This paper presents a simulation study of the problem of balancing a planar double pendulum in which the lower body (the leg) has been modified to include a spring-loaded passive prismatic joint. Robots of this kind can travel by hopping, and can also stand and balance on a single point. The purpose of this study is to investigate the degree to which a balance controller can cope with the large and rapidly changing forces from the spring. It is shown that good performance can be achieved using an existing balance controller if the spring-loaded joint is instrumented so that its position and velocity can be taken into account when calculating the state variables needed by the balance controller.