Some friends of mine worry about the health of their father, whose weight puts strain on his heart, has given him on-off problems with diabetes etc. We're not talking 29 stone here, but the combination of the heart strain and the pressure his weight puts on his knees makes walks of any length a problem. It occured to me that bikes can be gentler exercise, being more efficient and not having the same impact on the knees. He couldn't cope with a regular bike, but I've suggested to them that he might be able to ride a trike; not only would it remove stability problems but trikes can have a tweedy, vintage image which would appeal to him (and psychological motivation is important).
Does this sound reasonable? If so, can people suggest suitable models/manufacturers?