I'm sitting here bored and I think I've figured out why Arthur may have a bad back that doesn't deal with his age.
What if his spine was broken or injured somehow and didn't heal properly. That would cause him to have a bad back like he has and would only act up after so much movement. It's not what I'm going with, but it's a reason he may have it.
Or it may be because he's just an old man, but hey, whatever you like.
