I think your question is too vague to give a good answer. I know a girl who's body is slowly breaking down, she's in a wheelchair for life and her bones have become extremely weak, she's broken her leg just by moving it wrong.
If she does something beyond her limitations, like tries to reach over and pick up a bookbag with 50lbs of books in it when she fully knows that it'll cause problems, then she should not receive compensation. If, however, some jackass dropped that bookbag on her lap, he should probably be liable (esp. since she makes sure that ppl know she's very weak)
I think it also depends on the injury. It's pretty easy to scam these days. For a while my family owned two houses and we rented the other one out...we offered the tenants a very good rate that was basically just barely enough for us to pay the mortgage with. We had agreed that they would be responsible for general upkeep, including mowing the lawn and keeping the driveway cleared of snow and ice.
Then, after a few snowstorms, the guy supposedly slipped and sprained his ankle in the driveway...and we got sued.