Mechanical impedance is a concept that is used to model biomechanical propertiesof human joints. These models can then be utilized to provide insight into the inner workings of the human neuromuscular system or to provide insight into how to best design controllers for robotic applications that either attempt to mimic capabilities of the human neuromuscular system or physically interact with it. To further elucidate patterns and properties of how the human neuromuscular system modulates mechanical impedance at the human ankle joint, multiple studies were conducted. The first study was to assess the ability of linear regression models to characterize the change in stiffness - a component of mechanical impedance - seen at the human ankle during the stance phase of walking in the Dorsiflexion-Plantarflexion (DP) direction. A collection of biomechanical variables were used as input variables. The R^2 value of the best performing model was 0.71. The second and third studies were performed to showcase the ability of a newly developed twin dual-axis platform, which goes beyond the limits of a single dual-axis platform, to quantify bilateral stiffness properties. The second study quantified the bilateral mechanical stiffness of the human ankle joint for healthy able-bodied subjects during the stance phase of walking and during quiet standing in both the DP and inversion-eversion directions. Subjects showed a high level of subject specific symmetry. Lastly, a similar bilateral ankle characterization study was conducted on a set of subjects with multiple sclerosis, but only during quiet standing and in the DP direction. Results showed a high level of discrepancy between the subject’s most-affected and least-affected limbs with a larger range and variance than in the healthy population.