The Monday Shakeout: How Much Taller Can Running Shoes Get?
By Matthew Klein
In this week's shakeout Matt talks about the growing stack heights of shoes in the market. What is the impact of this taller growth? Is there a point of diminishing returns?
Only a few years ago, running shoes had distinct categories around stack height. "Stack Height" is defined as the amount of material between the ground and where the foot sits within the shoe. Some companies measure this without the stock insole while others include the insole in this measurement. Most shoes had a moderate or traditional stack height with the heel being in the higher 20mm range to low 30mm range. Maximal shoes were a separate category with the heel height above 34-35mm and the forefoot in the high 20mm to low 30mm range.
Now, the majority of standard training shoes have stack heights that would categorize them as maximal shoes. In fact, the majority of running shoes on the market now fit in the maximal category and there is now a "Super Trainer" category of shoes that have even taller stack heights than what was once considered maximal. The tallest shoes now available include the Adidas Adizero Prime X Strung 2 and the Puma Fastroid Nitro (still not sure if this is a real running shoe after 40 miles....) which both feature heel stack heights of 50 mm or more.
What Happens When Shoes Get Taller
As shoes get taller, there are several challenges that occur in regards to design. The first major issue is weight. The taller a shoe is, the more material there is under the foot which inherently brings more weight. The second major issue is stability. Although foams are getting lighter and softer, the taller a shoe is, the wider the last or shape of the shoe also needs to be for inherent stability. If a taller shoe is too narrow, it becomes unstable and a risk for serious ankle sprains (a problem seen in the original Prime X when people were not used to shoes that high, soft and unstable). So they need to be wider, which requires even more material and contributes to the weight issue. The third issue is geometry. The more material there is underfoot, the more inherently stiff the shoe is and the larger the rockers necessary to allow an appropriate transition. This can make the shoe feel quite awkward, as after a certain point the steepness of the curves may be difficult to control or may feel extremely uncomfortable.
Impact of Stack Height on Body
There are also several issues in regard to the human body. The first is the force required to control and utilize such a tall shoe. Although it was initially thought that a taller stack height would create a longer lever arm and a greater mechanical advantage, those longer lever arms also require much higher levels of proximal force and power generation (Hebert-Losier et al., 2020). Maximal rockered shoes shift the work of shock absorption and propulsion up toward the hip and knee (Hebert-Losier & Pamment, 2023). Not everyone can generate the forces necessary to move such a tall shoe and may just walk away with sore or injured hamstring/hip muscles. Another issue is how the body reacts to being on such a tall, narrow and unstable surface. Most of the maximal and super-maximal shoes are quite soft. We know that the softness and geometry can alter biomechanics and internal joint loading, with most runners landing stiffer with more cushioned shoes and the strides lengthening with highly rockered shoes. This can increase joint stress at places like the knee while increasing stress on the hip flexors and hip joint going past normal hip extension ranges of motion. Add in the instability of these softer and sometimes narrower platforms and you also have more stress at the hip and ankle stabilizers to keep the body on the platform.
A shoe is only good if it can be appropriately utilized and controlled. While stack heights are getting higher, there are limits to what the body can handle and what can be designed safely. We will likely see more super trainers and shoes over 40 mm of stack height in the coming years, but that will likely be the limit. There may be new advances, but I would bet they occur more in the foam or midsole composition area rather than stack height levels. I also hope shoes don't get any higher because some of these shoes are beginning to feel more and more like stilts.
References
Franz, J. R., Wierzbinski, C. M., & Kram, R. (2012). Metabolic cost of running barefoot versus shod: is lighter better?. Medicine & Science in Sports & Exercise, 44(8), 1519-1525.
Hébert-Losier, K., Finlayson, S. J., Lamb, P. F., Driller, M. W., HanzlÃková, I., Dubois, B., ... & Beaven, C. M. (2022). Kinematics of recreational male runners in “super”, minimalist and habitual shoes. Journal of Sports Sciences, 40(13), 1426-1435.
Hébert-Losier, K., & Pamment, M. (2023). Advancements in running shoe technology and their effects on running economy and performance–a current concepts overview. Sports Biomechanics, 22(3), 335-350.
Hoogkamer, W. (2020). More isn’t always better. Footwear Science, 12(2), 75-77.
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NEXT: The Impact of Footwear on Masters Runners