The effects of misalignment of the center of rotation of the ankle on stiffness measurements

Misalignment of the center of rotation of the ankle with the axis of rotation of an actuator results in significant error in measured ankle torque during passive ranging of the ankle. As a result, estimates of ankle stiffness must account for this misalignment torque. We quantified misalignment torques and foot plate forces in the human ankle with measured displacements in two orthogonal directions. The resulting foot plate forces were consistent with the forces predicted by the mechanics of the system. These data will be used to align the center of rotation of the ankle with the actuator axis of rotation. This alignment procedure will improve the validity of experimental comparisons when monitoring longitudinal changes in the mechanical properties of the ankle following spinal cord injury.