As a child, Young-Hui Chang would watch National Geographic TV shows and marvel at cheetahs chasing down gazelles, apes swinging through trees, vampire bats slicing through the air.
After beginning his academic career in mechanical engineering, Chang’s interest in animal locomotion returned. He wrote his doctoral dissertation on how humans run in strange environments, such as reduced gravity.
Now the director of Georgia Tech’s Comparative Neuromechanics Laboratory and an assistant professor in the School of Applied Physiology, Chang studies how animals—particularly humans—move.
“As scientists, we still don’t understand a lot about how we walk,” Chang says. “A 2-year-old can do it instinctively, but from a scientific perspective, we’re still trying to grasp what the nervous system is trying to do.”
The motion of running has been compared to that of a bouncing ball, but Chang likens it to a pogo stick, bouncing up and down. Still, human legs are far more complex than the toy’s linear springs.
The human leg has “motor redundancy,” Chang says, which means there are more muscles than are needed. Because each step is different, different muscles are used every time a foot touches the ground, each muscle helping the others compensate for injuries or changes in terrain.
“We know we have a destination or speed as a goal, but what are the physiologically relevant targets that the nervous system actually monitors?” Chang asks. “Is it overall force on the ground? Is it foot position?”
To answer those questions, Chang and his team study runners on a specialized treadmill in their lab. It’s actually two treadmills, side by side, and a subject runs with one foot on each belt. The instrumented treadmills measure the force of each footstep while a motion-capture system measures muscle activity.
On the other side of the lab, the team studies the movement of rats using an X-ray system that captures video at 500 frames per second.
Typically the team brings in healthy young people—there are plenty of those to find around campus, Chang notes—to serve as control subjects. They also test out the motion of amputee subjects.
“We’ve brought in some elite people with limb loss, Paralympic caliber,” Chang says. “It’s been really impressive watching them. It’s pretty humbling, actually. They’re coming in with a prosthetic leg, and they can run circles around most of us.”
Chang hopes that a more thorough understanding of how legs move will improve rehabilitation techniques and prosthetics in the future.
One of his more interesting discoveries is that, contrary to the old proverb, most creatures learn to run before they walk.
“Most people in the field would be surprised to hear that,” Chang says. “Running is relatively simple. Most of your joints are doing the same thing at the same time. When you’re walking, joints are doing different things at different times. Different muscles are being activated. In terms of the physics … like a rolling coin, there’s some stabilization just by moving faster.”
Chang notes that baby chickens can take as long as two weeks to learn to walk, but they run almost as soon as they’re hatched.
One of the pitfalls of Chang’s research is that he’s always analyzing how he and others move.
“I do think about it a lot when I run or when I am out in public,” he says. “I’ll be at the mall and analyze people as they walk by, try to guess what’s wrong with them if I see a limp.”
He describes his research as curiosity-driven, understanding just for the sake of understanding. An upcoming project will look into the mechanics behind flamingos’ iconic one-legged pose.
“It might seem very esoteric to study these different and even strange experimental set ups, but they are all carefully thought through to add to the larger puzzle of gaining a fundamental understanding of how locomotion is controlled,” he said. “We’re making some really good strides, no pun intended.”
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