Variable stiffness mechanism and limb support device incorporating the same

[0038]A natural human ankle varies its stiffness based on the activity being performed by the user, such as walking at various speeds, rising to stand from a seated position, sitting down, ascending and descending stairs, walking on uneven terrain, and / or running. The ankle can vary between, for example, relatively low stiffness and relatively high damping during slow walking, which can allow for easier transitions, and relatively higher stiffness and relatively lower damping when walking at faster speeds, which can provide greater energy efficiency. Some currently available prosthetic feet are adapted for either high damping, e.g., via a hydraulic mechanism, or high energy efficiency, e.g., via carbon fiber leaf springs. However, such prosthetic feet typically have fixed damping and / or spring characteristics optimized for a particular gait speed on level ground and may not adapt well to other speeds or activities. While prosthetic feet including an electric motor in series with a