Heavy layers of body armor, a proven lifesaver of U.S. troops, also may be an impediment to winning the fight in Afghanistan, where 17,000 additional American forces are being sent to quell rising violence.

Weighing as much as 34 pounds each, the protective vests hinder American forces hunting down more agile insurgents who use the country's rugged peaks and valleys to their advantage, according to military officials.

The proper balance between troop safety and mobility will be examined this week during a series of oversight hearings by the House Appropriations defense subcommittee. Beginning Tuesday, senior Army and Marine Corps leaders are scheduled to testify on a wide range of subjects, including force protection, readiness levels and ergonomic injuries.

When body armor is added to the assault rifles, ammunition, water and other essential gear troops are required to carry, they can be lugging as much as 80 pounds into combat. Besides moving more slowly, overburdened troops tire more quickly and are prone to orthopedic injuries that can take them out of action, the officials say.

But convincing a war-weary public of a less-is-more approach won't be easy, they acknowledge. If a commander decides the gear shouldn't be used for a particular mission and a service member is killed, there could be a backlash, said Jean Malone, deputy director of experiment plans at the Marine Corps Warfighting Lab in Quantico, Va.

"We've got to have the internal fortitude to come back and say: 'We have the data. We made the right decision. We can't guarantee you that nobody will die in this war,'" he said.

Paring down the amount of armor could actually make troops safer on the battlefield, officials say. Speed and maneuverability give them the best chance of killing or capturing the Taliban and other militants before they can set roadside bombs or get in position for an ambush.

"Being able to maneuver and fight and chase down a fleeing enemy; that's actually where your protection is (versus) armoring up and being more static," said Brig. Gen. Tim Hanifen, deputy commanding general of the Marine Corps Combat Development Command at Quantico.

The loads carried by modern American troops are equivalent to those "the medieval knight wore into and out of battle back in the year 1000 until about the 16th century," he said.

Bomb-resistant vehicles that are light and nimble enough to handle Afghanistan's primitive roads are also needed, according to Hanifen. Trucks that worked well in Iraq, which has a comparatively sophisticated transportation network, may be less suitable in harsher terrains.

As troop levels are surging in Afghanistan, so are roadside bomb attacks, according to the Pentagon's Joint Improvised Explosive Device Defeat Organization.

In January and February, 52 IED attacks in Afghanistan killed 32 coalition troops and wounded 96 more, according to preliminary figures from the organization. During the same two months in 2008, 21 IED attacks killed 10 troops and wounded 39.

Body armor has become a focus of Marine Corps efforts to lighten troop loads because it weighs so much more than the other gear. The standard kit consists of hardened composite plates inserted into a ballistic vest. The vest and plates protect the upper body from armor-piercing bullets and shrapnel.

Personal armor made of substantially lighter composite materials that are more effective than current models won't be available for several years. So the Marine Corps is looking for near-term solutions.

The Marine Corps is buying 65,000 vests called "scalable plate carriers" that weigh under 20 pounds. The carrier, which uses the same plates as the standard vest, doesn't cover as much of the torso. About 14,000 of the plate carriers have been fielded and the feedback has been positive, according to Marine Corps officials.

Over the next two weeks, the Marine Corps Warfighting Lab is conducting an experiment at Camp Pendleton, Calif., to assess the risks of using less armor. The results of the trials will help guide battlefield commanders who make the final call on what gear troops should use.