The only thing I can think of is maybe when you visit your parents, you could mention to your mom that you'd like to help her clean the house. And if there are heavy things maybe ask a neighbor to help you move it for them. And even if she isn't "up " to helping you clean, maybe you could go ahead a clean one or two rooms for her....

I know, lots of times, when mother was alive, and when I'd visit her, I'd clean just for the sake of helping her. She loved it when I helped her...