The arch
running across the width of the human foot might be a big part of the reason that
people can walk and run upright, a new study suggests.

People have
a prominent arch along the insides of their feet from ball to heel — a
structure that helps make feet stiff to withstand forces on the foot caused by walking
and running. But there’s another, less obvious arch. Bones in the middle of the
foot, called metatarsals, are arranged in a curve across the foot’s width. This
bend, called the transverse tarsal arch, stiffens the foot lengthwise and may have evolved more than 3.4
million years ago
,
a step toward ancient hominids gaining the ability to walk and run on two feet
unlike other primates, researchers report February 26 in Nature.

Scientists knew that the arch on the inside of the foot, called the longitudinal arch, makes the foot more rigid, thanks to the arch’s shape and elastic tissues stretching beneath it like a bow and string. How much the bend across the metatarsals helps make feet more firm was unknown.

Foot bone diagram
Human feet have two arches that help people walk upright. One arch runs along the inside of the foot (red dotted line), while the other, the transverse tarsal arch, crosses the top of the foot (solid blue line).M. Venkadesan
Foot bone diagram
Human feet have two arches that help people walk upright. One arch runs along the inside of the foot (red dotted line), while the other, the transverse tarsal arch, crosses the top of the foot (solid blue line).M. Venkadesan

The role that the transverse
tarsal arch plays in foot stiffness is like what happens when a piece of paper
is somewhat curled. “Hold [a dollar bill] with your fingers at one end of its
length, and it flops down,” says Madhusudhan Venkadesan, a mechanical engineer
at Yale University. “But press down with your thumb to slightly curl it along
the width, and the bill will stiffen and become straighter.”

Knowing how
human feet evolved to walk and run could help experts design better prosthetics
or treat people with flat feet. Most prosthetic feet, for instance, are
designed for walking. Those who want to run need something different — a stiff
prosthetic shaped like a blade. 

The study
is “exciting for those of us that live and die for foot evolution,” says
Patricia Kramer, a paleobiomechanist at the University of Washington in Seattle
who was not involved in the work. The study is an excellent example of combining
traditional biological anthropology with engineering principles to better
understand aspects of the foot, she says.

Venkadesan and his colleagues
tested the stiffness of three types of curved structures, including a thin
sheet, mechanical structures mimicking feet and two feet from cadavers, and
found that a transverse arch makes materials more rigid. When the researchers
cut tissues between the bones that comprise the transverse arch in cadavers,
the foot’s stiffness went down by up to 54 percent.

While it’s clear that the
transverse arch is important for foot stiffness, exactly how important will
need further study, Kramer says. Experiments with the mechanical mimics of
human feet, for instance, didn’t account for the parts of the foot that touch
the ground — details
that might be important in calculating the foot’s overall stiffness, she says.
“A modified human foot does not tell you how a hominin foot worked, but it
could be used to validate a model that then is modified to represent a hominin
foot.”   

The researchers also examined
skeletal remains and fossils from ancient human ancestors, searching for the
first appearance of curved foot arches. Nonhuman primates like chimpanzees — and probably their last shared
ancestor with humans — have
much flatter feet than people do. But around 3.4 million years ago, a humanlike transverse tarsal arch had developed in a foot from an unknown human ancestor (SN: 3/28/12).

The appearance of that central arch “probably played a big part in us becoming bipedal,” Venkadesan says. The structure “adds an essential and missing ingredient for the stiffness of human feet.”