This paper presents a novel method to characterize mechanical impedance of a human joint. We employed a non-parametric stochastic identification method and an interactive robot. This method allows us to determine several important properties including a) joint impedance in the direction of the joint principal axes, b) the spatial joint impedance structure, and c) the passivity properties of the joint. The method is applicable to studies of multivariable mechanical joint impedance, both for multi-joint impedance and single joint multi-DOF impedance. The method was successfully applied to the human ankle, and we showed that the determined ankle impedance precludes the utilization of a 2 nd order model. In addition, the impedance structure exhibited a characteristic peanut shape, being weak in inversion-eversion, and the ankle has passive and dissipative behavior over wide range of frequency.