Aims: Using a combination of simulation and empirical examples we hope to explore the consequences of many of the assumptions used in the previously mentioned methods. We specifically will investigate the suitability of displacement models (vertical growth, and bone mass accrual) to characterise first derivates, i.e. peak height velocity and peak bone velocity, and second derivatives i.e. growth acceleration, and contrast the conclusions made under different methods. In addition, we intend to investigate the simultaneous relationship between changes in body composition and bone growth using cross-lagged methods.

We aim to emphasize the importance of using appropriate methods in order to make robust conclusions.

The primary exposure in all analyses are time, outcomes include height, long bone length, bone mass, fat and lean mass. Similarly confounding variables are the same as the potential outcomes, as changes can occur simultaneously.