Last weekend, I began to understand why some football players have brain injuries that can be as severe as combat vets.

I was the guest on a three-hour, live radio broadcast from Bozeman, Mont., and I was discussing the study out of Boston that found neurodegenerative brain diseases among former soldiers and athletes, but none among a control group that had no history of concussions or traumatic brain injury. One small study showed that any brain injury could increase the risk of chronic traumatic encephalopathy. Another larger study showed that professional football players were four times more likely than the general population to suffer from CTE.

Then the station got a call from Corey Widmer, and we spent the next half hour discussing his eight-year career as a middle linebacker with the New York Giants … and the number of times he got his bell rung.

“I was pretty much in the trenches for the whole time,” Widmer told me. “The middle linebackers typically aren’t covered, so you’ve got to go over the guard who tends to be a pretty big guy. The impacts are a lot more than any other position except for fullback.”

Widmer said linebackers average about 240 pounds while the guards were averaging 310 to 320 pounds.  “We’d run at each other for 10 or 15 feet, picking up some pretty fair momentum, and then collide,” he said. “We got some of the biggest impacts.”

At Montana State University, where he’s currently in the Hall of Fame, Widmer was taught to tackle with his head up. “I always got dinged around. I remember my rookie year, I hit a guy from Kansas State and we both got knocked out. We were sitting there staring at each other and waiting to see who could get up first,” said Widmer.

That didn’t change when he played professional football. “When I hit, I hit with my face, and I would literally bend the steel face mask every game,” he said. “Sometimes it would pancake the whole helmet.”

The Rydell helmets had a plastic lining that would freeze rock-solid in cold weather, he said. That was common in Montana and New York. And the turf used to freeze in Giants Stadium, so when his helmet hit the ground, his brain took another bruising.

That’s what happened when Widmer suffered a concussion so severe that the coaches held him out of the game and sent him to medical specialists for a battery of tests. Afraid of losing his job (and a million-dollar paycheck), he stormed out of the doctor’s office. Shortly after that, he found himself in the middle of a fight on the practice field.

“It was at that moment that I noticed something had changed,” he told me. “I felt more aggravated at times. It was then that I became a complete believer that concussions can change personalities.”

Widmer misses football these days, the adrenalin-charged games, the 18-hour days leading up to each game, and the responsibility for never letting his teammates down. Today, he’s looking for something as fulfilling and exciting as football.

“I used paragliding to fill the void,” he said., “It’s a pretty extreme sport.”

But five years ago, Widmer had an accident in the Chilean Andes and broke his back. While he’s back on his feet again, he needs another challenge.

Sound familiar?  This echoes the stories that scores of vets have told me about how they felt in combat and the letdown they felt when they finally got home.

“Football is basically controlled combat,” neuroscientist John Medina, author of the best-selling “Brain Rules,” told me the other day.   Widmer agrees. And the NFL is looking for ways to protect its athletes better.