Aug. 24: This story has been corrected.

This story was co-produced with NPR and is slated to air on All Things Considered. (Check local listings.) It was also co-published with Stars and Stripes.

A new military study has found that almost 6 percent of soldiers who took hand-to-hand combat courses at a Texas Army base were struck in the head and suffered symptoms the Pentagon says are consistent with concussions, also called mild traumatic brain injuries.

Over the last decade, hundreds of thousands of soldiers have taken such classes — called "combatives" — at bases nationwide before deploying overseas.

Researchers stress that the study is relatively small, drawing from classes at Ft. Hood with just under 2,000 soldiers. And they haven't finished the study yet. But the preliminary results have sparked concern among brain specialists inside and outside the military, suggesting that some soldiers went to war in Iraq and Afghanistan having suffered mild traumatic brain injuries in training — and might have been more vulnerable to long-term consequences from additional concussions later.

"The more hits your brain takes, the less likely it will be that you will have a full recovery," said Dr. Alex Dromerick, director of neuroscience research at the National Rehabilitation Hospital in Washington, D.C.

Retired Lt. Col. Michael Russell, who is leading the Army study, said he wouldn't comment on it until the final version is released.

Col. Carl Castro, the director of the Military Operational Medicine Research Program, which funded the study, said the final results might dictate changes to improve safety. Castro said there is no acceptable number of concussions for a training program, if there's any way to avoid them.

"Even 1 percent of soldiers would concern me," he said. "I'd say we need to do something. We don't want soldiers getting injured while training, if we can prevent it."

Mild traumatic brain injuries have been called the "signature wound" of the wars in Iraq and Afghanistan. More than 244,000 such injuries, both from explosions and accidents, have been diagnosed among troops since 2000. Reports published in 2010 by ProPublica and NPR found that because of missed diagnoses and underreporting, the true figures are likely far higher.

Most people recover from concussions quickly, but some suffer lasting effects, such as memory loss, difficulty reading and frequent headaches.

The study, which began last summer, was designed to assess computerized tests for detecting concussions, comparing the military's testing tool to another one used by professional sports leagues. (An investigation published last year by ProPublica and NPR found that because of flaws and misuse, the testing program is ineffective.)

Researchers recruited soldiers who reported concussion symptoms, and asked them to take the two tests. Their test results were compared to those produced by soldiers who hadn't been concussed within the previous six months or had no concussion history.

All soldiers must take at least 22 hours of the first level combatives course during basic training. The fighting techniques are drawn from Brazilian Jiu-Jitsu, boxing, wrestling and various martial arts.

Some troops receive additional, advanced training, but researchers say their observations suggest that soldiers at level one may be at the greatest risk for concussions, partly because of inexperience.

One of the early training routines, the "clinch and punch" drill, requires one soldier to throw a punch and another to try to avoid it by "clinching" the attacker's arms. But the "clinchers" sometimes don't know how to properly defend against the punch and get hit repeatedly in the head, researchers said.

It's especially a problem for female troops, who often are much smaller than their male opponents, said retired Army Col. Harvey Watson, a psychologist who is conducting field research for the Army study.

"I can tell you that it appears as if women in those kinds of drills become concussed — percentage wise — more often than men," Watson said. "Imagine this guy at six-two or six-three, who weighs two-hundred and ten pounds, who's given the word to punch the clincher, who's a woman of five-three or five-four, a hundred and ten, a hundred and thirty pounds. He's smacking her pretty good, in the head."

Officials at Fort Hood did not respond to multiple requests from ProPublica and NPR to observe combatives classes there, but reporters were allowed to watch advanced students learn how to teach the combatives course at Fort Benning in Georgia.

In late March, the class gathered in a large building with walls adorned with brightly colored banners displaying the names of winning regiments in inter-military fighting tournaments.

The soldiers started off with stretching and warm-up exercises, and then moved on to drills that teach kicking, punching and grappling techniques.

In one, 10 soldiers paired off, facing each other in a large circle.

"Combo two!... Get Ready!... Begin!" the instructors yelled out.

The soldiers took turns performing a series of combinations, deflecting, then delivering, kicks and punches. Most of the soldiers had already deployed to Iraq or Afghanistan or both, some multiple times. Several said they had never had to fight hand-to-hand, but said the training had been useful, especially when detaining or searching suspected insurgents.

One student, Sgt. First Class Nkosi Campbell, who deployed twice to Iraq, described an incident in which a detainee tried to escape by rushing a soldier in his command. Instead of using his weapon, Campbell said, the soldier subdued his assailant with a combatives move.

"I was impressed," Campbell said.

Instructors monitored the soldiers closely, telling them not to strike each other with full force. The floors were covered with thick mats and the class members wore padded helmets, boxing gloves and shin-pads.

Each soldier gets a checklist of warning signs that could signal a potential concussion, including disorientation, nausea, ringing in the ears, balance problems and headaches.

"This program is about providing soldiers with the tools necessary to survive combat and safely come home and be with their families," said Capt. Jason Cumiford, then the commander of the combatives school at Fort Benning and a competitor in the inter-military tournaments. "It does nothing for us to get these guys hurt in training."

Despite the safety precautions, ProPublica and NPR reporters observed a soldier get kicked in the head during a sparring match toward the end of the class. He wasn't knocked unconscious, but he looked dazed. A medic ran over to check him, shining a flashlight into his eyes, asking him to follow his finger. The soldier sat out for the remainder of the class and was later sent to a clinic for further evaluation.

Staff Sgt. James Hanson, the master trainer at Fort Benning, said that he continues to change the course to make it safer. He said he has increased the number of breaks between drills, and teaches the soldiers to mix in punches and kicks to the body with blows to the head.

Hanson and other officials at Fort Benning told ProPublica and NPR that soldiers haven't sustained many concussions at the classes there. A researcher working on the study, who wanted to remain unnamed because he's not authorized to speak publicly about the research, said they haven't gathered enough evidence to confirm that.

Dromerick, of the National Rehabilitation Hospital, said that the military must find the right balance between ensuring soldiers' safety and providing them with proper training.

"If they can become effective in the battlefield and they don't have to suffer concussions, or the concussions can be rare or less severe, then that's obviously the way to go," he said.