This paper presents quantification of multidimensional ankle stability in relation to mechanical environments having different levels of stability. This study, for the first time, explores the range of stiffness-defined haptic environments over which young healthy individuals can maintain stability despite aggressive perturbation. Ankle stability was quantified in 2 degree-of-freedom (DOF) of the ankle, in both the sagittal and frontal planes. Importantly, the magnitude of negative environmental stiffness that the subjects could maintain stability is 4 times as great in the sagittal plane as in the frontal plane. In addition to managing a wider range of unstable environments in the sagittal plane, subjects were also more efficient at regaining stability after perturbation and less sensitive to changes in the environmental stiffness. Outcomes of this study would be beneficial to the design and control of robots physically interacting with human lower extremities, such as lower-limb exoskeletons and powered ankle-foot orthoses.