dramatic video showing josh nava stumble off the field. needing emergency surgery to stop the bleeding, swelling, and pressure on his brain. >> he looks like he's in pain. >> reporter: little is known about how hard nava was hit but the helmet he wore contains sensors able to catch that data. but a school official said the concussions. the long-term effects of multiple concussions or even just one concussion. >> reporter: so is a team of researchers from university of north carolina who determined jordan to be an at-risk player for head injury. hard hits to his head from early in the season flagging him, turning him into a subject in their three-year study aimed at reducing the risk of concussions. revealing a lot more than cara knew. >> i didn't realize in the beginning that he was the one getting more impacts than anybody else. impacts mean. >> we're trying to better understand if we can change the behavior on the field. >> reporter: dr. kevin goskowicz leads the study known as bemo, short for behavior modification. >> this is footage of