"I couldn't understand why I was laying on the ground because it happened so fast," Stewart said. "But as I reached for my rifle I saw that my legs were just gushing blood."
Ultimately, the lower part of his left leg had to be removed. But, remarkably, the experience hasn't slowed him down, reports CBS News science and technology correspondent Daniel Sieberg.
A standard prosthetic has allowed him to get around and even practice Ju-Jitsu. After leaving active duty, Stewart is now a student at Columbia University, but he wants to take his recovery a step further.
"I've gotten used to doing kind of complicated things with it," he said. "Like I know how to jump on it, I know how to stand on my toes. there's a little brain inside of it that's trying to think when I do those things."
That little brain inside of it is the brainchild of MIT robotics professor and himself a double-amputee, who lost his legs after a mountain-climbing accident when he was 17.
"Now I'm climbing a different mountain," Herr said.
Today Herr is re-creating the subtle relationship between human anatomy and movement, muscle and balance using computer models that can be sent to the prosthetic. In a sense, the device has a mind of its own.
For Stewart a critical feature is relief from lower back pain commonly experienced by users of prosthetic devices.
"It's giving you that extra push ... or it's thinking and telling how much push you need," he said. "So that you don't have to do the extra work with your hip and back. God that feels so ... I mean, that's a big improvement."
Stewart was recently one of the first to receive the new prosthetesis as part of the MIT program, funded by the U.S. Department of Veterans Affairs.
"Our obligation is to make every effort to give them this technology so they can live very fully," Herr said.
So what's next for this technology? Stewart says he's ready to enlist in the army of the future - fusing man with machine.
"The idea that I could maybe have a prosthetic that's attached to me and that I could move by thinking ... um, gosh, I mean I grew up on G.I. Joe and the chance to be a cyborg," Stewart said.