Repeated deployments and health problems have driven much of the increase in soldiers listed as non-deployable, said Gen. Peter Chiarelli, the Army vice chief of staff. A brigade has about 3,500 soldiers.
"These are folks who had a knee problem after the first (combat) rotation," he said, "and then, finally, after the third one of humping a rucksack in Afghanistan at 10,000 feet, the doc says, 'I don't care if you're going to deploy again, the fact of the matter is you're going to (stay back until you) get your knee fixed.' "
Nearly 70% of the Army's current roster of 460,000 enlisted soldiers have been to war — half of them once, nearly a third of them twice, 13% with three combat tours and 4% deployed four times.
Although the Army tries to make up for the missing soldiers by adding those from other units, Army records from 2008 show the shortages hurt overall readiness.
When Army brigades deploy, scores of soldiers remain back for many reasons, Army data show.
Some are assigned jobs back home, such as running motor pools or conducting training, while others require additional training and will deploy later. Some are held back to meet the Army's goal of allowing soldiers at least 12 months at home before deploying.
The largest group are soldiers with health problems, Army data show. They are either temporarily sidelined for issues such as rehabilitation or surgery, or are awaiting medical review to determine fitness for remaining in the Army.
Precise numbers for the Army are not available, but between 2006 and 2008, bad backs, strained knees and other ailments increased from 1.4 million cases in the overall military to 1.9 million, according to Defense Department records.
Mental health disorders increased by 67% during that time from 657,144 cases to 1.1 million, those numbers show.
Longer recuperation times between deployments should help soldiers recover, Chiarelli said. Recently, a brigade that had 28 months to rest had only 4% of its soldiers unable to deploy, he said.
At the peak of combat activity in Iraq and Afghanistan in recent years, soldiers had only about 12 months between deployments to train, spend time with their families and recuperate. That has increased to 14 to 15 months on average, with other brigades experiencing longer periods at home.
In addition, the Army is increasing its ranks from about 500,000 when the Iraq war began to about 570,000 next year.
"With the drawdown in Iraq and the growth that we've completed, we're starting to see (time between deployments) stretch out and that's only going to help us," said Gen. George Casey, Army chief of staff.