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The automobile is a remarkable achievement of mechanics. But in the end, it’s got nothing on the human leg.

“It’s amazing that in a car you have to have special components, an electric motor in a Prius or gas-powered engine in a Corvette. You also have to have brakes and shock absorbers,” says Rodger Kram, associate professor of integrative physiology at the University of ŷ���ڱ���Ƶ Boulder.

“Well, to walk and run we have to use all the same equipment. … (The leg) is an elegant device with an elegant structure that does all three things — motor, brake and bounce.”

So what more natural field of inquiry for scientist-runners than the locomotion of humans and animals? Kram, a long-time runner, and Kristine Snyder, who earned her Ph.D. at ŷ���ڱ���Ƶ-Boulder in 2011 and is now a post-doctoral researcher at the Neuromechanical Laboratory at the University of Michigan, aren’t content to merely admire muscles and tendons; they want to know how they work and how that information can be used to prevent injuries, develop prosthetics and more.

And in “The role of elastic energy storage and recovery in downhill and uphill running,” published this year in the Journal of Experimental Biology and co-written with ŷ���ڱ���Ƶ Ph.D. graduate Jinger Gottschall of Pennsylvania State University, they wanted to gain a better understanding of how muscles and tendons — which store elastic energy in much the way a spring compresses and stretches — work together when runners get away from the flats.

“If you run here (in Boulder), you can’t help but be interested in going uphill and downhill,” Kram says. “It’s hard to avoid.”

Using data collected from 15 healthy recreational athletes running on a treadmill, Snyder’s rigorous applied mathematical abilities, and her previous research on optimal stride frequency for runners, the study yielded results both expected and unexpected.

Snyder’s previous investigations of stride frequency — two steps, one by each foot — found that human bodies pretty much find their own natural frequency, in the range of 85 to 90 per minute.

“Some people have a fascination with steps, thinking you have to have 180 per minute. But you can just relax. You don’t have to think about what all the books say you should do when running uphill or downhill. Just do what comes naturally,” Kram says.

And here’s the not-so-surprising news about running uphill: leg muscles have to perform more work from tendons when runners are going uphill. Tendons provide the spring or “bounce” runners have at their disposal when running on even ground (think photos of Jamaican sprinter Usain Bolt, both feet off the track), but muscles have to provide more “motoring” power on an incline.

“But what’s interesting is that you can still use the ‘bounce’ at all while running uphill,” Kram says. Actually, Snyder’s calculations revealed that in uphill running, legs act as a combination motor and spring; runners are still significantly ‘springy’ when going uphill.

On level ground, “42 percent of the total work for a given step is done with elastic energy,” says Snyder, who ran cross country as an undergraduate at Bryn Mawr. On a grueling incline of 15.6 percent — equivalent to the steepest part of Boulder’s Flagstaff Road — that portion drops to 24 percent, indicating the increased work of muscles.

The results for downhill running provided some interesting insights, as well. But instead of acting as springs, leg muscles and tendons must act more like brakes and shock absorbers.

“That’s why you get so sore when you go downhill. You are absorbing all that energy,” Snyder says. Heading downward is less energetically “expensive” than running on flats or uphill, requiring less muscle power.

But the researchers found that if the decline is less than about 10 percent, you have to use more muscle work, and if it’s more than about 15 percent, the effort of shock absorption makes increasing demands on energy.

“There is a kind of sweet spot. If you are running downhill, you want that kind of angle,” Snyder says.

Hills, of course, may not be so obliging as to provide a sweet spot. But the research offers insights into everything from injury prevention to prosthetic design (ŷ���ڱ���Ƶ’s Locomotion Lab was involved in several studies analyzing the performance of amputee athletes, including Olympic sprinter Oscar Pistorius, who use blade-like, carbon fiber leg prostheses in track events). Assistant research professor, Alena Grabowski is already working on developing a motor powered prosthetic ankle for running and walking uphill, Kram says.

“Most human prosthetics are designed for walking and a small subset are good for running on flat ground,” he says. “Anecdotally, users report that they are not good for uphill and downhill. … So we can apply the biomechanics of how biological legs walk and run on hills to making artificial legs.

To see a report noting Rodger Kram’s work related to Olympian Oscar Pistorius, click .