I have a longitudinal data set of children from birth to 5 years. During the first year, %body fat is recorded every 3 months and then recorded every year until age 5.
We are interested in testing the association of the velocity of %body fat gained during the first year and the %body fat at year 5.
Originally I tested this by generating a variable for the change in % body fat during the first year and regressing it on %body fat at year 5.
I suspect that I can also do this using mixed models. Although I can fit the LMM. I am having difficulty constructing the needed hypothesis test. Is there a way that I can do this using marginal, contrast or some other way?

Thank you in advance,
Mario