Passive Stiffness Reduction in Human Ankle Joint Mechanical Impedance When Exposed to Externally Imposed Movement

Exoskeleton design is based on achieving representative torques for both active and passive joint mechanics for a given task. The latter has been typically estimated for static postures during relaxed muscle activity. However, some studies have shown that differences in the estimates during voluntary motion versus isometric postures have resulted in lower impedance estimates during active muscle activity. The objective of this study was to determine if the estimates for the passive joint impedance of the human ankle exhibited the same reduction during relaxed muscle states with an externally imposed movement over static posture estimates. System identification techniques were used to analyze the passive joint impedance as a second-order model with inertia, damping, and stiffness. The experimental results showed little differences for the inertia and damping coefficients; however, a 10 to 34% reduction in the passive stiffness estimate was observed during movement when compared to static posture estimates. It is postulated that the 22% increase in angular velocity of the perturbations due to the superimposed externally imposed motion may be a contributing factor to these statistically different estimates. This reduced passive stiffness yields possibly new insights into muscle mechanics and applications to exoskeleton design. With lower torque and power estimates for passive resistance during imposed movement, reductions in motor sizing and inertial effects could lead to improved overall performance.