Stack Heights — Just the height of the shoe?
One thing that always caught my attention while looking at shoes, and I'm sure you do too, is that stack heights are always mentioned one way or another. One of the main eye-catching detail is the allure of plush, thick midsoles. Models like the New Balance Fresh Foam More and Adidas Ultraboost 21 encapsulate this feeing.
But what do these stack heights mean? Are they actually useful for the consumer or just a vehicle of misinformation? Let’s take a small look into it…
By definition, stack height is the combined height of the insole, midsole and outsole of a shoe. Manufacturers have altered this by either adding material or componentry into the sole. In Layman terms, stack heights is the distance between your foot and the ground with the shoe on, with higher stack heights meaning that you are standing on more material underneath your foot. This is largely described in the heights at the heel and at the toes.
Shoe Heel/Toe Stack Heights
Hoka One One Bondi 7 31/27mm
New Balance 1080 v 10 30/22mm
Adidas Ultraboost 21 30.5/20.5mm
Saucony Kinvara 12 28/24mm
Nike Pegasus Air Zoom 37 24/14mm
Just for structure, this article is done more of in a FAQ manner, answering questions that I would think we would have about stack heights. The angle of how I talk about stack heights will be more from the point of how high stack heights affect us. Although not exactly interchangeable, this article will also be referring to shoes with high stack heights as maximalist shoes and it could be argued that I am talking about maximally cushioned shoes instead. There is no academic definition of maximalist shoes, but it is largely characterized as shoes with high stack heights and with great cushioning. An example of one of these shoes is the Hoka One One Bondi 7.
Expecting this to be a very wordy one, here is the Too Long Didn’t Read version:
The TLDR version:
- Higher stack height in shoes do not reduce impact forces while running and do not necessarily mean they are softer.
- However, it could distribute forces more evenly across the foot, and possibly reduce localized forces acting on the foot.
- In some sense, it could reduce injury to the surface of the foot (such as to the skin or other trauma-related injuries) but does not reduce musculoskeletal injuries (such as muscle sprains or bone stress fractures).
- Shoes that are built with higher stack heights and light can be more comfortable whilst running.
- Shoes with high stack heights can be ideally used for more easy day runs and for walking. Clinicians can consider to recommend shoes with higher stack heights to patients with fragile skin on the foot or just healed lesions.
- Do not totally change from a shoe with low stack height to high stack height or vice versa, immediately. Do it gradually.
Anything we have to know about stack heights?
Firstly, reported stack heights on websites are not iron clad accurate. Different companies have different standards of measuring stack heights. What’s more is that different batches of shoes coming from the same model can have their discrepancies in stack heights. Hence, we can never be too sure what the actual stack height of your shoe is unless you measure the stack height for yourself. (Even then, where do you measure it from?)
It is also hard to measure the effectiveness of stack heights between different shoes. If we look at 2 different shoes with the same stack heights, they probably will feel and perform differently. This has to do with the weight of the midsole material, the type of material, the componentry built into the midsole and onto the outsole and presence of flares or other structural additions. Stack heights should not be treated as an individual property except for limited instances, and should always be taken as part of the understanding.
Does higher Stack Height mean it is softer?
This is a hard question to find an answer to because firstly, what is “soft”?
My answer is: “No”.
But that is if you are measuring softness by how much of the material you can press down while running.
The reasons being is that, that is due to the stiffness of the material itself and the area of contact between your bare foot and the shoe. Both, which do not have much relevance to stack height.
In that vein, an increased stack height might actually increase the longitudinal bending stiffness of the shoe (Basically a property of “stiffness” used in a different context, and hence not to be confused with material stiffness). This could make quick lateral movements, such as skirting a clueless dog, a little harder.
However, this can be managed with the integration of flex grooves, seen in Trail Shoes, or a hindfoot to forefoot linking componentry, seen in the Adidas Torsion System. Some interesting research has also looked at how increased longitudinal bending stiffness can alter running mechanics for the benefit of the runner.
But if you are talking about the perception of softness and comfort, I would say: “Maybe?”.
This, I believe, is influenced by 2 things: The actual impact force experienced by the foot and the spreading of forces throughout the foot’s contact surface.
The impact forces from striking the ground, travels through the outsole, midsole, and insole then to the sole of your foot. As it travels through, energy is loss through different modes such as heat (Which is produced when cells making up the material rub against each other). This dissipation of energy is hypothesized to be proportional to the dampening coefficient of the material and the amount of material the force has to go through. A higher stack height means more material under the foot and as mentioned, this can increase the dissipation of these forces, possibly leading to a less overall magnitude of force acting on the foot. This perception can also be enhanced with that recoil effect of the midsole material as it bounces back to shape upon compression.
Increased stack heights can also distribute the impact forces more evenly across the foot. The geometrical construction of the shoe’s midsole can help in this aspect, and this phenomenon might be even more enhanced with more midsole material. Additionally, shoes with increased stack heights generally have greater outsole widths, possibly increasing the contact surface where the impact force from the ground hits the shoe, helping to further dissipate energy evenly across the shoe and foot.
However, this increased perception of softness and comfort does not mean our body receives/absorbs less force.
What do you mean? Doesn’t shoes with huge heights of cushioning supposed to lessen the impact of running and decrease musculoskeletal injuries?
The big mystery in running is that we do not land with the same force while wearing shoes of different stack heights. They key here is that we might be striking the ground harder or absorbing more impact forces up the leg with maximally cushioned shoes. Without getting too much into the jargon, research has been churning out results that show maximally cushioned shoes do not significantly decrease impact forces and instead might increase them.
Hannigan and Pollard (Hannigan and Pollard, 2019) found that maximalist running shoes were shown to increase average vertical loading rates and Ground Reaction Force impact peaks. Loading rates were also reported to be increased in downhill running (Chan et al., 2018). A recent study in Nature by Kulmala (Kulmala et al., 2018) also found that maximalist shoes as compared to conventional shoes was seen to have an increase in GRF impact peak forces and loading rates, with a greater difference seen at faster running speeds. (Loading rates are another type of variable researchers use to measure forces whilst running)
However, these results do not paint a full picture on the actual influence stack heights are on impact forces. Current research methodologies are divided on the cross effects of running speed, running patterns, presence of other features such as rocker bottom soles and heel-toe drops and on the actual variables being measured. There are not many papers out there with just stack height as the only independent variable. But what we can say with a strong confidence is that shoes with high stack heights do not necessarily provide a reduction in forces as compared to wearing ,well, other shoes of different heights. The overall meaning of this paragraph is encapsulated in the systematic review done in 2013 (Fong Yan et al., 2013).
This also does mean that because it doesn't see a reduction in impact forces being absorbed by the foot, it doesn’t mean that wearing shoes with high stack heights will decrease musculoskeletal injury risk.
Then what do shoes with high stacks do for us?
Possible uses for Runners:
Running with shoes with high stack heights are not inherently bad. What we do know is that increases in measured impact forces whilst running is not definitively going to give you an injury. Research has only seen an association and it is with a high variability amongst different people.
As mentioned above, shoes with high stack heights can reduce the localized pressures acting on the sole of our foot. Along with the perceived comfort we have from running in such shoes, it can make certain items in your training schedule more enjoyable/palatable. Recovery runs or any distance at an easy pace (or less than tempo) could be more suited for shoes with higher stack heights. If you binge watch shoe reviews like me, you will often hear them recommend highly cushioned maximalist shoes for when their legs are beat up or when they go for their easy day runs. Comfort is the main selling point for shoes with high stack heights and it can be used strategically as a runner.
Prescription of these shoes should also be with the caution with runners that have problems with their Achilles Tendon, Tibia or the range of motion of their Big Toe. Research articles from Hannigan (Hannigan and Pollard, 2020) has shown that maximalist shoes cause a greater eversion at toe off, which restricts the foot from utilizing a strong lever for effective toe-off and propulsion. (Although this might somehow be influenced by the low drop of the shoe instead).
Possible uses for Clinicians:
Borrowing the idea of reduced foot pressures with shoes with high stack heights and that the differences in impact forces are not that different at slower paces of running, shoes with higher stack heights can too be recommended to patients with a high susceptibility to digital ulcers or have a lesion that has healed or close to completely healing. This effect can also be enhanced with slotting an insole made of Poron 92 or Poron 4000.
It would be ideal if more research is done to find out how these shoes can affect walkers and for specific patient groups. A great outcome measure would be to find out whether the peak plantar pressure are actually significantly different between shoes with high and low stack heights.
However, higher stack height can introduce a slight instability while walking. Shoes with exaggerated stack heights should thus be advised with caution if it is to the more advanced in age or those who are not stable ambulating. (Shoes that use stiffer midsole materials or have full contact outsoles could counter this problem).
A word on Comfort
It has been shown that comfort, decreased muscle work and a lower incidence of injuries are correlated in running. Nigg’s 2015 comfort filter paradigm encapsulates that and is a recommended reading.
My personal thoughts.
The bottom line about stack heights is that they do not tell us what we want it to tell us. What I mean by that is that, shoes with high stack heights do not necessarily reduce the impact forces whilst running and hence, do not decrease injury risk (Although it doesn’t mean it increases injury risk either). But it does tell us some things.
On its own, it allows us to know that we are not drastically changing the kinetics of how we run, which could lead to possible injuries. Never should we do wholesale changes on shoes, and if a person wants to run with a shoe with a much higher stack height as before, they should gradually build up to it. Moreover, the value allows us to change the types of input we receive while we run, which is related to reducing the occurrence of running related injuries. (Do read my first article).
As we look at the shoes available in existence now, stack heights, by adjusting the amount of midsole in shoes, work with other features such as the drop of shoes and shoe componentry (Carbon Plates) to produce certain benefits for different circumstances. Examples include how minimalist shoes reduce work at the knee or how good performance shoes like the Vaporfly 4% are on races.
Hence, shoes with high or low stack heights are here to stay and is one property of a running shoe that is invaluable. With how shoes are made these days, a shoe with a high stack height can be more comfortable for the wearer. But the thing we need to consider is that these shoes usually come with a hefty weight penalty, which could increase metabolic cost, increase fatigue and negate the initial feeling of comfort as we run with these shoes. The best situation would be to find a shoe with a generous stack height and is sufficiently light for our easy day runs.
Afterword:
One can look no further and look at how the design of the Adidas Ultraboost 21 has kicked off with a much greater stack height and more Boost capsules under the foot. I expect people would like, hate, and use these shoes according to the points I have brought up above. Let’s look at how it performs intently together!
Again I do not earn money from any of this and I would greatly appreciate it if you have any comments to help in my learning! I am still a student, albeit a slightly more informed one. I recognise that I might have misunderstood some areas and I would be really happy to discuss them with you and to correct/further my knowledge. Do please also look at other credible sources of information to better your understanding on this topic.
P.S: A interesting note that I have chosen to leave out is that shoes with higher stack heights can possibly decrease the impulse of the force travelling up the leg. Because I feel that we must add this with a discussion on heel to toe drops and a whole paragraph on impulses, I have chosen to leave this point out for now. This is an additional point for the comfort of shoes with higher stack heights.
P.P.S: There is a lot to add on how stack heights and drop heights can influence the runner but that is another whole realm by itself. To be fair, we can’t be sure that drop heights bring about most of the biomechanical changes whilst running in shoes with higher stack heights. It would be a dream to just clinically test out Hoka’s range of footwear and see what it does for different types of runners.
References:
CHAN, Z. Y. S., AU, I. P. H., LAU, F. O. Y., CHING, E. C. K., ZHANG, J. H. & CHEUNG, R. T. H. 2018. Does maximalist footwear lower impact loading during level ground and downhill running? Eur J Sport Sci, 18, 1083–1089.
FONG YAN, A., SINCLAIR, P. J., HILLER, C., WEGENER, C. & SMITH, R. M. 2013. Impact attenuation during weight bearing activities in barefoot vs. shod conditions: a systematic review. Gait Posture, 38, 175–86.
HANNIGAN, J. J. & POLLARD, C. D. 2019. A 6-Week Transition to Maximal Running Shoes Does Not Change Running Biomechanics. Am J Sports Med, 47, 968–973.
HANNIGAN, J. J. & POLLARD, C. D. 2020. Differences in running biomechanics between a maximal, traditional, and minimal running shoe. J Sci Med Sport, 23, 15–19.
KULMALA, J. P., KOSONEN, J., NURMINEN, J. & AVELA, J. 2018. Running in highly cushioned shoes increases leg stiffness and amplifies impact loading. Sci Rep, 8, 17496.
