Nearly 40 percent of NFL players last season wore a helmet model that got the second-lowest rating for reducing the risk of concussions in a study by Virginia Tech researchers, according to an ESPN.com report. Riddell's VSR-4 helmet received just one star in a study of football helmets led by Virginia Tech professor of biomedical engineering Stefan Duma and released Tuesday. Another Riddell model, the Revolution Speed, was the only helmet that earned five stars, the top rating.