I believe the BMI was created for exactly the purpose you described, to track changes across populations. It was not intended to be a diagnostic tool at the ind

I believe the BMI was created for exactly the purpose you described, to track changes across populations. It was not intended to be a diagnostic tool at the individual level. That said, I suppose if a population generally becomes fitter and increases muscle weight, the overall BMI could become unreliable for the same reason it's not a great tool at the individual level.