Earlier quoted context omitted.
Devil's advocate: Wouldn't a person who prefers sitting be more likely to keep a job as a driver rather than conductor? An unhealthy person might think, "Ugh, I'd have to be on my feet all day." And a healthy person may not see that as a major negative.
Not everyone can be so choosey about the job they get
But some can, and maybe it's enough to impact the study. Or maybe not.