May 11, 2025

Written byJimmy McPherson

How To Measure Midsole Softness By Cutting Running Shoes In Half? [PDF]

At the RunRepeat lab, we take midsole testing to the next level by making an important sacrifice we carefully cut each shoe in half before measurement. This destructive testing method allows us to place our durometer directly on the exposed foam, giving us completely accurate softness readings without interference from outsole rubber or upper materials. While this approach is significantly more expensive since we purchase all shoes independently rather than accepting brand samples, we believe it̵7;s the only way to obtain truly reliable data.

The alternative method used by most running websites and YouTube channels – simply pressing the durometer against the exterior of the intact shoe – creates misleading results. When testing through the outsole, you’re not just measuring the foam’s properties but also the influence of the rubber compound and its thickness. This exterior method might show a shoe as being firmer than it actually is if it has a thick outsole, or softer if the outsole is particularly thin or flexible. Our cutting technique removes these variables, ensuring we capture the foam’s true characteristics exactly as runners experience them during foot strike.

The difference between surface measurements and our internal foam readings can sometimes be substantial – we’ve recorded variances of up to 15% on certain models, which could mean the difference between a shoe feeling comfortably soft or uncomfortably firm to a runner. That’s why we continue to invest in this meticulous approach, even as it requires us to purchase duplicate pairs for complete testing and review.

Our testing reveals a critical flaw in surface-level durometer measurements that most reviewers overlook. When you press a durometer against a shoe’s exterior, you’re not actually measuring the midsole foam – you’re measuring through multiple layers of outsole rubber, glue, and sometimes even plastic stability features. These materials dramatically skew results, as we’ve found variances up to 30% compared to our cut-in-half measurements.

The problem compounds with modern racing shoes featuring complex outsole patterns – those aggressive chevrons and pentagonal lugs create uneven contact points that make consistent readings nearly impossible. Even the slightest tilt of the durometer (often unavoidable on contoured soles) can register false firmness readings of 5-10% variance. This explains why two shoes with identical midsole foam might test completely differently if one has a thin racing outsole while another uses thick, protective rubber.

Our dissection method eliminates these variables by giving the durometer direct, flush contact with pristine foam – the only way to know exactly what your feet will feel during runs. The industry’s widespread use of surface measurements means most published hardness data requires this important context about its limitations.

Interior and Exterior Durometer Readings in Midsoles

Our testing methodology demonstrates why surface measurements fail to represent true midsole performance. Through rigorous analysis of 24 shoes measured both internally (via dissected midsoles) and externally (through outsoles), we’ve quantified substantial discrepancies that every runner should understand. The data reveals shocking variances – from a 24% difference in the racing-focused ASICS Metaspeed Edge+ to an extraordinary 485% variation in the trail-ready Nike Ultrafly.

These extreme disparities directly correlate with outsole design philosophy. The Metaspeed Edge+, optimized for road racing speed, employs minimal outsole protection, resulting in measurements closer to the foam’s true properties. Conversely, the Ultrafly’s aggressive trail outsole – designed for durability on rough terrain – creates a dramatic buffer that completely masks the midsole’s actual softness. This pattern holds true across our entire test sample: shoes with thicker, more protective outsoles consistently show greater divergence between surface and core measurements.

For runners seeking accurate comparisons, our complete dataset (available below) provides transparent hardness values measured directly on exposed foam, where lower numbers indicate softer materials. These findings underscore why cutting through marketing claims to test actual foam properties matters – the difference between what a shoe’s exterior suggests and how its midsole truly performs can literally make or break your running experience.

(Shore A Hardness “HA” | Lower = Softer)
(Sorted by Largest Difference → Smallest)

Why is it Essential to Cut Shoes?

In order to understand it properly let us divide this into 3 Examples.

Example 1 : ASICS Superblast

Our research demonstrates how external durometer measurements can be profoundly misleading due to varying outsole designs. The ASICS Superblast provides a perfect case study of this measurement discrepancy. When we measured the shoe’s exterior without cutting, the durometer registered 46.5 HA, suggesting a moderately firm ride. However, when we performed an internal measurement directly on the exposed FF Turbo foam, the reading dropped to just 17.5 HA – revealing the midsole’s true ultra-soft nature. This 166% difference between external and internal measurements shows how dramatically a thin outsole layer can distort perceived cushioning.

The implications are significant for runners: a shoe that appears firm based on surface tests might actually contain premium-level soft foam. This pattern holds true across categories – trail shoes with rugged outsoles show even greater variances, while racing flats with minimal protection come closer to accurate external readings. Our complete dataset reveals these trends consistently across multiple brands and models, proving that only direct foam measurement provides reliable hardness data. For runners making purchase decisions based on cushioning feel, this distinction between surface appearance and midsole reality could mean the difference between a shoe that matches their preferences and one that disappoints.

The Superblast’s hidden softness exemplifies why our dissection methodology matters – what you see (and measure) on the outside often bears little resemblance to what your feet actually experience during runs. These findings challenge conventional review practices and underscore the importance of precise foam evaluation methods.

Example 2 : Nike Ultrafly

The Nike Ultrafly perfectly illustrates why cutting shoes open provides the only reliable way to assess true midsole softness. This premium trail runner contains an ultra-soft 10.0-HA ZoomX foam core – one of the softest materials available. Yet when measured externally through its protective layers, the durometer registers a shockingly firm 58.5 HA, completely misrepresenting the actual cushioning experience. This 485% discrepancy stems from Nike’s innovative protective design: they’ve encased the delicate Pebax-based ZoomX in a dense, woven fabric shield to prevent damage from trail hazards like sharp rocks and roots.

While this armor successfully preserves the foam’s integrity, it completely obscures the shoe’s real performance characteristics when tested superficially. Only by dissecting the shoe and measuring directly on the exposed ZoomX could we confirm its authentic 10.0-HA reading – a value that finally explains why runners report such a plush, responsive feel on trails. This dramatic case study proves that external measurements often reflect protective elements rather than the actual midsole foam, potentially misleading runners about a shoe’s true performance capabilities. The Ultrafly’s hidden softness demonstrates why our cutting methodology provides essential insights that surface measurements simply cannot reveal.

Example 3 : On Cloud X and Hoka Mach X

The Hoka Mach X and On Cloud X serve as perfect examples of why external durometer measurements fail to capture true midsole performance. While their names both feature an “X,” these shoes couldn’t be more different in feel. Our internal measurements reveal the Mach X as exceptionally soft at 10 HA, while the Cloud X ranks among the firmest we’ve tested at 33 HA—a 230% difference in actual cushioning. Yet, if we had relied solely on external measurements, this dramatic contrast would shrink to just 24%, completely distorting reality.

This discrepancy explains why runners immediately notice the Mach X’s plushness versus the Cloud X’s rigidity within minutes of wearing them—a distinction surface-level testing utterly misses. The reason? Outsoles, stability frames, and protective layers mask the true nature of the foam beneath. Without cutting these shoes open (and sacrificing them in the process), reviewers and consumers alike risk being misled by numbers that don’t reflect real-world performance.

This case proves our method isn’t just about precision—it’s about truth in testing. When a 230% softness gap shrinks to 24% externally, it’s clear why dissection matters.

How are Durometers used for Measuring Shoe Softness?

The durometer serves as our precision tool for quantifying midsole softness, operating on the Shore hardness scale – specifically the Shore A variant for running shoe foams. This handheld device works by applying a calibrated spring force to press a standardized indenter (typically a conical or spherical tip) into the material’s surface at a defined pressure. The depth of this indentation gets mechanically converted into a hardness value, where greater penetration equates to lower Shore A numbers and softer materials.

We follow strict protocols: each test applies 822 grams of force for exactly 3 seconds on a perfectly flat, clean foam surface at consistent room temperature. The scale ranges from 0 HA (extremely soft, like gel) to 100 HA (rigid as hard plastic), with most running shoe foams falling between 15 HA (ultra-soft PEBA) to 50 HA (firm EVA).

What makes this measurement scientifically valuable is its reproducibility – when we retest the same exposed foam sample, results typically vary by less than 2%, ensuring reliable comparisons across hundreds of shoes in our database. This consistency proves crucial when distinguishing between foams that may feel subjectively similar but demonstrate meaningful performance differences in our controlled tests.

Durometer Measurement’s Dimensionless Nature

Shore hardness measurements require careful interpretation because they represent relative comparisons rather than absolute physical quantities. The durometer functions like a specialized comparator – when its indenter tip presses into material under standardized force, the resulting HA value indicates softness only in relation to other materials tested under identical conditions. Imagine it as giving a “softness score” rather than measuring an intrinsic property like weight or length. This relative nature means two key considerations emerge.

First, measurements only hold meaning when comparing results using the same Shore scale (we exclusively use Shore A for running shoes). Second, even within the same scale, instrument-to-instrument variation occurs due to manufacturing tolerances, spring calibration differences, and tip wear over time. Our lab controls these variables by using the same calibrated durometer for all tests, maintaining strict environmental conditions (23°C, 50% humidity), and following a rigorous testing protocol that includes multiple measurements per sample.

This methodology ensures our reported hardness values have consistent meaning when comparing between shoes, though the numbers themselves shouldn’t be interpreted as having standalone physical units. The system works because all measurements exist in the same controlled framework – much like how player statistics only compare fairly within the same league’s scoring system.

Conclusion

Our dissection-based durometer testing represents the current gold standard for midsole softness evaluation, despite certain inherent limitations. While the method doesn’t capture how outsole rubber thickness or embedded plates might modify the ride experience, it provides the most accurate assessment of a foam’s fundamental properties by eliminating the distorting effects of external layers. The alternative – surface measurements on intact shoes – produces demonstrably unreliable data, as our comparative testing has shown discrepancies up to 485% between external and internal readings.

This level of variance renders exterior measurements practically useless for meaningful comparisons, potentially leading runners to select shoes that feel completely different than expected based on published hardness values. Our approach prioritizes scientific rigor by isolating the foam itself from confounding variables, creating a standardized reference point for objective comparisons across brands and models. Until new technologies emerge that can non-destructively measure true midsole characteristics through multiple material layers, controlled dissection remains the only method that provides runners with transparent, accurate foam performance data to inform their purchasing decisions.

The choice is clear: either accept potentially misleading surface measurements or embrace the precision of direct foam assessment, even if it requires sacrificing test samples.

Disclaimer: The images used in this article are adaptations inspired by original visuals from RunRepeat. They have been modified for presentation purposes.

Author

Jimmy McPherson

Jimmy is the former founder of Midwest Soles, a sneaker and streetwear reselling company. He's a current business student with a continued passion for sneakers and sneaker reselling.