After all those years rebelling against your parents, maybe lying to them or at least learning how to distract them from what you are doing....
....now your mother is 90 and you are telling her what she needs to do.
It's obvious, what she needs to do, and she rebels. She acts just like you did. She is stubborn as a rock and equally responsive.
Having watched this for years as a family doctor, my technique with my mother was indirect. She was walking bent over with pain from her ovarian cancer. I asked what she was taking for pain.
"Ibuprofen." she responds.
Shit. Some time later. "Can I come with you to meet your doctor?"
"Yes." says my mother.
We go to Dr. Murphy's. My mother asks few questions and introduces me. Dr. Murphy says the cancer marker is up. My mother says, "I can take more chemotherapy, right?"
"Yes." says Dr. Murphy. She only answers the questions my mother asks.
I say, "My mother is losing weight. She is only taking ibuprofen. Would another pain medicine help her gain weight?"
Dr. Murphy looks at me then, eyes sharp. She looks at my mother. Dr. Murphy knows I am a family doctor and knows that a different pain medicine won't change my mother's weight. My mother is curled over in a position where her abdomen doesn't hurt as much.
"Yes." says Dr. Murphy. "Let's try some oxycodone instead of the ibuprofen." She writes a prescription.
Five days later my mother says, "I WAS hurting, but I didn't want to admit it. The oxycodone helps a lot more."
I don't know if I fooled my mother. But it was more about saving face: she was denying pain and Dr. Murphy had lost track of the fact that my mother would not admit pain.
My technique with my father was similar. As much as possible, I tried to get the experts, the doctor, the physical therapist, the respiratory therapist, to tell him what to do. My proud introverted father actually loved respiratory therapy and bloomed in it and was proud of his lung capacity, trained by singing and trumpet. He quit smoking his camels 3 and a half years before he died.
As a family doctor, I have 50 year olds taking exasperated care of 80 year olds. A 91 year old in frail health taking care of his 90 year old bedridden wife. Adult children trying to take care of stubborn parents who won't let help in the house or have forgotten that they've lost their memory. Home health can help, but it also helps to hire someone else to exercise with mom and talk her through it. If you can. Or find a friend with their own elderly parent and trade duties: you go encourage their parent through the exercise and have them work with yours. The parent will often put on a cheerful face for the therapist.
Blackmail works too. When my father was hospitalized for pneumonia, I went out to his house. To see whether I needed to call the Health Department to have it condemned. No, not quite that bad. He had a year's worth of mail, back to unopened Christmas cards a year ago, on the table. My sister and I went through it. It turned out that he was paying bills on line, though ignoring price increases, so the bills were paid. Sort of.
But....we told him that we'd gone to the house.
"You are officious." he said angrily.
"Yes I am." I said. "Will you be coming to my house to get well or going to a nursing home? And do you want me and Chris to clean your house up or will you be hiring people?"
Sullen silence as he glared at me and thought about it. "I'll hire people." he said.
Later on, after he and Chris had a fight about whether we were officious or not, he said, "Can I recover at your house?"
"Of course." I said. "We'll arrange home health and a rental hospital bed and oxygen."
In the end, the parent is an adult and they may balk and may refuse....
Sweet Honey in the Rock: https://www.youtube.com/watch?v=jFvDtzMpPM0
Dave van Ronk: https://www.youtube.com/watch?v=tcdBQwSEFi4