Dr. Jennifer Coughlin, an associate professor at the Johns Hopkins University School of Medicine and the study’s lead researcher, first observed the overtime work of the reparative brain cells in a pilot of the study that began in 2015. Testing four active N.F.L. players and 10 former pros whose careers ended within 12 years, Coughlin’s team found higher levels of a biomarker that increases as microglia activity does.
That chronic activity, she said, might be a sign that players are at risk of developing other problems linked to brain trauma, such as deteriorating memory, mood disorders or Alzheimer’s disease.
“We want to know whose brain is healing and why,” Coughlin said. “That could inform new treatments.”
To get more clarity, Coughlin and the researchers focused the study’s second phase on younger former players, who were less likely to have vascular disease or other indications that might independently muddy the interpretation.
Martin, who since the bullying scandal had battled depression that deepened after he left the N.F.L., wondered if football played a part. He reached out to the Concussion Legacy Foundation to learn more about any potential links, and the group pointed him to the Johns Hopkins study.