Stiffness Perception During Active Ankle and Knee Movement

Objective: Recently, human joint impedance-the instantaneous mechanical response to a perturbation-has been quantified during gait, providing new insight beyond the traditional biomechanical descriptions of kinetics and kinematics. However, the role of joint impedance in neuromotor control and the development of exoskeletons and other wearable robotic systems remains unknown. One approach to studying the role of impedance in neuromotor control involves characterizing the human ability to discriminate changes in external impedance properties. Thus, the purpose of this work is to quantify the minimum detectable change in the stiffness component of impedance when interacting with an external mechanical impedance at the human ankle or knee. Methods: A dynamometer coupled to subjects' right ankle or knee rendered the dynamics of a virtual rotational spring-mass-damper system. The minimum detectable change, or just noticeable difference, was determined via a weighted up-down staircase method in which subjects compared the stiffness values of two different controller configurations. Results: We found that subjects could reliably detect stiffness changes of at least 12% at the ankle and 13% at the knee. Conclusion: Stiffness errors or variations produced by an external mechanical device will be undetected if they remain below the 12-13% threshold. Significance: Our results provide novel insight into how the sensorimotor system senses joint impedance, information that may improve the design and control of impedance-based wearable robotic technologies.