I was recently invited to trial an underwater treadmill at Swimotion in Groombridge. I have always wanted to try one since reading how Alistair Brownlee used one prior to the Olympics when he had Achilles problems.
Swimotion is run by Jon Yates , a swimming coach who works in Sevenoaks, and he has invested in an Endless Pool with the treadmill incorporated. I started with just the treadmill moving and found that the tendency is to run more on my toes, putting more effort into going up and down, than perhaps my normal running style which propels. It was quite easy, surprisingly without feeling that the water was resisting. Jon then introduced the current which I found encouraged me to adopt a good running posture as you need to use your core to push against the force, leaning slightly into it and reducing the tendency to go up and down rather than forward. Again my legs never felt tired, although my brow showed that I was working hard. After 30 mins my legs did not feel particularly tired or achy, probably looser if anything.
So this facility could certainly be used for assisting recovery from surgery to replace any joints in the leg, as it allows movement plus strengthening, but no trauma to the limb. It could be very helpful to have a session once or twice a week post hip or knee surgery, to encourage mid range movement and strength.
It does not replace actual running as it certainly made me run differently to how I normally would as I found it quite hard to do a heel strike. It could be a good place to start if you are trying to encourage forefoot running without damaging your calf muscles. If you use orthotics normally you can still use this equipment as you will not have the same ground contact.