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The transverse arch may be more important for foot stiffness than the longitudinal arch, researchers report.

Walking and running subjects our feet to forces in excess of body weight. Scientists have thought the foot’s longitudinal arch was why our feet don’t deform under such load. The new research, however, suggests the transverse arch may play an equally important role.

The researchers found that the transverse arch is a bigger source of foot stiffness than what researchers previously found was due to the longitudinal arch. They also discovered that the transverse arch evolved to become almost human-like over 3.5 million years ago.

This research motivates further work into the role of the transverse arches in podiatry and evolutionary anthropology, the researchers say. These insights could also inspire new designs for prosthetic and robotic feet.

The foot skeleton has a blue line showing the transverse arch over the width of the foot and a red-dotted line showing the longitudinal arch under the foot. A blue arrow under the ball of the foot skeleton is marked "ground reaction forces" and red lines going up and down at the ankle are marked "ankle forces." Top right: A black piece of paper is bent to mirror the longitudinal arch. Bottom right. A weight sits on a piece of black paper bent to mirror the transverse arch. — A schematic of the foot skeleton showing the arches and typical loading pattern. The image at the top right is a sheet of paper bending under a 5g weight. The bottom right shows the same sheet but with a transverse arch supporting 500g of weight. (Credit: U. Warwick)

The role of the transverse arch may be understood in simpler terms by looking at a thin paper sheet. When you hold the short edge flat, the sheet is floppy and droops under a little weight. But curl the edge a little and even 100 times as much weight is not excessive.

“Flat thin objects like paper sheets bend easily, but are much difficult to stretch,” says Shreyas Mandre from the math department at the University of Warwick. “The transverse curvature of the sheet engages its transverse stretching when attempting to bend it. This coupling of bending and stretching due to curvature is the principle underlying the stiffening role of the transverse arch.”

Because the foot serves multiple mechanical functions, however, its structure is more complicated than the paper sheet. Therefore, “flattening” the foot to test the hypothesis of curvature-induced stiffening may have unidentified confounding variables. To overcome this difficulty, the researchers disrupted the underlying principle while keeping the transverse arch intact.

“Understanding of the underlying principle enabled us to build mechanical mimics of the foot comprising springs that imitated the elastic tissue of the foot. Disrupting the transversely oriented springs in these mimics had the same effect as flattening them,” explains coauthor Ali Yawar of Yale University.

“We disrupted the underlying principle of curvature-induced stiffening in human cadaveric feet by transecting the transverse tissue, which reduced the mid-foot stiffness by nearly half,” says coauthor Carolyn Eng of Yale. In comparison, experiments in the 1980s on disrupting the stiffening mechanism due to the longitudinal arch only showed a reduction in stiffness by about 25%.

This research also injects new interpretation of the fossil record of human ancestral species, especially pertaining to the emergence of bipedalism. The researchers formulated a measure of the transverse arch to account for variations in the length and thickness of the feet. They used the measure to compare related species such as the great apes, human ancestral species, and some distantly related primates.

“Our evidence suggests that a human-like transverse arch may have evolved over 3.5 million years ago, a whole 1.5 million years before the emergence of the genus Homo and was a key step in the evolution of modern humans,” explains coauthor Madhusudhan Venkadesan.

It also provides a hypothesis for how Australopithecus afarensis, the same species as the fossil Lucy, thought to not possess longitudinally arched feet, could generate footprints like humans researchers discovered in Laetoli.

The work appears in Nature.

Funding for the work came from a Young Investigator award by the Human Frontiers Science Program. Additional researchers from the University of Warwick, the Nonlinear and Non-equilibrium Physics Unit at the Okinawa Institute of Science and Technology Graduate University, and Yale contributed to the work.

Source: University of Warwick

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Prosthesis lets amputees feel knees and feet

A new interface that connects a leg prosthesis with the residual nerves in the user’s thigh allows above-the-knee amputees to feel their prosthetic foot and knee in real time, a new study shows.

While walking, people with intact legs feel when they move their knee or when their feet touch the ground. The nervous system constantly draws on sensory feedback of this sort to precisely control muscles. People using a leg prosthesis, however, don’t know precisely where the prosthesis is located, how it is moving, or what type of terrain it is standing on.

They often cannot trust their prosthesis completely when walking, leading them to rely too often on their intact leg, which in turn reduces their mobility and causes them to tire quickly. A simple walk on pebbles or sand, for example, can prove very exhausting for people using a prosthesis.

Furthermore, people with amputations can experience phantom limb pain, a condition that existing medications often cannot treat.

Savo Panic, a volunteer fitted with the device, says he wakes up at night due to the phantom pain: “The toe that I don’t have hurts. My big toe, foot, heel, ankle, calf—they all hurt, and I don’t even have them.”

The new prosthesis benefited the amputees in a variety of ways, the researchers report in Nature Medicine.

“This proof-of-concept study shows how beneficial it is to the health of leg amputees to have a prosthesis that works with neural implants to restore sensory feedback,” says Stanisa Raspopovic, a professor at the Institute of Robotics and Intelligent Systems at ETH Zurich.

Tactile and motion sensors

To provide the nervous system with sensory information, the scientists began with a commercially available high-tech prosthesis: they attached tactile sensors to the sole of the prosthetic foot, and collected the data on knee movement provided by the prosthesis’s electronic knee joint.

For the three months that the experiment lasted, surgeons placed tiny electrodes in each volunteer’s thigh and connected them to the residual leg nerves.

“The goal of the surgery was to introduce electrodes in the right places inside the nerve to allow the restoration of lifelike sensory feedback, and to allow the stability of the electrodes,” says Marko Bumbasirevic, professor and orthopedic microsurgeon at the Clinical Centre of Serbia in Belgrade, who was the clinician responsible for the electrode implant. Scientists at the University of Freiburg developed the electrodes and the prosthesis came from the prosthetic company Össur; both actively involved in the project.

The research team developed algorithms to translate the information from the tactile and motion sensors into impulses of current—the language of the nervous system—which went to the residual nerve. Then nature does the rest: the signals from the residual nerves are conveyed to the person’s brain, which can sense the prosthesis and helps the user adjust their gait accordingly. The machine and the body are finally connected.

Walking on sand

As part of the study, the volunteers underwent a series of tests—alternating trials with and without neurofeedback. The results made it very clear just how advantageous the feedback was: walking with neurofeedback was physically much less demanding, evident from the significant reduction in the volunteers’ oxygen consumption while walking.

Also, brain activity measurements during the trials showed walking with neurofeedback was also less strenuous mentally for the volunteers, who didn’t have to concentrate as hard on their gait, which meant that they were able to devote more of their attention to other tasks.

In one difficult test, the volunteers had to walk over sand—and the feedback enabled them to walk considerably faster. In surveys, the volunteers said the neurofeedback greatly increased their confidence in the prosthesis.

‘I don’t feel any phantom pain’

The interface with the nervous system can also stimulate the nerves independently of the prosthesis. Before they started the trial, both volunteers complained of phantom limb pain. Over the course of a one-month therapy program with neurostimulation, the scientists managed to considerably reduce this pain in one of the volunteers.

Panic said his pain disappeared completely. “Since I have started this treatment program, after having received electrical stimulations, I don’t feel any phantom pain.”

While the scientists view these outcomes optimistically, they point out the need for a longer investigation with in-home assessments and a greater number of volunteers, in order to provide more robust data that they can use to draw more significant conclusions.

For the time-limited clinical study, the signals from the prosthesis moved along cables through the skin to the electrodes in the thigh. This meant that the volunteers had to undergo regular medical examinations. To eliminate this need, the scientists intend to develop a fully implantable system.

“At SensArs, we’re planning to develop a wireless neurostimulation device that can be fully implanted into the patient like a pacemaker, and that can be brought to the market,” says Francesco Petrini, CEO of SensArs.

Additional researchers from EPFL, the Sant’Anna School of Advanced Studies in Pisa, the University of Montpellier, and the company mBrainTrain contributed to the project.

Source: ETH Zurich