Why runners don’t fall down on bumpy terrain

New research digs into how runners stay upright on uneven or bumpy terrain.

If you go running over a trail in the woods or a grassy field, there are countless bumps and dips in the terrain, each with the potential to trip you up. But typically, runners manage just fine. It’s a remarkable physical feat that we tend to take for granted, but researchers may have some answers.

With a specially made running track and mathematical modeling, the lab of Madhusudhan Venkadesan found that when running on uneven terrain, humans mostly rely on the body’s mechanical response for stability rather than consciously plot out their footsteps to find level ground.

Further, the researchers found that the runners were just as efficient in their movements and physical exertion as when running on flat ground.

The findings appear in eLife.

Even without occasional hazards like steep drops, runners must contend with gentler, but still uneven ground that can be destabilizing. So why aren’t trails typically littered with toppled runners?

One possibility is that visual cues allow runners to carefully observe the land to step on mostly level areas. On the other hand, running played a huge role in human evolution, particularly in how it benefited humans in hunting. That means sight cannot be devoted solely to find areas to step on; it’s also needed to watch out for the prey, trees, or other obstacles to avoid, and decide which path to take.

“Imagine running and constantly looking at the ground right in front of you to decide where to place your step,” says Venkadesan, associate professor of mechanical engineering and materials science at Yale University. “You can’t be devoting all your attention just to that problem, because your vision is needed for many things.”

Because an actual trail in the woods doesn’t have the controlled conditions necessary for a scientific study, Venkadesan commissioned the construction of an uneven trackway from a company that specializes in climbing walls.

The researchers heuristically designed different levels of unevenness of the terrain to mimic the kind of uneven trail that outdoor runners often encounter. They outfitted the 70-foot-long, 3-foot-wide track with technology that can track and measure where on the terrain the runners’ feet were landing. This included a sensor in one area part of the track to measure the forces experienced by the foot.

“With this, we could measure in a few of those steps how forces are felt by the runner,” he says. “In addition, we measured key landmarks on the body using 3D-motion capture. So we could ask, what does the center of mass movement look like? Are they meandering around and trying to find a path, or just going straight down the middle?”

Nihav Dhawale, a recently graduated PhD student from Venkadesan’s lab, developed a mathematical model of a runner who would try to find the most level path through the uneven bumps, while still matching the runner’s step length and width. This model would ask whether there’s a feasible path through the trackway that can minimize the unevenness, and thus ask whether the real runners find that path.

As it turns out, the runners weren’t choosy about where they put their feet.

“What we found was people appear to land their feet wherever they like,” Venkadesan says. “They don’t seem to care about the unevenness.”

So, in that case, how do runners manage to stay upright? The researchers found that, rather than trying to find specific level areas, the runners minimized the horizontal forces experienced when they land, and therefore used their body’s intrinsic mechanics to reduce the destabilizing influence of the terrain’s unevenness.

The mathematical modeling showed that the runners kept their legs as compliant as they could, and doing so allowed them to minimize the horizontal forces when their feet touched down (that is, the sliding, scuffing forces upon impact). The same authors had predicted in an older paper that low horizontal forces would drastically mitigate the instability associated with running on uneven terrain. As a result, the runner has several steps to make small corrections to regain full stability.

“The corrective action doesn’t have to occur in milliseconds or within a single step,” Venkadesan says. “It could occur over a few steps, and that’s adequate to maintain stability. So in a sense, we are letting the mechanical response of our body buy the brain extra time to control stability.”

Source: Yale University

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Team finds black hole ‘table for two’

Astronomers have discovered a galactic table for two—a pair of unusually close black holes that are feeding together after their respective galaxies collided.

The finding could have a profound impact on our understanding of later-stage galaxy mergers and suggests that the phenomenon of side-by-side black holes occurring during a merger may be more common than previously known.

“Relatively few dual black holes like this have ever been confirmed,” says Meg Urry, professor of physics and astronomy at Yale University and director of the Yale Center for Astronomy & Astrophysics. “This pair has the closest separation yet measured, only about 750 light years.”

Urry, part of the international research team that made the discovery, is coauthor of the new study in The Astrophysical Journal Letters and presented at the 241st meeting of the American Astronomical Society in Seattle on January 9.

A considerable body of research exists on the early phases of galactic mergers, which occur when gravity slowly draws two or more galaxies together. However, relatively little is known about the later stages. A key component in such mergers is the behavior of black holes—areas of space that have intense gravity and can grow by gobbling up gas and dust from their immediate surroundings.

For the study, astronomers enlisted a variety of powerful instruments to observe the late-stage merger of the galaxy UGC4211, located 500 million light years from Earth in the constellation Cancer. Using multiple instruments enabled researchers to observe the side-by-side black holes in different wavelengths and gather a more complete picture of the phenomenon.

Urry contributed data from the W.M. Keck Observatory’s OSIRIS near-infrared field spectrograph in Hawaii; Yale has maintained a years-long association with Keck that has yielded significant data.

“It’s super important that we can make these kinds of observations with Keck,” Urry says. “First, with Keck’s NIRC2 instrument, to survey the remnants of galaxy mergers to find hidden dual nuclei—supermassive black holes that will eventually merge—and then, in this particular case, to confirm the presence of two galactic nuclei with Keck’s OSIRIS near-infrared field spectrograph.

“OSIRIS found broad infrared lines in the southern nucleus, confirming it is certainly an active galactic nucleus, and measured the velocity offset between the two nuclei.”

Data from the Atacama Large Millimeter/submillimeter Array (ALMA)—an international observatory co-operated by the US National Science Foundation’s National Radio Astronomy Observatory—enabled astronomers to find the exact location of the two black holes in the UGC4211 galaxy. Additional data came from the Chandra and Hubble telescopes, the ESO’s Very Large Telescope, and the Dark Energy Camera Legacy Survey (DECalS) on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory.

“Simulations suggested that most of the population of black hole binaries in nearby galaxies would be inactive because they are more common, not two growing black holes like we found,” says Michael Koss, a senior research scientist at Eureka Scientific and the lead author of the new research.

Koss added that the use of ALMA was a game-changer, and that finding two black holes so close together in the nearby universe could pave the way for additional studies of the phenomenon.

Urry and her colleagues says that if close-paired binary black hole pairs are indeed commonplace, there could be significant implications for future detections of gravitational waves, as well.

“It will help us develop estimates of black hole merger rates for future gravitational wave detectors,” Urry says.

Source: Yale University

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