By Paul Freary
What is stack height? In recent years the terms ‘stack height’ and ‘heel to toe drop’ have become commonplace in the running and running shoe community.
Stack height became a more commonly used term after Eliud Kipchoge ran the sub-2-hour marathon. The record brought particular attention to the shoes he wore, a pair of prototype Nike shoes, the Alphafly. The effect of the shoes and their deep cushioning and carbon plate attracted lots of attention, so much so that World Athletics decided to introduce new rules regarding the overall thickness of a shoe's cushioning.
The overall thickness of the shoe's cushioning, from the inner sole to the outsole is referred to as stack height. World Athletics decided that the limit for the overall stack height of a shoe (for road running) would be set at 40mm. Coincidentally, this limit was the same height as Kipchoge’s shoes!
Before this ruling, stack height wasn’t something that was mentioned with specific measurements too much. After all, you could simply look at a shoe to see how much cushioning it had.
Adidas - Prime X - MAX Cushioning, 50mm STACK, Medium DROP - 8mm
What is heel to toe drop? As I said, the stack of cushioning in the shoe is measured from the ground and includes the thickness of the sole, cushioning and any lining. It’s usually measured in the centre of the heel.
This thickness is measured in the same way in the forefoot, under the ball of the foot.
The difference in these two measurements is referred to as the ‘drop’ or heel to toe drop.
Again, heel to toe drop wasn’t something often mentioned until the dawn of ‘barefoot running’ or shoes that didn’t have any cushioning, just a thin piece of rubber. These shoes placed the foot ‘flat’ on the ground and the term ‘zero drop’ came into use.
Historically most running shoes would be manufactured with around a 12mm drop. Nowadays you will find running shoes can range from ‘zero drop’ (where the shoe has no difference in heel or forefoot cushioning) up to 12mm.
It is worth noting that the drop refers only to the difference in thickness between the heel and forefoot of the shoe, and is not an indication of the thickness, or stack of cushioning. Don’t assume that if a shoe has a very thick or cushioned midsole it will have a higher drop.
Is Heel to Toe Drop Important?
As with any claims around running shoes, it’s always best to consider fit and feel above anything else when buying new shoes.
Although you’ll hear various claims regarding the ideal drop (most probably in marketing and advertisements) any such claims should be viewed with caution. Given the significant variation exhibited across every single one of us, blanket approaches cannot be applied here.
In various studies examining shoe drop and injury rate, no clear association has been found.
Altra - Paradigm 7 - HIGH Cushioning, 30mm STACK, Zero DROP
What Drop Should I Use?
I think the most important factor when changing shoe brands or drop heights is adaptation. If you are running regularly, your body adapts to your running stride and gait.
Different shoe drops may load certain joints and muscles differently. As such, if you are currently uninjured there is no reason to change the drop of your shoe.
If you do want to try new shoes with a different heel to toe drop, then be mindful of allowing the body time to adapt to the change.
It’s also important to consider your foot strike. A heel striker would experience the biggest difference when changing between different drop shoes.
A mid or forefoot striker would notice less difference in different drop shoes as the drop is measured by the height of the heel compared to forefoot cushioning. This difference is less when landing further forward in the shoe.
An Interesting Perspective - Shoe Size and Heel Drop
As many manufacturers quote measurements for stack and drop based on a standard sample size of a US8, this can change slightly in smaller and larger sizes.
With this in mind, a 10mm drop in a very small shoe would have a greater difference than a 10mm drop in the largest-sized men's shoe with the same drop. In a smaller shoe, the angle of the drop would be steeper than the larger shoe! Okay, we are talking about small differences, but it’s worth considering if thinking about the effect of drop on your gait.
HOKA - Stinson 7 - MAX Cushioning, 42mm STACK, Low DROP - 5mm
Shoe Types - Low to High Stack
The market currently has a wide variety of shoes available from low to high stacks of cushioning.
Generally speaking, training shoes have been typically, what would now be referred to as medium stack.
Road racing shoes, before the recent rise of the ‘super shoe’ or carbon plated shoes were usually considered to be lower stack or more minimalist shoes, with light weight being their most important consideration.
Today most daily training shoes fall into, what I would consider a medium stack, at around 20-25mm of stack.
Low-stack shoes would, of course, be less than 20mm in height and I’d consider high-stack to be shoes in the 25 to 30mm range, with maximal cushioned shoes being those at 30mm plus.
Different Running Shoe Brands
This isn’t a complete list, but here’s a selection of popular brands and where most of their most popular models sit in terms of stack height and drop.
*generalisation of models within the range of each range
NNormal - Kjerag - Medium Level Cushiong, 23.5mm STACK, Low DROP - 6mm
Heel to Toe Drop - Running Speed
The speed at which you are running can also play a part in footwear selection in terms of stack and drop and here’s an interesting point.
When most runners start to run at a very easy pace, they tend to ‘bounce along’ a little landing on their forefoot. As the speed increases and the distance covered, they may move into a heel-striking gait and then, if increasing the speed again to a sprint, move up onto their toes.
When racing, many will naturally adopt a more mid-foot strike and in doing so, the heel to toe drop is less of a consideration.
Again, the foot strike and gait of every individual one of us will vary and thus, the best feeling shoe to compliment our gait will do too.
Higher or Lower Drop for Running Injuries
Running Injuries: Achilles
As with all injuries, it’s important to seek professional medical advice, but from experience and common research, we know that a higher drop is an often-cited option in the treatment of Achilles-related injuries.
A shoe with a higher heel to toe drop reduces the load on the Achilles tendon at impact and thus, is usually recommended. A heel raise placed within a shoe is another solution, again raising the height of the rear of the foot and reducing the overall strain on the tendon.
Running Injuries: Runner’s Knee
Again it’s important to seek professional medical advice, but the help drop of a shoe is often referred to in the cause and treatment of runner's knee (pain under the kneecap at the front of the knee).
If experiencing this condition, a shoe with a lower heel to toe drop is thought to reduce the load on the knee and potentially help.
As always, when changing from one type of shoe to another, slow adaptation is the best way to approach any change. For example, if running in a high drop shoe (12mm), it might be best to move to a mid (8mm drop) shoe before progressing to a low (4mm) or even zero drop model.
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