Dr sport studies gross motor development in children; specifically, dr sport is interested in how children's ball throwing changes over the course of development. he is interested in studying

He is interested in studying physical development.

So, Dr Sport wants to see how these children changed and what caused them to throw the ball in a different way. This has to do with the changes in and on their bodies, and isn't connected at all with their personality, cognitive, or social development, but rather with their bodily functions.