Older cats can become quite thin and some develop arthritis so it is important to take care when grooming them.
If your cat hasn’t got much of a coat, very gently use a comb to ensure you completely remove all dead hair or knots from the coat, as if you leave them, they may produce hairballs if they ingest the loose hair. If they are short haired you can use a gentle grooming mitt on your cat’s body to help lift the shedding hair.
Although this won’t remove the knots, you should find your cat doesn't mind the mitt so much. You could also try a grooming tool called the Zoom Groom, which removes loose hair, helps with blood flow and massages and stimulates the skin to help it rebuild when you brush your cat.