How do you exercise someone who won’t walk?

My mother has Alzheimer’s/Dem and bad knees. She has not been able to straighten her back or legs so she doesn’t walk anymore without assistance. Now her legs and knees are swelling more. How can I excersize her legs and how can I keep her legs and back straight enough for her to walk upright? Has anybody dealt with this? She keeps her muscles in her back neck and knees in a constant state of forced flex. How can I get her to relax her muscles and straighten up? Has anyone used CBD oil? Lots of questions, sorry lol