Scientific American - 09.2019

ADVANCES

16 Scientific American, September 2019

SHANE KING AND MAURA EVELD

Vanderbilt University

BIOMECHANICS

Falling

for Science

A devious treadmill prompts

stumbles to study balance

A study participant walks briskly on a

treadmill, video cameras recording his

every move, when a 35-pound metal block

suddenly appears in his path. Special eye-

glasses prevent him from seeing it, and he

stumbles, lurching forward—until he is

caught by a safety harness. One trip down,

dozens more to go.

Researchers developed the treacherous

treadmill to study how people regain their

footing after tripping. They knew this usu-

ally means taking an exaggerated step that

allows the central nervous system to reori-

x³îîšxU ̧lāÜä`x³îxß ̧…ßDþžîāDU ̧þx‰ß­

footing, says Michael Goldfarb, a mechani-

cal engineer at Vanderbilt University and

co-author of the treadmill study. But “the

way you do that changes depending on

ÿšDîā ̧øßZU ̧lāÜä[` ̧³‰øßDîž ̧³ÿDä

when you stumbled,” Goldfarb adds.

People with prosthetic legs often

struggle to recover from a stumble and

thus fall at far higher rates than the gener-

al population. Understanding how people

trip and recover on two legs could help

researchers design better prosthetics.

To trigger genuine tripping, research-

ers had to deliver the heavy blocks surrep-

titiously. Goldfarb and his colleagues’

apparatus, described in June in the Journal

of NeuroEngineering and Rehabilitation,

works because it can place a heavy block

on the treadmill so delicately that partici-

pants do not perceive it until they have

already tripped. An algorithm determines

where to place the block so the research-

ers can observe stumble responses at dif-

ferent points in a subject’s gait.

The surprise is key, but participants in

this kind of study know they will eventually

be tripped up—which could confound

results, says Mark Grabiner, a biomechani-

cist at the University of Illinois at Chicago,

who was not involved in the new work.

Researchers have taken various approaches

to this problem in the past, sometimes not

even telling participants they are in a trip-

ping study, Grabiner says. The new study’s

stealthy design is “an incremental improve-

ment over existing technologies”—a step in

the right direction—he adds.

In the next phase of the study, Goldfarb

says, his team will use the tripping data to

Çß ̧ßD­ßx‹xĀžþxäîø­U§xßxäÇ ̧³äxä… ̧ß

a variety of situations in prosthetic limbs.

—Jim Daley

Researchers successfully

triggered 190 stumbles.

ECOLOGY

Footsteps

of Giants

Frog generations thrive

in elephants’ footprints

As herpetologist Steven Platt trudged

îšß ̧øšîšxäxDä ̧³D§§ā‹ ̧ ̧lxl%Dā?D

Inn wetland in Myanmar (formerly Bur-

ma) during a 2016 dry-season expedition,

something strange caught his eye: Frisbee-

sized pools brimming with clusters of frog

eggs and wriggling tadpoles.

The watery pockmarks were old ele-

phant tracks. Platt, who works at the Wild-

life Conser vation Society, realized that in

the parched landscape these puddles may

be a lifeline for the next generation of

frogs. “It made me wonder how important

these tracks—really, tiny little ponds—

might be for all the smaller things that are

out there,” he says.

Elephants are often cited as ecosystem

engineers. They knock over trees, trample

brush, prune branches and disperse seeds,

enhancing biodiversity and helping main-

tain savannas and forests.

Many researchers focus on these big-

picture impacts, but Platt realized other

important ones may be right at the ele-

phants’ feet. When he returned to the site

in 2017, he found tracks in the same spot—

and the tadpoles and eggs were back, too.

Resembling a series of frog-sized Jacuzzis,

the tracks appear to act as small breeding

sites linking together larger wetland patch-

es during the dry season, Platt and his co-

authors reported in May in Mammalia.

Such microcosms of life are probably

commonplace, Platt says, but almost “no

one bothered to look before.” A 2017 paper

published in the African Journal of Ecology —

possibly the only other study that has

xĀD­ž³xlUž ̧lžþxßäžîāž³‹ ̧ ̧lxlx§xǚD³î

tracks—supports his hunch: its authors

found dozens of invertebrate species and

îDlÇ ̧§xäž³x§xǚD³î… ̧ ̧îÇߞ³îäD³lDß-

cially created puddles in Uganda.

According to Chris Thouless, who directs

the Elephant Crisis Fund at the Kenya-based

³ ̧³Çß ̧‰î3Dþxîšx§xǚD³îäD³lÿDä³ ̧î

involved in the new research, the Myanmar

‰³lž³äDßxÙD³D­Dąž³lx­ ̧³äîßDîž ̧³ ̧…

the interconnectivity in the natural world,

between the largest and one of the smallest

creatures in the landscape.” But habitat loss

and poaching threaten elephants throughout

their range, Thouless says, and scientists do

not know whether frog populations will

crash if elephants disappear from the land-

scape—or whether “new ecological relation-

ships will develop that re-create at least part

of the lost complexity of the system.”

Platt guesses that at least some of that

complexity is irreplaceable. “As the ele-

phants go,” he says, “probably a lot of

relationships we don’t even know any-

thing about at this point go, too.”

— Rachel Nuwer

Illustration by Thomas Fuchs