How Far Can Running Embrace Mechanical Propulsion?

Brands have long been tinkering with mechanical propulsion concepts. In 2013, Adidas debuted the Springblade, a shoe it said was six years in the making. In place of a foam midsole, the Springblade had 16 individually tuned, elastic polymer “blades” engineered to compress and release energy upon impact with the ground and during the push-off phase of a new stride—in a motion specific to the runner’s footstrike and the surface below—with the intent of helping propel the runner forward. It was an interesting concept but an incomplete and flawed one. While springy, the shoes were also heavy, unstable, and clunky to run in.

Christian Freschi, an avid runner who owns an aeronautical design and production company in Toulouse, France, has taken mechanical propulsion even further. A few years ago, his start-up brand, ENKO Running, designed a mechanical running shoe with a twin shock-absorbing system designed to store energy upon impact and return it at the start of a new stride.

Like a traditional running shoe, the G4.1 has a mesh upper, standard lacing system, rubber toe bumper, and reinforced heel. But set off from the bottom of the shoe is a hinged mechanism that acts as midsole and outsole as the runner goes through a gait cycle. The company claims that the shoe reduces impact by 70 to 80 percent and lasts three to four times longer than a standard running shoe. And it’s the world’s first production-model running shoe that can be tuned to a runner’s weight. The price? A cool $372. Although no validating science has been revealed, the shoes were favorably reviewed by several media outlets and bloggers.

Another possible avenue takes cues from the vast changes in adaptive equipment used by Paralympic athletes. Dr. Alena Grabowski, a physiologist at the University of Colorado, studies the interaction of physiology and biomechanics during human locomotion—specifically how assistive mechanical devices, such as adaptive prostheses and exoskeleton systems, influence walking, running, hopping, and sprinting.

One of only a half-dozen researchers on the planet who specializes in studying lower-limb prostheses for runners, Grabowski, director of CU’s Applied Biomechanics Laboratory, has dedicated her career to helping elite amputee athletes such as former South African sprinter Oscar Pistorius, aka the “Blade Runner,” and German long jumper Marcus Rehm, aka the “Blade Jumper,” address a controversial question with the potential to make or break athletic dreams: Should runners with prosthetic legs be able to compete against nonamputees?

Her research was instrumental in the case to allow Pistorius to be the first double-amputee runner to compete in the 2012 Olympics against able-bodied runners. Grabowski’s efforts showed that Pistorius’s “blade” prosthetics did not give him an unfair advantage over able-bodied runners because his prostheses pushed off with less force than a biological limb would (although many still argue that athletes such as Pistorius don’t have to contend with lower-leg muscle fatigue, don’t have to pump blood quite as far to their lower extremities, and carry less weight below the knee).

The next blade-enabled runner who could make Olympic headlines is American Blake Leeper, a 29-year-old sprinter who was born without legs below the knee. I bumped into Leeper in 2018 at Grabowski’s lab while I was sniffing around for Nike’s top-secret ZoomX Vaporfly Next% shoes, which were quietly being tested at the lab next door and would in 2019 supplant the Vaporfly 4% as the world’s most efficient marathon shoe.

As I watched him run on a treadmill at more than 25 miles per hour, his mechanics were fluid, efficient, and flowing. I captured it on slow-motion video, and when I watched the replay, I was amazed to see that his blades appeared to barely impact the treadmill deck while still producing a propulsive forward drive. In contrast to runners in shoes, there was no sloppiness or inefficient lateral rolling or settling to the deck in the way that Leeper’s foot blades impacted the surface. Instead, it was a pure smoothness that continued up through the rest of his kinematic chain as his knees, thighs, hips, shoulders, and arms moved in perfect harmony. Because he was so efficient, he could run with more power and thus much faster.

I felt as though I were watching poetry in motion and wondered aloud whether what Leeper could do on blades could ever possibly be mimicked by a runner wearing shoes. That’s the goal, Grabowski offered, but so far it’s been rare to find a runner who moves with so little energy loss while wearing running shoes because a runner’s foot interacts with the treadmill surface based on how it interacts first with the materials of the shoe. Leeper’s interface was much simpler, which led me to suspect that his blades didn’t give him an advantage as much as running shoes put most runners at a disadvantage.

It was a heady moment, especially watching as Leeper took off the blades after the workout in much the same way that a runner unlaces his or her shoes after a hard session. It all begged some fundamental questions: What is a running shoe? Isn’t it something that we wear on our feet to provide cushion, support, traction, and propulsion to ultimately promote running efficiency? And isn’t that the definition of giving ourselves a competitive advantage every time we lace up our shoes?

Along those lines, Grabowski is developing a new generation of prostheses—a type of blade—for the purpose of allowing able-bodied athletes to run significantly more efficiently. The contraptions are affixed to a runner’s legs and feet like a mechanical boot, in much the way that a prosthesis fits over the stump of an amputee. They have springy blades on the bottom to help propel the runner forward and allow greater efficiency via a standard, albeit modified, running gait. Grabowski admits that the initial prototype has many limitations—not the least of which is its weight. Each leg extension weighs about 38 ounces, more than four times the weight of a lightweight running shoe. Furthermore, it reduces the body’s ability to move dynamically and eliminates one of its primary propulsion-accentuating movements: plantar flexion in the foot. But, at least hypothetically, it starts to create a scenario for understanding how running might become more efficient, even if it’s light years from being practical and even though the IAAF, the sanctioning body for competitive running, would likely never allow it.

“I’m not convinced that it’s an advantage or that it’s better,” Grabowski says. “You lose sensory capability when you have something springy under your feet that you can’t totally control or has stiffness. It’s challenging to move in it, and balance is all over the place.”

Grabowski isn’t the only one who has thought about that concept in a laboratory setting. A few years ago, Rezvan Nasiri, a graduate student at the Cognitive Systems Laboratory at the University of Tehran in Iran, was out on a training run when he realized how much energy he wasted each time one of his feet hit the ground. He began considering ways to harness the energy before it was lost.

He and fellow researchers began tinkering in the lab with ways to couple a runner’s hips so that the energy created as one leg swings backward at the end of a stride could somehow be transferred to the other leg as it moves forward. Ultimately, the goal was to find a way to decrease the metabolic cost of running and a runner’s muscular output.

The team developed a lightweight harness for a runner to wear around the waist. The device includes metal straps affixed to the runner’s thighs just above the knee that are connected to a metal loop arching out of his or her lower back, creating a spring effect that gathers and transfers energy from one hip to the other while running.

They tested male runners on a treadmill, having them run 10 minutes at about 9 minutes per mile both with the device and without it. Running with the device proved to be much more efficient, decreasing a runner’s energy cost by an average of 8 percent. That’s a huge change, especially when compared to the 4 percent gains from the Nike Vaporfly 4% shoes that Eliud Kipchoge used to smash the marathon world record.

If nothing else, the experiment proves that it is possible to make runners more efficient through scientifically advanced accessories. But, just as with Grabowski’s device, it begs the question of where to draw the line when it comes to running shoes. Where do the limits of human propulsion end and man-made technological advances—whether midsole foams, energy-saving carbon-fiber plates, or springs—take over?

“It’s interesting, for sure,” says Kram, who studied Nike’s Vaporfly 4% and ZoomX Vaporfly Next% shoes and reviewed the University of Tehran study that was published in IEEE Transactions on Neural Systems and Rehabilitation Engineering in October 2018. “At a basic level, it’s something that allows a runner to use their own muscular energy more effectively to run more efficiently. And that’s one of the challenges that every running shoe manufacturer is looking at as it develops new models of performance-oriented shoes, too.”

Adapted from Kicksology: The Hype, Science, Culture & Cool of Running Shoes by Brian Metzler with permission from VeloPress.