But do they establish a causal arrow? If you're in chronic pain because you're dying of something you're not exactly going to be likely to do a lot of cardio w

But do they establish a causal arrow? If you're in chronic pain because you're dying of something you're not exactly going to be likely to do a lot of cardio workout. :) Without care you might conclude, for example, that rock climbing prevents becoming a quadriplegic since there aren't any quadriplegic rock climbers... when in fact to the extent that they're related at all rock climbing sometimes causes it.