Carrying heavy combat loads is taking a quiet but serious toll on troops deployed to Iraq and Afghanistan, contributing to injuries that are sidelining them in growing numbers, according to senior military and defense officials.

    Rising concern over the muscle and bone injuries - as well as the hindrance caused by the cumbersome gear as troops maneuver in Afghanistan's mountains - prompted Army and Marine Corps leaders and commanders to launch initiatives last month that will introduce lighter equipment for some U.S. troops.

    As the military prepares to significantly increase the number of troops in Afghanistan - including sending as many as 20,000 more Marines - fielding a new, lighter vest and helmet is a top priority, Marine Corps Commandant Gen. James Conway said recently. "We are going to have to lighten our load," he said, after inspecting possible designs during a visit to the Quantico Marine base.

    Army leaders and experts say the injuries - linked to the stress of bearing heavy loads during repeated 12- or 15-month combat tours - have increased the number of soldiers categorized as "non-deployable." Army personnel reported 257,000 acute orthopedic injuries in 2007, up from 247,000 the previous year.

    As injuries force more soldiers to stay home, the Army is having a harder time filling units for upcoming deployments to Afghanistan and Iraq, said Gen. Peter W. Chiarelli, the service's vice chief of staff.

    "There is no doubt that [in] our non-deployable rates, we're seeing increase," he said. "I don't want to see it grow any more."

    The number of total non-deployables has risen by an estimated 2,000 to 3,000 since 2006, putting the current figure at about 20,000, according to Chiarelli. "That occurs when you run the force at the level we're running it now," he said.

    "You can't hump a rucksack at 8,000 to 11,000 feet for 15 months, even at a young age, and not have that have an impact on your body, and we are seeing an increase in muscular-skeletal issues," Chiarelli told reporters last month.

    The top U.S. commander for eastern Afghanistan, where the bulk of U.S. troops in the country operate, has issued a formal request, known as an operational needs statement, for lighter body armor for troops there. The new equipment, called a "plate carrier," would protect vital organs and weigh less than 20 pounds. It would not include additional pieces that troops currently use to shield sides, shoulders, arms, the groin and other areas - pieces that, with a helmet, weigh about 35 pounds.

    Commanders would determine in what circumstances troops could wear the lighter gear, which would make it easier to maneuver when pursuing insurgents over rugged terrain at high altitudes.

    "Our dismounted operations are occurring at very high elevations, 10,000 feet and higher, where the air is thinner and it is difficult already to maneuver. You add to that body armor, ammunition and the full load that soldiers carry - it is difficult," said a military official familiar with the request. "You are operating against an enemy that is very agile - running around in tennis shoes, if that - and they are fleet of foot and can move faster and elude us," said the official, who spoke on the condition of anonymity because the request had not yet been approved.

    Pietro Tonino, chief of sports medicine at Loyola University Health System in suburban Chicago, agreed that the loads troops carry would "absolutely" predispose them to muscular-skeletal injuries over time.

    "They will get stress fractures or overuse injuries of the back, the legs, the foot," Tonino said. "Recruits get these stress fractures in their feet all the time just from walking."

    The military has added to its protective gear in recent years to guard against improvised bombs and other threats common in Iraq and Afghanistan, but that has come with a trade-off, as soldiers and Marines routinely carry more than half their body weight into combat.

    Individual Marine combat loads - including protective gear, weapons, ammunition, water, food and communications gear - range from 97 to 135 pounds, well over the recommended 50 pounds, a 2007 Navy study found.

    In Afghanistan, soldiers routinely carry loads of 130 to 150 pounds for three-day missions, said Jim Stone, acting director of the soldier requirements division at the Army Infantry Center at Fort Benning, Ga. In Iraq, where patrols are more likely to use vehicles, loads range from 60 to nearly 100 pounds, he said.

    "It's like a horse: We can load you down, and you just don't last as long," Stone said.

    Injuries - the bulk of them muscular-skeletal - are the main cause of hospitalizations and outpatient visits for active-duty Army soldiers, leading to about 880,000 visits per year, according to Army data. The injuries include sprains, stress fractures, inflammation and pain from repetitive use, and they are most common in the lower back, knees, ankles, shoulders and spine. They are one of the leading reasons that soldiers miss duty, said Col. Barbara Springer, director of rehabilitation under the Army surgeon general.

    The overall injury rate for active-duty soldiers has increased slightly to 2.2 injuries per soldier each year, according to Bruce Jones, director of injury prevention at the Army Center for Health Promotion & Preventive Medicine at Aberdeen Proving Ground.

    Jones confirmed that soldiers "are now carrying heavier loads on our back, so there is a greater opportunity for overuse injuries." And with the rapid pace of deployments, he said, "you get a chronic back injury, then you don't recover before the next cycle.... You have to go back to theater 100 percent fit," able to wear the life-saving armor every day.

    Sgt. Waarith Abdullah, 34, is struggling to recover at Fort Stewart, Ga., from a lower-back injury that he says was caused by the strain of wearing body armor for long hours each day during three deployments to Iraq and Saudi Arabia.

    Abdullah's injury flared up painfully during his most recent 15-month deployment to Balad, Iraq, where he had to maneuver to search vehicles and stand for 12-hour shifts in guard towers.

    "That takes a toll on you, because you have to maintain your center of gravity wearing all that stuff and doing your job," said Abdullah, of Miami. He wore a Kevlar helmet, body armor with four plates, a throat and groin protector, and shoulder pads, while carrying 10 pounds of ammunition, a rifle, a flashlight and other gear.

    "At times, I did think the equipment we were wearing was heavier than usual, but I'm a soldier and I still do my job," he said. "I think it could be lighter and stronger at the same time."

    During the deployment, Abdullah was allowed to go without armor for 30 days, but the pain returned when he started wearing it again. He returned last July to Fort Stewart, where he is in physical therapy. He is still unable to wear armor but hopes to recover in time for his next deployment.

    Maj. Neil Vining, an orthopedic surgeon at Winn Army Community Hospital at Fort Stewart, said many of those sidelined have debilitating lower-back pain. "If their condition makes them a danger to themselves or others, if they couldn't wear their armor or extricate themselves or others from danger, then they are non-deployable," he said.

    After two tours in Iraq, Staff Sgt. James Otto, an Army mechanic, has undergone nine months of physical therapy, traction and medication for back pain. He hopes that in three to four months he will be able to wear his vest again and switch to a different job so he can stay in the Army. In November, an Army board gave him a six-month probationary period in which he has to prove he can "wear the vest and shoot a weapon again," he said.

    Further evidence of the frequency of the injuries, which have forced some to leave the military, has come up in studies of veterans.

    Carroll W. McInroe, a former VA primary-care case manager in Washington state, said he has seen such injuries in hundreds of veterans from today's wars. "Our infantry should not be going into battle carrying 90 to 100 pounds on their backs," he said. "The human muscular-skeletal system is simply not designed for that much weight, and it will break down over time."

    Army experts say some units are adopting more strenuous exercise routines to prepare soldiers for the strain.

    At Fort Drum, N.Y., the 10th Mountain Division readies its troops for Afghanistan using aggressive strength training. Command Sgt. Maj. Joe Montour said the training, which involves pull-ups and other drills while wearing full body armor, helped reduce injuries by 45 percent.

    Also, the Army is now deploying a physical therapist with most active-duty combat brigades, said Lt. Col Nikki Butler, a senior rehabilitation specialist. And the Army recently held its first two-day summit devoted to tackling the issue.

    "We refer to soldiers as tactical athletes," Butler said. "You want to help take care of them early so they can get back in the game."

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    Staff researcher Julie Tate contributed to this report.