A Gravity Balancing Passive Exoskeleton for the Human Leg

A gravity balancing lower extremity exoskeleton is a simple mechanical device composed of rigid links, joints and springs, which is adjustable to the geometry and inertia of the leg of a human subject wearing it. This passive exoskeleton does not use any motors or controllers, yet can still unload the human leg joints of the gravity load over the full range of motion of the leg. The underlying principle of gravity balancing consists of two steps: (i) Locate the combined system center of mass of the human leg and the exoskeleton, (ii) Add springs to the exoskeleton, one between the center of mass of the combined system and the fixed frame representing the trunk, the others within the links of the exoskeleton so that the potential energy of the combined system is invariant with configuration of the leg. Additionally, parameters of the exoskeleton can be changed to achieve a prescribed level of partial balancing, between 0-gravity and 1-gravity. The goals of this paper are as follows: (i) briefly review the theory for gravity balancing and present laboratory prototypes of gravity balanced machines, (ii) describe the design of a lower extremity exoskeleton that was fabricated using this principle, and (iii) show the performance of the exoskeleton on both healthy human subjects and a stroke patient by comparison of leg muscle EMG recordings, joint range of motion, and measured joint torques. These results strongly suggest that human joints can be unloaded from gravity using these exoskeletons and the human joint range of motion can be significantly increased. Potential applications of gravity balancing exoskeletons include: (i) gait training of stroke patients, (ii) characterization of long-term effects of zero gravity on the human musculature, (iii) human performance augmentation during assembly tasks.