Compliant Knee Exoskeletons and Their Effects on Gait Biomechanics

The human knee joint exhibits a spring-type behavior during the stance phase of walking at the preferred speed, which is both subject-specific and gait-specific. This observation led us to hypothesize that the human knee joint could partially adapt to an externally-applied tuned mechanical stiffness during the stance phase leading to reduced muscle involvement and energy expenditure. We also hypothesized that a spring, which is tuned to the body size and gait speed, in parallel with an impaired knee joint during the stance phase can partially restore the natural spring-type behavior of the knee joint. Three experimental and theoretical steps were taken to test these hypotheses.;First, a series of statistical models were developed that can closely characterize the moment-angle behavior of the knee joint using a set of measurable parameters including body weight and height, gait speed, and joint excursion. It is explained that these models can be used to tune the components of knee exoskeletons/orthoses and prostheses to the body size and gait speed of users, as well as general applications in understanding gait biomechanics. The statistical models of the knee joint were used in the next steps of this research to tune the stiffness of the experimental exoskeletal devices throughout the experimental sessions.;To experimentally test the first main hypothesis, a pair of quasi-passive knee exoskeletons was developed. When worn on a healthy subject, each exoskeleton implements an interchangeable spring in parallel with the knee joint during the stance and allows free rotation during the swing phase. The exoskeletons with a range of stiffness were used in a series of experiments on healthy individuals to study the mechanics and energetics of human gait in interaction with exoskeletal impedances in parallel with the knee joint. Healthy lower extremity joints showed substantial adaptation to the exoskeleton stiffness/assistance suggesting that replicating the natural behavior of a joint could be a viable method for the design of lower extremity exoskeletons to reduce muscle involvement and energy expenditure. It was also observed that a healthy knee joint can fully accommodate external assistance only to a certain level, above which the knee joint adaptation saturates and biarticular effects emerge.;To test the second hypothesis, a compliant stance control orthosis was developed that implements a spring in parallel with an impaired knee joint during the stance and allows free rotation during the swing phase. It was found that a compliant stance control orthosis can restore the natural spring-type behavior of an impaired knee joint during the stance phase. The compliant stance control orthosis showed higher gait speed and more natural kinematic patterns when compared with the state-of-the-art stance control orthoses that rigidly lock the knee during the stance phase. The findings of this research also showed that a friction-based latching mechanism can be a viable option in the design of lower extremity assistive devices that require engagement and disengagement of passive components.