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.

Most of the studies that I've seen either seem to be looking at high-acceleration impacts or are trying to quantify impacts received in "daily life". I have repeated impacts flagged in my head as "result unknown" - I haven't come across anything that I can remember that would give legit thresholds for how hard a repeated impact has to be before it would cause damage (other than the 10g figure noted above). People seem to agree that repeated impacts have the capacity for great harm, and I remember seeing that people with a certain gene appeared to be more prone to symptoms if subjected to repeated impacts, but that's pretty much all I remember. Let me know if you find anything.