Then you'd notice something else: that he's a workout machine — in part, because he's part machine.
Halling has the world's first bionic knee. Whether it's in the gym or out for a hike near his home in rural North Carolina, the whirring electric motor is the sound of the future ... right now.
"For me to go through an airport, I'll get stopped three or four times and someone will say, 'Hey, I just have to ask, what is that, what's going on?,'" Halling says.
Now retired from the Special Forces, Halling lost his leg in Somalia when a rocket-propelled grenade hit his chopper. Hollywood summed up that battle in three words: "Black Hawk Down."
"I lost some really good friends, and I saw my missing limb almost as a scratch and me being fortunate that's all I lost," Brad says.
Since then, he's had a variety of prosthetics, but nothing like his new bionic one.
It's dubbed the "Power Knee" and here's how it works: A transmitter strapped to Halling's good leg sends a signal to a microprocessor. It then "learns" how he's moving and tells an electric motor in the bionic knee to copy that fine muscle movement.
The Power Knee knows exactly how far to pick the leg up and how far to extend the leg in front of Halling so he can walk. The motor inside is almost as powerful as his leg muscles used to be.
Researchers have applied that "smart-motor" technology to an ankle joint, too. It's capable of monitoring the ankle's position a thousand times a second.
Steven Burns needs it. He's an industrial firefighter who lost his foot in a motorcycle accident. But you'd never know it to watch him with his bionic foot now.
"You have to plug it in every night, just like your cell phone, but that's probably the only inconvenience," Burns says with a laugh.
These limbs aren't experimental. Starting this week, both will be on the market.
The price is steep — about $20,000 for the foot and close to $100,000 for the knee — though the manufacturer is confident that they will soon be covered by insurance.
But it's the beginning of the bionic age.
"It needs to get to the point where one day, someone wearing a pair of pants, could run alongside you, do everything that you do, and no one would know," Halling says. "We're getting there; we're getting there."
Getting there, one step at a time.