It wasn't a joke, I didn't explain myself properly. When some people pick up their heavy suitcase after slouching for a few hours, their back locks in and the airport has to put them in a wheelchair and take care of them and their luggage. Having people walk a few minutes more actually reduces the chances of that happening.

I don't think he was joking, but rather implying that people with stiff backs could be more likely to injure themselves.