Yeah, I thought of the same, a 3% year-on-year improvement must necessarily hit some ceiling at some point. The study didn't get into _how_ to achieve a 3% improvement, but from how I understand it, looked at the average year-on-year decline, stability, or improvement, over a period of 7 years.

My own take-away is that there's even more data that confirms "being fit" is about a continuous effort, and that putting in an effort (even quite minimal) to stay fit comes with a whole range of positive health benefits.