I guess my question is this. Neither Han's friend nor Tulip's story are scientific control group studies -- they are anecdotal. Would Han's friend or the step instructors have had a hip replacement if they chose a different line of work? We don't know that.

Just because someone is a fitness instructor and they also had a hip replacement doesn't mean that being a fitness instructor caused the need for a hip replacement.

Being overweight can lead to the need for a hip replacement.

Auto-immune disorders (like RA) can lead to the need for a hip replacement.

Childhood injuries can later lead to the need for a hip replacement.

I don't have time to find this (but I bet Zen could). What is the percentage of adults over the age of 50 (or 60 or whatever) who has to have a hip replacement? I bet it's actually pretty high.