When you lift a heavy box, are you ever concerned about getting a hernia? Well, guess what? This is, at best, a half-truth. And it brings us back to a crucial point of distinction for interpreting science and understanding how things happen: there is a difference between causation and association. Causation makes something happen; association means that two things go together, but not necessarily that one thing caused the other to happen. Hernias and heavy boxes happen to be associated with one another, but lifting heavy boxes doesn’t cause a hernia.
The other picky-doctor detail here has to do with the definition of a hernia. A hernia is technically an opening or a weakness in the muscle wall of the belly or groin. That opening or weakness is there whether or not you lift heavy things. This opening or weakness is not there because you lifted anything. Lifting did not cause the hernia. Now, here is where the association comes in. When you lift something heavy, it increases the pressure in your abdomen. If you already have a hernia, this type of increase in pressure can cause some of the contents of your belly (a little sac of your intestines, in fact) to bulge out through the hole. Lifting can cause this bulge to pop through or become more prominent, and so this makes your hernia more obvious to you. But even if you never lifted anything or engaged in any form of exertion, that defect or opening in your muscle wall would still be there. In other words, your hernia would still be there.
Exertion is clearly not the only, or even the most important, factor involved with hernias. In two studies investigating what people think caused their hernias, it is clear that the majority of people with hernias do not think that exertion or heavy lifting was involved. In a study of 129 patients with 145 hernias, only 7 per cent reported that they thought their hernia was attributed to a muscular strain or exertion like heavy lifting. In another study of 133 patients with 135 hernias, 89 per cent reported a gradual onset of symptoms for their hernia. Of the 11 per cent of people in the study who did think that the hernia was tied to exertion, the medical examiners could find no evidence to support that claim. Lifting something heavy and discovering you have a hernia may go together, but it was probably not the lifting that put that hole in your muscle wall.