I did my undergrad engineering capstone project at the beginning of this year creating a linear accelerator to subject networks of mouse brain cells to repeated 50g acceleration loads, based specifically off of football helmet impact data.

I was only assisting the PI running the research so I hadn't read all of the literature, but from what I know the jury is out on a good model of risk from repeated head impacts. We can tell you pretty well what the risk is for single impact events, but expect a few years for the first characterization of repeated trauma to be published. This is based on my lab's timing of course - I'm not sure how far along other labs are with this.