  Springs are commonly used in wearable robotic devices to provide assistive
joint torque without the need for motors and batteries. However, different
tasks (such as walking or running) and different users (such as athletes with
strong legs or the elderly with weak legs) necessitate different assistive
joint torques, and therefore, springs with different stiffness. Variable
stiffness springs are a special class of springs which can exert more or less
torque upon the same deflection, provided that the user is able to change the
stiffness of the spring. In this paper, we present a novel variable stiffness
spring design in which the user can select a preferred spring stiffness similar
to switching gears on a bicycle. Using a leg-swing experiment, we demonstrate
that the user can increment and decrement spring stiffness in a large range to
effectively assist the hip joint during leg oscillations. Variable stiffness
springs with human-selectable stiffness could be key components of wearable
devices which augment locomotion tasks, such as walking, running, and swimming.
