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This is really a stunner, because we now know the effects of a concussion are cumulative - having one concussion puts you at high risk of the next one being serious. So these guys are going into war already at risk of a serious head trauma:

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.