The design of a biped which makes use of the natural frequencies of the moving components, and thus has a minimal energy expenditure. The biped will have a modular structure, i.e. components can be easily replaced. The purpose is to enable designers of components for prostheses and orthoses and members of rehabilitation themes: - to get more insight in normal and pathological walking, and - to test prototypes of walking aids in a laboratory situation before trying on patients. Furthermore, it is expected that a robot as proposed will also be an attractive feature in an amusement park. In this stage of the project the aim is a biped with stiff knees and ankles. Addition of knee and ankle joints will be realised in a second stage.