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.