CHARACTERIZATION OF MULTIJOINT FINGER STIFFNESS - DEPENDENCE ON FINGER POSTURE AND FORCE DIRECTION

The two-dimensional static stiffness of the index finger was measured with the interphalangeal joints in flexed and extended postures. The s tiffness of the relaxed finger was compared with the stiffness when vo luntary force was exerted in different directions. The finger stiffnes s was found to be anisotropic, with the direction of greatest stiffnes s being approximately parallel to the proximal phalange of the finger. This direction was relatively unaffected by finger posture or directi on of finger force. Finger stiffness was more anisotropic when the int erphalangeal joints were extended than flexed. The stiffness was most anisotropic when the interphalangeal joints were extended and force wa s being exerted in the direction of pointing, while it was least aniso tropic when the interphalangeal joints were flexed and force was being exerted in directions normally associated with pinching and tapping a ctions. The stiffness of the individual finger joints was computed and the relation between stiffness and joint torque was examined. Previou s studies, which examined single finger joints in isolation, had found that joint stiffness varied in a linear fashion with net joint torque . In contrast, we did not find a monotonic relation between joint stif fness and net joint torque, which we attributed to the need to vary th e amount of cocontraction of antagonistic muscles when controlling the direction of finger force.