A man with mechanical legs is standing a few feet away, but Bruce Wiggin’s attention is fixed in the opposite direction.
There, on a monitor against the wall, an orange stick figure sways gently above a bobbing red line.
“Ready to roll, Greg?” Wiggin asks the man.
“I’m ready.”
The massive treadmill beneath Dr. Greg Sawicki’s feet begins to whir. In sync with his motion, the stick figure on the screen swings, the red line now a series of peaks and valleys.
Even with his back to the test subject, Wiggin, a Ph.D. student in biomechanical engineering at N.C. State, can see more than he ever could with his own eyes. Above his head, eight infrared cameras track a collection of marble-sized sensors on Sawicki’s right leg, rendering him as a virtual wireframe. Pressure sensors beneath the treadmill track impact in three dimensions while a force gauge in the device strapped to Sawicki’s leg measures the strength of every flex.
This is what observation looks like in the Physiology of Wearable Robotics laboratory in Raleigh, a joint project of the Department of Biomedical Engineering at NCSU and UNC Chapel Hill. By using gadgets and devices adapted from other disciplines, motion researchers here are no longer limited to measuring only what they physically observe.
“It was only five years ago that physiological measurements in this detail were first made on a person wearing a device like this,” Sawicki, director of the lab and assistant professor of biomedical engineering, said after stepping from the treadmill and detaching the spring-loaded brace from his calf. “Until then, it was like, ‘Hey look, they can walk.’ It was more like stopwatch and ruler type of measurements.”
Using the data they’re collecting, Sawicki and his team are working to create more effective orthotics for patients suffering from mobility impairments like strokes and spinal cord injuries. And as they do that, they’ll have to use their inventions to help deconstruct basic motor functions most of us mastered by the time we were just a few years old.
Learning to walk – and run
That’s no easy task, namely because the interplay between the muscles and tendons in the legs is so complicated.
Take the calf, where Sawicki and fellow researcher Dominic Farris are focused. It’s made up of three muscles, including two, called the gastrocnemius, balled up like fists at the top. Instead of expanding and contracting itself, the gastrocnemius makes your Achilles tendon do all the work, helping you push off from one step to the next.
“The muscle part of it is actually staying pretty much the same length, and it’s stretching the tendon like a catapult,” Farris said. “That’s really good, because that’s a really efficient way for the muscle to contract.”
As you come to the push-off stage, the “catapult” releases, firing back with all the energy stored from your body weight. This energy-efficient model of motion – essentially allowing the tendon to act like a spring – sets the calf apart from other muscle groups.
“You don’t normally think about a muscle becoming spaghetti to allow the spring to get out of the way and then becoming rigid to allow the body to stretch a spring,” Sawicki said. “But the calf, the ankle muscle, seems to be doing that most of the time.”
Keeping that efficiency intact may be one reason we naturally shift from walking to running when we start moving about 4.5 miles an hour. In a study published in January in the Proceedings of the National Academy of Sciences, Farris and Sawicki used a small ultrasound device originally designed as a rectal probe to take cross-sections of the calf muscle while participants switch from walking to running.
Maximizing efficiency
The imagery revealed that the faster we walk, the faster the muscle shortens. That’s inefficient – more work, less force. Around 2 meters per second, the researchers found most participants started running, prompting a change in gait that reset the calf muscle to a much slower and more static pace.
“So now the tendon is doing most of (the work) again,” Farris said. “You’re getting more bang for your buck that way.”
For some patients though, achieving efficiency like this isn’t possible. Conditions like muscle weakness on one side of the body can set normally symmetrical motion out of sync, shifting the burden to areas like the hips, where movement is more costly.
“One of the hardest things for people who have suffered a stroke or perhaps have a partial spinal cord injury or anything else that affects their gait is that they’re using a lot more energy to get around,” Farris said. “It’s super tiring for them just to go out and do their shopping or anything you or I do.”
But “bio-inspired” orthotics like the ones created in Sawicki’s lab can apply a better understanding of human motion to normalize these flaws.
Wiggin’s device, for example, uses a carbon-fiber and aluminum brace connected to a regular running shoe by a spring. Mimicking the Achilles tendon, the spring stretches when the ankle is at an angle specified by a clutch, which acts like the gastrocnemius. Accounting for its extra weight, Sawicki’s team found the device can reduce energy consumption by about 5 percent, according to their measurements of oxygen intake.
“If I were a baby boomer and I just retired, I might be used to hiking 20 miles when I was 25,” Sawicki said. “I just slap these on and expend the same amount of oxygen that I did when I was a young person going the same distance.”
Widespread use of devices like these to improve quality of life is on the horizon, Sawicki said. In the meantime, they’ll use the devices – coupled with all the observation tools at their disposal – to add to our understanding of human motion.
“We build these devices so we can learn more about human physiology,” he said. “There are certain questions that you ask that you couldn’t otherwise ask unless you use the exoskeleton as an experimental device.”
