Detroit Lions receiver Calvin Johnson took a helmet-to-helmet hit in a game against the Minnesota Vikings on Sept. 30. After undergoing a series of tests per the NFL's concussion protocol, medical personnel cleared him to return to the game.

Now, nearly two weeks later, Johnson admits that he did suffer a concussion.

"Our evaluation was that he was not concussed," Coach Jim Schwartz said in an interview. "He was thoroughly checked. We're very strong in our evaluation. He was cleared to go back in the game, and he was on a protocol after that, and he was cleared then. We're very strong in our evaluation, and ... as an organization, I think we have some credibility when it comes to concussions."

While the medical personnel's assessment may have been carried out correctly, the problem lies in that the criteria used for concussion evaluation is too subjective, according to a report published in the Oct. 2 edition of the Journal of Neurosurgery.

The study, part of a larger ongoing investigation into the effects of repeat head impacts conducted by Brown University, Dartmouth College and Virginia Tech, examined 450 student-athletes playing football or ice hockey at the schools using sensor-equipped helmets. The impact readouts from the helmets were compared to assessments made by trainers and physicians.

Of the more than 486,000 head impacts recorded during a five-year period, a total of 48 concussions were diagnosed. Of those, only 31 could be tied to a specific impact, and an immediate diagnosis was made just six times, with many athletes not experiencing symptoms for several hours after the incident.

"The term 'concussion' means different things to different people, and it's not yet clear that the signs and symptoms we now use to make a diagnosis will ultimately prove to be the most important pieces of this complicated puzzle," said study leader Ann-Christine Duhaime, director of the Pediatric Brain Trauma Lab at Massachusetts General Hospital. "Some patients who receive a diagnosis of concussion go on to have very few problems, and some who are not diagnosed because they have no immediate symptoms may have sustained a lot of force to the head with potentially serious consequences."

The findings suggest that concussions occurring in sports differ from other medical contexts, where the cause is more evident and the diagnosis simpler. The study's authors recommend replacing the term "concussion" within a broader concept of a "concussion spectrum" to better address the range of factors that contribute to head injuries in athletes.

Better diagnostic criteria may have helped the Lions' medical staff sideline Johnson, but the situation also sheds light on the "tough guy" image that still perpetuates sports and causes athletes to downplay potential symptoms.

"It's a part of football. You get concussed, you gotta keep on playing," said Johnson in a radio interview. It's not just football; NASCAR driver Dale Earnhardt Jr. admitted this week that he has driven in at least five races with a concussion.

In addition to the potential harm returning to action means for a concussed player (or, Earnhardt Jr.'s case, other drivers on the track), the decision sets a poor example for young athletes. In light of recent events, NFL Commissioner Roger Goodell took the opportunity to address the issue while speaking to students at Centreville High School in Clifton, Va., on Wednesday.

"You've got to make sure that if you're injured and don't feel right, if something hurts, your head or your ankle or anything else, you've got to make sure you tell your coaches and your parents," he said. "Make sure they understand what happened to you. It's not about football. There's nothing wrong with raising your hand and saying, 'I don't feel good.' "