Advice on training with an injury

ok im currently in a leg cast that goes up to my knee and have been for 3weeks, i have fractured my talus in three places (bone near the ankle on the foot) i really feel fat and lazy and am desperate to get out on my bike when its off! but for now i cant, i had the idea of buying a turbo trainer today and just having rides in my house to keep fit? i can get on and off my bike and can pedal fine in the cast (rode backwards stationary)
what im asking you is do you reckon it will do my foot any harm? i do not want to slow the recovery process or do more damage but surely my foot is rigid in a cast and all im really moving is my knee?
opinions appreciated
thanks

It's only been 3 weeks, I've been waiting for 3 months before my physio is fInally sort of happy for me to get on a fake bike. Am still working on convincing them to let me do some rounds in a park. I'd ask the doc or physio. Everybody is different; some can get back on their bikes quicker than others, so don't risk it before consulting your docs. The last thing you wat is to be in that cast for longer than you need.

3months thats harsh man, its just depressing that im slobbing around getting unfit and theres nothing i can do, i will try and get in contact with my physio and ask.
thanks

I appreciate how desperate you are to continue with the cycling, but whilst you may be happy to do some turbo training, you must be aware that because your lower limb is immobile, your venous return is poor, meaning that any metabolites of exercise may very well pool in your injured limb.
The lower limb is massively important in returning deoxygenated blood back to the heart, and the calf muscles are crucial for this. Because one of them is out of action, any strenuous activity involving the lower limbs may be detrimental to healing, this early on in your injury.
Knock yourself out with upper body exercise, maybe try rowing with your lower limbs fixed, or upeer body ergo's.
Eat well and hydrate and your recovery should be quick.

tell me about the weight gain big fat lazy ass self conscious feeling I have been building over the last few months. You are not alone. Everytime I go to the physio, I hear peopel asking when can they get back to cycling / football / basketball / running etc etc I ask the same question everytime too. But I couldn't agree wirh pistaboy more, every single bit of your body relates to each other somehow, while you might feel you wanna remain in shape now, you must think about it in a long term.

Good luck and be patient.

thanks for your comments, seems like im just going to have to stick to the upper body workouts. and its really helpful to get an real body perspective on whats actually happening to my leg and what happening when i exercise it so thanks for that pistaboy, and ExTra its good to know im not alone its so downgrading!