension, viscoelasticity, electromyogram

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