I'll go with the spine, for pretty much the same reasons except more.
And there seems to be an understanding that "bad back" is an extremely common thing that can happen to anyone. For example, when JFK was president, many of his health issues were covered up because they wanted to project the image of a young, healthy President--except it was pretty well known that he had a bad back. In other words, in the 1960's, it was considered perfectly normal for a man in his forties to have a "bad back" for some reason or another, even during an era when chronic health conditions were stigmatized.
(It probably helped that JFK's back injury was partially caused by his heroic WWII exploits, namely swimming back and forth to rescue the crew of his sunken PT boat and sometimes towing their unconscious bodies to shore with the strap of their life vest between his teeth--thus elevating his disability to the status of a heroic war wound.)