The Monday Shakeout: The Science of Sole Width and Why it's So Popular in Footwear Today
By Matthew Klein
It's one of the defining characteristics of running shoe footwear in recent years. With the rise of stack height has also come in the rise of sole width as the go-to mechanic to balance this characteristic out. Matt discusses in today's Monday Shakeout why sole width is so impactful in today's shoes.
While a great amount of attention gets paid to the foam types, rocker geometry and plates, the width of the sole is often missed. Sole width refers to how wide the bottom part of a shoe is, including both the midsole and outsole. This is an important consideration when figuring out whether a shoe may work for you.
How Sole Width Affects Shoes
There are two big ways that sole width plays a role in footwear. The first thing it affects is the stability of a shoe. The more narrow the sole is, the less stable it will be. The wider the sole is, the more stable it will be. Often when we see a wide sole, the extra width comes from sole flare. However, this is not always the case with increasingly taller midsoles. The reason that a wider width increases stability is basic physics. The more material medial or lateral to your foot, the wider the platform and the more resistance there is to motion. A narrow base is inherently less stable as there is less material to resist motion. Many stability and neutral shoes throughout the years have claimed to have stable components in their shoes, only for them to be almost or completely negated thanks to a narrow sole in that area.
The second thing sole width will impact is the stiffness of the sole. A wider sole will most often impact stability in the frontal (side-to-side) plane of motion. What little research out there also suggests that this mostly impacts the frontal plane, reducing side-to-side motion within the foot (Ma et al., 2022). Anecdotally, it can also impact the stiffness and transition in the sagittal (front to back) plane. That is why wider-soled shoes also need to be appropriately rockered to help facilitate forward motion. Areas that have more material will often have less flexibility. Areas that have less material will often have more flexibility.
Perfectly Balanced
While the focus on stability footwear has been centered on posts, sidewalls, and guide rails, sole width has quietly played a major role in shaping stability as we know it. This is true not only in stability footwear, but also neutral shoes as a way to provide a stable base that works for a larger population since it tends to be non-intrusive. As shoes have gotten taller, it's been the easiest way for companies to balance the popularity of having more cushion underfoot without doing anything too dramatic to stabilize the shoe.
Comparatively, a narrow sole will have less material to cradle the foot and less material to stop it from rolling off the platform. This is commonly executed by companies with the addition of a sidewall to a narrow platform, which may cancel any form of stabilization out. A wide sole creates inherent stability with more material for the foot to sit on and not roll off of. The heel and midfoot are common areas companies like to narrow to save weight. This type of geometry can come at the sacrifice of stability.
Instead, companies should look to keep the sole width wider while using strategically placed cutouts to reduce weight. Central guidance lines and grooves have been a common way to do this, with some sort of plate required to maintain the integrity of the midsole. Asymmetrical cut-outs can work, but adequate stiffness is needed to not overly bias the foot in one direction.
Obviously, sole width by itself is not the only factor in stability. The compliance and resilience of the midsole and outsole materials, the sole geometry, the presence of sidewalls/guiderails/posts, upper security and far more all impact the stability of a shoe. The stability is often a unique result of all these pieces and sole width plays one part in this. In summary, those looking for shoes that are more or less stable should keep sole width in mind. A wider platform is inherently more stable, whereas a narrow platform is inherently less stable. Which one will work best for you will depend on your unique needs!
References
Ma, R., Lam, W. K., Ding, R., Yang, F., & Qu, F. (2022). Effects of Shoe Midfoot Bending Stiffness on Multi-Segment Foot Kinematics and Ground Reaction Force during Heel-Toe Running. Bioengineering, 9(10), 520.
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