A video of the explosion showed grass flattening under the force of an invisible shock wave, swelling in advance of the fireball that scorched the crash-test dummy.

That same shock wave rolls up inside a soldier's helmet, which actually may focus the wave's force and increase the risk of brain injury.

A computer simulation, shown by a researcher Tuesday in San Antonio, showed the blast hitting the body, then rolling into the space between the head and the helmet and curling around the cranium, building to the strongest force as it met itself at the back of the head and combining with another pressure wave on the outside at certain points on the skull.

The kind of injury such a blast causes — traumatic brain injury, or TBI — has been called the signature wound of today's battlefield, where hidden explosives often slam a helmeted cranium against a wall or roof of a Humvee and jostle the brain inside the skull.

The RAND Center for Military Health Policy Research recently estimated about 320,000 service members have experienced a traumatic brain injury during deployment in Iraq and Afghanistan.

Naval Research Laboratory scientist David Mott presented his team's research Tuesday at the American Physical Society's Division of Fluid Dynamics meeting in San Antonio.

Using sensors and computer models on standard Marine Corps light helmets, researchers from the NRL and Allen-Vanguard Technologies tracked the shock waves' path.

The sensors potentially could provide information to emergency workers after an injury occurs, or store the cumulative history of events, Mott said.

“They're thinking that having that information on what happens over an entire deployment may be useful,” he said.

It also may be useful in future helmet designs, he said, as scientists try to understand how explosions damage the brain.

Even a mild brain injury can cause headaches, insomnia, difficulty thinking and mood swings. Severe injury can result in the loss of cognitive and bodily functions and emotional problems.

Mott's computer simulation showed the blast as it moved around the head.

“You get a high pressure from that initial blast wave hitting any unprotected surface, and then you get focusing under the helmet as the blast wave penetrates the helmet,” he said.

One of the more surprising results, Mott said, was that when a shockwave hits a helmeted head from the side, it can combine with other shock waves wrapping around the outside of the helmet, and cause higher pressures to the far side.

Mott emphasized his computer model doesn't include padding between the helmet and the head, so the true effect of blasts on human bodies is likely to be different.

That padding is a crucial part of protecting the soldiers, said Dr. Bob Meaders, who runs a program out of his house in Montgomery, north of Houston, to send helmet kits, purchased through donations, to Iraq and Afghanistan.

Meaders began by sending helmet upgrades to his grandson's Marine company, he said.

“When they got into Iraq ... by word of mouth we were absolutely flooded,” Meaders said.

The padding he sends, he said, is as effective but more comfortable than what the military furnishes.

He testified before the House Armed Services Committee on the issue in 2006, and set up Operation Helmet to respond to the demand.

“We're closing in on 41,000 sets right now,” he said.

The organization is more than 350 troops behind in answering requests because money is tight, Meaders said.

But he hopes a recent interview he gave to Dan Rather will increase attention on his organization “so we can keep these guys out on the pointy end of the spear fixed up with what they need.”