1. How Much Force Does One Foot Take With Every Step?
Each time a foot lands, the ground pushes back up on it. In a classic experiment, twelve healthy men walked at 1.0 to 3.0 meters per second and ran at 1.5 to 6.0 meters per second over a force-measuring floor. At its peak, the upward push on one foot was about 1 to 1.5 times body weight when walking and about 2 to 2.9 times body weight when running, and it grew as the speed went up [2]. For someone who weighs 30 kilograms (66 pounds), a running peak of 2 to 2.9 times is like carrying 60 to 87 kilograms (132 to 192 pounds) on a single foot, for a moment, with every stride.
A foot built from many small bones has to hold that load without folding in the middle. It does this partly with arches, curved shapes held in place by bones and bands of tissue. Anatomy texts describe three: an inner arch and an outer arch running along the length of the foot, and a transverse arch running across its width [3]. Most of the attention goes to the inner one. So which arch keeps the foot stiff?
2. Why Does Curling a Sheet of Paper Make It Stiff?
Hold a sheet of paper by one short edge so the rest sticks out sideways. If it is flat, it flops over. Now curl the paper slightly across its width, like a shallow trough, and hold it the same way. It stays out straight. You have not changed the paper or its thickness. You have only added a curve, and the curve resists bending in the direction at right angles to it. The same trick is used with a flexible dollar bill: curl its long edges a little and the middle can push back against pressure [4].
A foot is like that sheet in one respect. If its sole were flat, it would bend like a hinge between the heel and the ball with each push-off. A curve across the foot, through the tissue linking the long bones of the midfoot, works like the curl in the paper. That is the idea behind the 2020 report in the journal Nature: the transverse arch accounts for more than 40 percent of the stiffness of the foot's length [1] [4]. It is a share of the foot's stiffness, not 40 percent of body weight, and the paper is a way of picturing the idea, not a model of a real foot.
The research team also tested curved shells, thin elastic sheets, in models. Bending a shell across its width added more stiffness than bending it along its length, whatever its size or thickness [5].
3. How Did Researchers Measure What the Transverse Arch Contributes?
Models are one thing, and real feet are another. In the foot experiments, the researchers bent feet donated from human cadavers and measured how stiff they were [4]. Cutting the ligaments that tie the foot together across its width reduced the foot's overall stiffness by about 40 percent. Cutting the ligaments along the length reduced it by about 23 percent [5]. In other words, the crosswise connections mattered more than the lengthwise ones, which fits the 40 percent figure.
There are limits. These tests used the feet of a small number of people who had died, and a foot bent on a lab bench is not the same as a living foot running down a street. The percentages are best read as results from one set of experiments, not as fixed values for every foot or every movement. Researchers also point out that in living people the transverse arch works together with other parts of the foot, so its role is hard to isolate [4].
4. If Stiffness Is So Useful, Why Does the Arch Still Flex?
A rigid foot would push off well, but it would not absorb a landing. A real arch does both. When weight comes down on the foot, the band of tissue along the sole stretches and the arch lowers a little, which helps soften the impact and spread the load [6].
The flexing may also save energy. In a 2016 experiment, people ran wearing insoles that limited how much the length-wise arch could compress, by about 80 percent. At 2.7 meters per second (about 6 miles per hour), the energy their bodies used rose by about 6 percent. But for walking, and for running up a gentle slope of 3 degrees, there was no difference [7]. The researchers concluded that the arch compressing and springing back helps lower the cost of running [7].
That number belongs to one speed and one kind of insole. It does not mean arch supports in shoes make runners 6 percent less efficient, or that the arch is responsible for the same share of energy in every situation. What the pieces suggest together is a foot that is stiff enough to push against and springy enough to take a landing, with the curve across the foot helping to supply the stiffness and the arch along the foot doing much of the flexing.
5. How Can You See the Curve at Work Yourself?
For the first test, take two sheets of paper. Hold one flat by a short edge so the rest sticks out over the edge of a table or past your hand, and watch it droop. Curl the second one slightly across its width and hold it the same way. Then try curling a third sheet in a different direction and compare. Which direction of curve makes it hold out best?
For the second test, sit on a chair, take your shoes off, and look at your bare sole. Then stand up, put your weight on both feet, and look again. Does the curve on the inside of the foot change? Do not do this on a wet floor. Everyone's feet are shaped a little differently, and a higher or lower arch is not a contest. If you want to dig deeper, search for the transverse tarsal arch and ask a librarian or an adult to help you find the original study.
When you read about numbers like these, check what each one is a percentage of. In this article, "more than 40 percent" is a share of the foot's lengthwise stiffness, and "about 6 percent" is a rise in the energy cost of running. Neither is a share of your weight.