Demonte Bolden has 20 minutes of his life for which he can’t account.

After taking a knee to the head in spring 2007, the 6-foot-6, 300-pound University of Tennessee Volunteer lay unconscious on the football practice field.

The former Tyner Academy star had dizzy spells and saw spots for most of the next week.

His concussion, which also came with headaches, was part of the price of doing business, Mr. Bolden said.

“I’m a football player; that’s my job,” he said.

Congress has convened committees to discuss the dangers of head-related injuries, especially in football. And doctors say the dangers for young athletes may be even greater.

“The younger the brain, the longer it takes to recover,” Dr. Micky Collins, assistant director of the University of Pittsburgh Medical Center’s sports concussion program, told the Chattanooga Times Free Press.

A concussion occurs when a person’s brain violently is rocked back and forth inside the skull because of a blow to the head or upper body. More than 1.2 million high school athletes play football each year in America, and about 110,000 will suffer concussions, according to the Hughston Sports Medicine Foundation. Almost 70 percent of those injuries result from blows to the side or back of the head rather than the front or top, experts say.

That study also noted football has the highest risk of concussion of any sport other than boxing.

“There’s no way to prevent it, because it’s just a violent sport,” said Eric Westmoreland, who played in the NFL for six seasons after a stellar college career at Tennessee and playing at Marion County High School. “You know that when you step out there to play. Guys are going to hit helmet-to-helmet on every play, whether it’s the linemen or whatever, so there’s always that risk.”

During a football career that began in pee-wee leagues, Mr. Westmoreland accumulated a laundry list of injuries ranging from four knee surgeries, another on his foot, six dislocated fingers, a separated shoulder and a hip contusion. He also suffered three concussions in the National Football League over a four-year span.

He continued playing after sustaining the first one, but the other two were severe enough to force him to the sideline. The second was the result of a helmet-to-helmet collision with the 260-pound running back Jerome “The Bus” Bettis, a former Pittsburgh Steeler, and the third was after being hit in the side of the head by a 320-pound offensive tackle.

“Everything went black for a second when I hit (Mr. Bettis),” Mr. Westmoreland said. “My vision went blurry, and I stumbled when I tried to walk off the field. I was dizzy for a couple of days.

“But that last (hit resulting in a concussion) knocked me out,” he said. “I woke up face down with our trainer asking if I could move. I missed the rest of that game, that week of practice and the next game. I had problems being out in the sun and had to wear dark glasses everywhere for a few days after that because of headaches, and I even got nauseous for a while.

“A lot of times you don’t realize you’re hurt because of the adrenaline,” Mr. Westmoreland said. “You don’t feel it until later.”

During a hearing before the House Judiciary Committee in October, the National Football League was criticized for its lack of research on the subject. The hearing came after an NFL-sponsored study showed a link between multiple concussions and neurological disorders. The study reported that former NFL players under age 50 are 19 times more likely to show symptoms of dementia than nonplayers of the same age.

Researchers have found brain damage in 10 deceased former NFL players that more commonly would be associated with boxers who had taken numerous blows to the head, according to the NFL study.

Just this week, Philadelphia Eagles running back Brian Westbrook’s future as a player was questioned after he suffered a second concussion in a three-week span. Doctors since have said they expect him to make a full recovery, and he has indicated he may return to action this season.

Several recognizable players at the collegiate level also have suffered concussions this season. The most notable of those is University of Florida quarterback and former Heisman Trophy winner Tim Tebow, who was knocked unconscious by a hit from a Kentucky defender in September.

The nation’s leading rusher, Fresno State’s Ryan Mathews, will be held out of Saturday’s game after suffering a concussion last week against Nevada, his coaches have said.

Earlier this month, California Golden Bears running back Jahvid Best, once a Heisman Trophy candidate, suffered a concussion after a scary fall on the back of his neck when he tried to leap over an Oregon State defender into the end zone and was knocked unconscious. His helmet was knocked off as he landed, and as he lay on the ground his left arm locked awkwardly in the air. Compounding the concern was the fact that Mr. Best had suffered a concussion the week before.

“It takes at least two weeks to recover from a concussion, but that’s on average,” Dr. Collins said. “There is a tremendous variability on how long it takes, depending on severity and the individual. And once someone has a concussion, their chances of a second one are much greater, and it doesn’t have to be as hard a blow.”