I have a male akita named Bear that started pooping in house about a year ago in another house after 10 years. Before that he would wait no matter how long. I had him checked by vet and muscles are fine. He seems to know what he is doing and will lay where he poops up against the wall. I removed carpet and cleaned and put new tile already and he found another place to lay and poop. He acts like he knows it is wrong and just does it anyway. At that time he got sprayed by a skunk and I don't think his old eye sight is quite as sharp either. He is folding one weaker back leg under and sitting on it getting stiff also. I am trying pain meds this week.

Pain meds are a good idea. My Old English Sheepdog's legs aren't the best, and when didn't want to get up, she would just go where she was. Not to say that your dog's legs are bad or anything. Try that Dr. Frank's Joint Pain Relief for dogs and cats - I saw a noticeable difference in my dog when she got up and down

And if pain isn't the problem, maybe your dog is stressed out. I think any dog would be if that happened to it.

Try the pain thing still. This sounds like how my dog was... same scenario except different type of dog. Good luck.