Three numbers tell you how a running shoe rides: stack, drop, weight
In short
Three numbers decide how a running shoe rides: stack height (how much foam is under the foot), drop (the heel-to-forefoot height difference) and weight. A recently reviewed model came in at 39mm stack, 5mm drop and 7.7 ounces (about 218g) in a men's size 9 with a roomy toe box, and the reviewer described it as launching forward at speed while feeling slightly unstable at walking pace. Every property that helps there works against you under a barbell — thick foam compresses, and a rocker rolls your center of mass forward.
Most shoe reviews leave you with an impression. What the shoe actually feels like is mostly contained in three numbers on the spec sheet — stack height, drop and weight. For the model reviewed here those were 39mm, 5mm and 7.7 ounces (about 218g) in a men's size 9. Those three alone describe the shoe.
What does a 39mm stack mean?
Stack is the thickness of foam under your foot, and 39mm puts a shoe in high-stack trainer territory. More foam means less landing impact but also more distance between you and the ground, and combined with a rocker shape it produces a strong sense of rolling forward. The reviewer's note lands exactly there: the shoe fires forward once the pace picks up, yet feels a bit unstable while walking around.
Why is a 5mm drop worth noticing?
Drop is the height difference between heel and forefoot. What makes this figure interesting is that the shoe comes from a brand known for zero-drop, wide-toe-box models — 5mm is raised for them and low for a speed trainer. A lower drop shifts load forward, putting more of it through the calf and Achilles, so anyone not used to it needs an adaptation period.
Where do weight and toe box actually show up?
218g is light for that stack height, and weight multiplies with every stride, so the difference accumulates over distance. Toe box width works the opposite way — it only shows up with time. Feet swell over a long run or walk, and a narrow front that is fine at 30 minutes becomes a problem at two hours. A roomy toe box is a long-session spec, not a race-day one.
So can you squat in it?
No, and the spec sheet already says why. A 39mm slab of foam compresses, and it does not compress evenly side to side. A rocker exists to roll your center of mass forward. The logic in why walking shoes and lifting shoes should be different pairs applies unchanged. The deadlift adds one more penalty — a 39mm stack means pulling the bar that much further.
The Muscle Index is calculated from squat, bench and deadlift one-rep maxes. If your platform compresses or rolls forward, the weight you actually lift comes out lower, and that lower number is what stays in your score. Which is why handling both running and barbell work in one pair of running shoes is the most common setup and the most expensive one. Check your shoes before you run the numbers.
The reviewer's sense of instability came at walking pace. Standing still under a maximal load is a harder test than that, not an easier one.
Frequently asked questions
What should I look at first on a running shoe spec sheet?
Stack height, drop and weight. Those three predict the ride far better than the foam's marketing name or the upper material.
Is a 39mm stack high?
It is high-stack trainer territory. Landing impact drops, but your foot sits further from the ground, which can feel slightly unstable at walking pace.
What changes with a lower drop?
Load shifts forward, placing more demand on the calf and Achilles tendon. Moving suddenly from a high drop to a low one needs an adaptation period.
Can I squat or deadlift in high-stack running shoes?
Not advisable. Thick foam does not compress evenly, which lets the bar drift, and a rocker sole rolls your center of mass forward. In the deadlift, the stack also adds to the distance you pull.
Is a wider toe box always better?
It helps over long sessions. Feet swell during extended running or walking, so a narrow front feels fine early and becomes the limiting factor later.
Source: Men's Health