When an improvised bomb on an Afghan road, tore both of his legs off, Downing's military career came to a brutal halt.
But like other American veterans being treated at Walter Reed Army Medical Center in Washington, Staff Sergeant Michael Downing said he has turned the page and is fiercely dedicated to relearning basic movement
"I waited until my son was old enough to remember who I was if something really bad happened. He's almost nine years old," the 42-year-old tattooed sergeant told AFP in his small dimly-lit room, recalling how he volunteered to be deployed to Afghanistan's Logar province.
"I knew what I was getting into when I signed up. I did my job, I am done. I have no regret," said Downing, adding that he would probably retire from the army, like 80 percent of Walter Reed amputees.
Downing was wounded in September.
"We got hit by an IED (improvised explosive device).... It went off right underneath me, I was gunning" in an armored Humvee, Downing said, adding that he was thrown from the vehicle and landed 40 feet (12 meters) away.
The Taliban then attacked his detachment. "I did shoot with my pistol to try to make them duck a little bit," he said. "During the fight, a Navy corpsman was working on me the whole time we were getting shot at.... He put a tourniquet on what was left of my left leg and gave me morphine."
Since 2001, the US-led wars in Iraq and Afghanistan have killed nearly 5,000 US soldiers, but also wounded 30,000 others, including nearly 900 amputees, most of whom have been treated at Walter Reed, the country's largest military hospital.
"Everything you take for granted, getting out of bed, going to the bathroom, cooking a meal, they teach you how to do this with your handicap," said Downing, who was awarded a Purple Heart for his combat wounds.
"You have to learn how to walk again. The walking, I am getting there, it takes time," he added, noting his five hours of therapy per day. He also had 16 surgeries after shrapnel fractured his elbow, broke five ribs and cracked his vertebra.
After enlisting in the US Army in 1985, Downing served for several years before taking a 13-year break. He then volunteered again "specifically because of September 11," 2001 attacks on US soil, he said.
But he whispered to a civilian visitor planning to travel to Afghanistan: "You don't want to go, things are getting worse."
New US President Barack Obama has called Afghanistan the central front in the fight against terrorism, rather than Iraq, and has approved the deployment of 17,000 additional troops, joining 38,000 US troops already there, to take on Taliban insurgents.
Staff Sergeant Earl Granville, 25, will also no longer return to combat. A veteran of Bosnia, Iraq and Afghanistan, he lost his left leg a year ago, close to the Afghanistan-Pakistan border.
In a corner of the large physical therapy center for amputees, between two weight machines, a dozen artificial limbs are lined up, fitted with brand-name sneakers.
Granville joined the army when he was 17 years old. "My motivation, honestly, was free college. Then 9/11 happened, and I really enjoyed the army so I stayed in."
His calf now replaced with a prosthesis, he says he is happy and was lucky.
"I was the team leader, so I sat in the passenger seat of the Humvee. And then, a major wanted my job, so he took my seat and I went for the gunner position. Then we hit an IED. I went unconscious. That major and the driver were killed," Granville explained.
"When it happened to me, I thought it was the end of the world. But when I came here, I saw people like me. It helps," he said, smiling, while a physical therapist was working with a young patient on the neighboring bed.
He went skiing in December.
"We really try to ensure that the latest technology is made available to our soldiers," said Lieutenant Colonel Paul Pasquina, a doctor and the head of Walter Reed's orthopedics and rehabilitation department.
"The guys who have lost their limbs are going to need care for the rest of their life. We need to be committed to provide that care for the rest of their lives."