Abstract
The excitability of the stretch reflex system can be examined by the gain of the stretch reflex, which includes short latency and long latency components. However, the wrist joint impedance characteristics, such as stiffness, viscosity, and inertia, must be estimated simultaneously in order to analyze the relations between the excitability of stretch reflex system and the wrist joint impedance characteristics. In this paper, we developed a method satisfying this demand using a linear motor system. While the subject maintains a wrist joint torque isometrically, an external disturbance to estimate the wrist joint impedance and to elicit the stretch reflex was applied to the subject's hand. The wrist joint impedance is estimated by a least square method, and the stretch reflex is recorded from the flexor carpi radialis muscle using a surface electromyogram technique. The experimental results showed that the stretch reflex excitability was quasi-linearly facilitated by the increase of the wrist joint torque. Also, the viscosity and the stiffness of the wrist joint were closely related to the stretch reflex excitability, while the inertia of wrist joint was almost constant. Indeed, the natural frequency and the damping coefficient of the wrist joint were linearly related to the increase of the wrist joint torque.
