A decade ago, runners had a method of choosing shoes that was simple, scientific, and wrong. It was all about pronation to make sure your shoe was allowing your foot to roll inward just the right amount with every step. In the midst of the upheaval of the barefoot revolution, one of the first victims was the pronation paradigm: despite two decades of increasingly chunky shoes that control pronation, runners were repeatedly injured.
The challenge since then has been figuring out what to replace it with. The temporary solution that many people (including myself) chose was a suggestion from the University of Calgary biomechanic Benno Nigg – one of the original proponents of the pronation paradigm in the 1980s – that runners should rely on what he did referred to as “comfort filter”. “The idea is that when a shoe is comfortable, your bones and joints are likely to move the way they’re supposed to, which lowers your risk of injury. It’s practical and simple, but it’s also scientifically untested and sounds suspiciously like a cop-out: we’re out of ideas, so just run into what feels good.
All of this lurks behind a recent study testing a refinement of Nigg’s idea. The study was published back in January 2020 in the journal Scientific Reports, but I only came across it recently. It comes from a group of employees at the German Sport University in Cologne, the University of Massachusetts Amherst and the Brooks shoe company. It is far from providing a definitive answer to choosing the right running shoe, but it does offer an important first step: experimental data.
The theory tested in the study is called the “habitual motion path”. This is the idea that (a) each person’s joints prefer to move in different ways, and (b) the shoe that allows your joints to move as tightly as possible, choosing this preferred path will minimize your risk of injury . It builds on Niggs earlier “preferred movement path” and was developed by several researchers, including Steffen Willwacher and Gert-Peter Brügemann from the German Sports University, Joe Hamill from UMass and Matthieu Trudeau from Brooks.
One way to understand the first claim about unique movement patterns is to move the corpse legs by running motion. Here is a graphic from a 2019 article about the usual path of motion, showing the movement of the knee joint in six cadaver legs flexed back and forth. The three axes of the diagram show three different angles that describe the movement of the knee, with each (dead) subject being a different color:
(Figure: Shoe Science)
The movement of each subject is very repeatable: the knee joint moves the same way every time it bends and stretches. The movement of each subject is also completely different from the other subjects. The proportions of your limbs, the stiffness of your tendons, and your history of movement all merge to make your knees unique. We all have our own familiar path of movement.
The second claim – that this habitual path of motion says something useful about running shoes – becomes much more difficult. First you need to decide how you want to measure the habitual path of movement in living people. To do this, they measure the joint angles while the test persons do an unloaded half-squat. The idea is that a half squat looks like you bend your knee while running, but puts quite a little strain on your joints, just like the typical everyday activities like walking and getting up from a chair that our joints have and soft tissues are presumably in Adapted to handling over time. You can compare the angles measured while half-crouching with the angles measured while running and calculate a number that indicates how far your running movement deviates from your usual path of movement. The more you deviate, so the theory goes, the more stress you will put on joint areas, tendons, and other structures that are not used to it.
In the latest experiment, Willwacher and his colleagues had 12 volunteers complete a series of three 75-minute runs under different shoe conditions. Immediately before and after the run, they had an MRI to measure the thickness of cartilage in their knees. This running time compresses your cartilage temporarily, so putting greater stress on your knee should result in more crushing of the cartilage.
The main finding of the study is that runners with a greater deviation from the usual path of movement, measured from their half-crouch, had a significantly greater loss of cartilage volume in three parts of the knee during the run. This was based on a comparison between the six runners with the greatest deviation (average 12.5 degrees) and the six runners with the smallest deviation (average 6.2 degrees), averaged over all three runs. It’s an intriguing indication that the idea of a habitual path of movement corresponds to real biomechanical effects.
It becomes even more interesting if you abandon the three treadmill attempts. One wore normal neutral shoes, either the Brooks Launch or the Brooks Glycerin. Another was in a launch with small plastic tubes that were rammed into the midsole to make the inside of the shoe stiffer. The third was in a launch with tubes that make the outside of the shoe stiffer. This is what these two shoes looked like:
(Photo: Scientific Reports)
Both altered shoes should change your path of movement. But which is better? Well it depends. Three of the runners had the smallest deviation, which should theoretically be the best choice, in the unchanged neutral shoe. Two of them performed better with the stiff outer midsole, while seven performed better with the stiff inner midsole. Viewed from the opposite perspective, six runners had the highest deviation (i.e. worst choice) in the neutral shoe, while three in the other two shoes each performed worst. There’s no obvious pattern: everyone reacts differently.
But do the changes in angle correspond to lighter loads on the knee? The results were again encouraging. When comparing the shoe with the highest deviation of each runner with the shoe with the smallest deviation, the shoe with the highest deviation produced a significantly greater cartilage compression in the medial-tibial area of the knee joint. Bingo: Proof that you should choose a shoe that minimizes the deviation from your usual path of movement.
OK, that’s optimistic reading of these results. The first major caveat is that this is a very small study with a large number of potential outcome measures, none of which is “Was this runner injured?” The assumption that changes in biomechanical variables would lead to real outcomes such as reduced risk of injury exactly led the running shoe industry down the pronation path. So this is an early proof of the principle at best, not a definitive answer.
There is also a practical question. If a habitual path of movement is the way to go, what is the practical process of trying shoes in the store? Brooks took the idea and called it Run Signature. With a tablet app, they have set up 30 running specialty stores around the world, with which the sales representative can estimate your usual path of movement and then see how much it changes in different shoes. But when I asked Trudeau, a trained biomechanic who heads Brooks Running’s Future Concepts team, he was noncommittal about the tablet system: “The scientist in me hesitates,” he said.
Even in the laboratory, it is difficult to get the joint angles properly measured. Typically, a series of markings are made on the runner, sometimes holes are drilled in shoes and clothing, and a large number of carefully placed cameras are used. Achieving adequate accuracy with a tablet is a big question, so Trudeau is more interested in the future prospects of markerless gait analysis technology. But he’s also cautious for deeper reasons. Estimating the habitual path of movement with a half-squat is “a good first attempt,” he says. However, there may be better ways to monitor joint movement in a variety of common tasks such as climbing stairs, standing from a chair, and so on. Possibly use wearable sensors to get a more holistic picture of how your joints prefer to move.
And the biggest question is whether shoes really matter. Nigg himself has argued that 80 percent of running injuries are caused by so-called training errors: essentially too much, too soon. Aside from the specific number, I think the mood is largely true. Trudeau disagrees either. He assumes that shoes are probably just a minor factor in running injuries – but he adds that they are a factor that we can control. Whatever happens to this idea, it’s important not to over-sell it. Even a shoe that fits perfectly with your usual path of movement won’t make you invincible.
As preliminary as these results are, I find them really interesting. Maybe that’s because almost all of us wear shoes, so we need a method that goes beyond color and brand to choose between them. According to Trudeau, even nigg himself sees the idea of the comfort filter as “a crutch until we can figure out how to use biomechanics”. It’s nice to know that there are some advances in biomechanics and the prospect of more. In the meantime, I’ll stick to comfortable shoes.
If you’d like to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Strangely Elastic Limits of Human Achievement.
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