Abstract: A limb support device includes a variable stiffness mechanism. The limb support device can be an orthotic or prosthetic device. The variable stiffness mechanism can include, for example, a rate-sensitive or speed-dependent material or a damping mechanism. The variable stiffness mechanism causes the limb support device to exhibit different properties when the user of the limb support device is walking at high or fast walking speeds compared to low or slow walking speeds. The limb support device can exhibit high damping and energy absorption, and therefore stability, at slow speeds, and high energy return at faster speeds.