Athletic fields, like boiled eggs, have a range of hardness.

Too soft, and they slow down runners. Too hard, and they increase the risk of concussion.

Because of that, the NFL, the NCAA and researchers at multiple universities are paying attention to how, and how often, playing surfaces contribute to injuries.

An analysis of 459 concussions documented in the National Football League over the 2015 and 2016 seasons found that 19% were caused when a player’s head hit the ground.

An examination of 41,000 National Collegiate Athletic Association football injuries from 2004 to 2009, which wasn’t limited to concussions, attributed more than 7% to contact with the field.