Sounds like a bit of a technomyth to me or a hypothetical discussion at best. One would be hard pressed to hit 60kph on a normal bicycle let alone a set or spring loaded stilts - how would one translate the vertical thrust fro the springs into horizontal motion? You can get spring loaded stilts for fun which allow you to jump 6ft+ vertically and take massive strides. There are research projects out there developing exoskeletons to enhance human performance but it is more in the load-carrying arena.