dear calistair,

extremely happy to know about yourself and your interest towards designing something unique to aid rehabilitation.

well, going through the spectrum listed by you - "broken/sprained ankles or broken femurs or hip-replacements" I wish you work on a model to aid rehab of the ankle/foot.

ankle/foot is one area that I have not been able to work on to my utmost satisfaction. I agree one can use therabands, wobble boards, manual resistance so on & so forth, yet a device designed to strengthen the ankle/foot musculature in a more specific manner would indeed be a welcome move.

well this is just my personal opinion. on the other hand if you need any help or further inputs please feel free to contact me. I will be more than happy to share my technical expertise.

cheers,

thomas