What does she complain about? Pain? How long since she broke her knee? Did she have therapy with a physical therapist? If not, maybe that would help her.
You can refuse to fetch and carry things that she has the ability to get on her own; for example instead of bringing her a tray make her come into the dining room for meals. If this is a matter of just walking for exercise then you need to figure out how to add a little incentive, maybe something like "three times up and down the hall and we can have some ice cream", or taking her out to walk to somewhere worth the effort.