According to a Brown University press release:
The report doesn’t just count impacts overall. It also provides a breakdown by player position. The researchers measured not only how many hits the running backs or linebackers endured, but also the location the hit on the helmet. Where a hit occurs can make a big difference in how vulnerable a player is to damage from that impact.
Offensive linemen suffer nearly three times as many blows as the quarterbacks they protect, but about half the time the impact is on the front of their helmets, suggesting that they can control where their impact will be as they jump off the line at the beginning of a play. Quarterbacks, by contrast, experience blows from behind more than from any other direction. As any football fan has observed — and as Crisco’s study quantifies — when the quarterback is hit, he is often blindsided and unprepared.