Walking is the basis of human locomotion. With an increasing aging population, the need for devices that assist human walking are in demand. While several simple devices exist such as canes, braces, and walkers, to assist the elderly or individuals suffering from gait impairment, these devices often limit the user’s performance and increase their energy expenditure during walking. Recently autonomous exoskeletons that reduce the user's energy expenditure without limiting their performance have been developed. Unfortunately, these exoskeletons are heavy and have limited operating time due to their actuation mechanisms and power demands.

This thesis focuses on the development and testing of passive lower limb exoskeletons that assist level walking. Passive or un-actuated exoskeletons, are naturally lighter and fully autonomous since they do not rely on electro-mechanical actuators to assist the user. This reduces the environmental impact and provides accessibility to populations without access to a power source. Like the mechanical wristwatch, passive exoskeletons utilize the potential energy of the user’s natural motion to “power” the assistance. This potential energy is otherwise dissipated in the body in deformation of soft tissues or as heat. By harnessing the potential energy and releasing it at properly timed intervals, it is possible to reduce the metabolic energy expenditure of the user and reduce the onset of fatigue.

Three passive exoskeleton devices to assist the user during level walking were developed using two approaches. The first approach attempts to minimize the step-to-step transition cost of walking, where energy is lost due to collision of the leading leg with the ground. The approach focuses on a method of reducing the energy lost during the collision and using the energy to assist the trailing leg. The second approach examines assistance of the leg joints during walking. This approach follows an inter-joint energy transfer paradigm where potential energy at one joint is used to assist a different joint. The performance of each device and their effects on the biomechanics and energetics of the user are presented. Contributions of this thesis further our understanding of the mechanics in human locomotion and interactions with passive exoskeletons.