Passive Wrist Joint Impedance in Flexion - Extension and Abduction - Adduction

The purpose of this study was to find a reproducible estimate of passive wrist joint impedance. Six healthy subjects were exposed to constant velocity perturbations in flexion -extension and abduction -adduction. Subjects were asked not to interfere; absence of muscle activity was verified through EMG recordings. Torque / angle relations showed that the static impedance of the wrist joint could conveniently be described by four linear stiffness curves, corresponding to each side of the neutral position in both DOF. These stiffness values were all significantly different. They were also reproducible for each subject and comparable between all subjects. The results also showed a consistent hysteresis loop. The stiffness values are comparable to previous studies, although to our knowledge no previous study recorded a significant difference between stiffness on each side of the neutral position. This difference was hypothesized to be caused by the sum of muscle volume that has to be lengthened. Hysteresis was linked to history dependent behavior of the muscles.