Measurement and reliability of 3D end-point stiffness of the human arm

The endpoint stiffness of the human arm was characterized in three-dimensions (3D) and both the reliability and error properties of the estimates were evaluated. Endpoint stiffness was estimated by multiple-input multiple-output frequency response functions between 3D stochastic displacements imposed on the hand and the evoked 3D forces. The response functions, estimated while the subject maintained one of seven constant endpoint forces, were identified with high coherence and were highly repeatable above approximately 1 Hz. Analysis of potential mechanisms for poor characterization below 1 Hz indicate that small changes in the subject´s contraction level during the imposed perturbation were likely responsible. We found that the horizontal components of the stiffness were similar to those previously identified under planer, two-dimensional, conditions. In addition, coupling between the vertical and horizontal components was small for the posture tested. Overall these results indicate that characterization of three-dimensional stiffness of the arm is feasible.