"The effect of shoe bending stiffness on running economy is runner-specific" Chollet et al. (2022)
THE CARBON-PLATED RUNNING SHOES TREND
In the never-ending quest for enhanced performance, running shoes designed for long-distance runners have gone through significant changes over the past few years: from new midsole geometries and cambered outsoles, lighter, more resilient foams, to perhaps the most widespread and talked about change – carbon plates. If you are on the hunt for a shoe that promises the holy grail of an improved PB (personal best) thanks to its carbon plate, you would be hard pushed to ignore the overwhelming range of options being sold by footwear brands today.
Carbon plates in shoes have been credited with helping runners improve their performance and in some cases, crush records. Scientific studies have shown that changes in bending stiffness directly influence running economy (which we will call RE from now on) and therefore have a direct effect on running performance. But does this hold true for all runners? At all paces, in all races and varying situations?
Given the widespread and democratised presence of carbon-plated running shoes at worldwide racing events, we here at Salomon decided to delve deeper to better understand the relationship between RE and shoe bending stiffness. We took this initiative to acquire an informed understanding of the role it plays in running performance - for all runners, not just high-level, high-frequency marathon racers.
OUR STUDY WITH 96 RUNNERS
We conducted a study to test the impact of bending stiffness on RE and discovered that carbon plates in shoes are not automatic and universal performance-enhancers for every runner out there.
In fact, while high-level runners saw their performance improve significantly with increased bending stiffness, some runners measured little or no change with the carbon plate, while for others it was even detrimental to their RE.
Armed with a heterogenous sample of 96 runners, two shoe prototypes with differing longitudinal bending stiffness, some treadmills and gas exchange analysers, we measured how overall running performance and RE was affected by the differing levels of stiffness. Running Economy is, simply put, the relationship between a runner’s oxygen consumption and their velocity. It’s the efficiency with which a runner converts their oxygen consumption into forward motion, so for any given pace the less energy and oxygen used, the better the performance. The higher the RE, the less efficient the runner, and therefore the less efficient the shoe.
We analysed everything to see if there were other criteria by which we could cluster the results, and then describe these functional clusters from morphological, physiological, and biomechanical data. The test was conducted using two shoe prototypes: the “Flex” and the “Stiff”. The upper, midsole foam (EVA) and midsole geometry (heel height: 34.7 mm, drop: 9mm) were identical in both prototypes. For each shoe, a plate was embedded in the midsole. While the shape of the plate was identical, the material and therefore bending stiffness differed: the Flex prototype had a polyamide plate (like the Salomon Phantasm), while the Stiff shoe had a stiffer, fibre composite plate (like the Salomon Phantasm CF). Shoe mass was similar for both samples and each pair was limited to 50km usage to ensure that shoe wear didn’t influence the results. Within the sample of runners tested, 40% were females; 25% of the runners had a PB of less than 35 minutes on an official 10KM race; 25% had a 10k PB between 35 and 45 minutes; 25% had a 10K PB between 45 and 55 minutes; and 25% had a 10K PB over 55 minutes.
Runners were fitted with portable gas exchange analysers and placed on a treadmill to first assess their maximal oxygen consumption (VO2 max) and aerobic speed (MAS) before then performing two 5-minute treadmill runs at 75% VO2 max (marathon pace) with the two different prototypes. RE and stride kinematics were recorded during both trials.
ONLY 30% OF RUNNERS IMPROVE THEIR RUNNING ECONOMY
The main takeaway from these tests is the following: about 30% of runners saw RE improve with the increased bending stiffness while 27% saw it deteriorate, with results ranging from 3% improvement to 3% deterioration. And finally, 43% of runners showed no change whatsoever in RE when bending stiffness increased.
Looking at the differences between these three clusters, the data suggests that runners with an improved running efficiency with the stiffer prototype also recorded greater maximal aerobic speed compared to those with no change or decreased RE. The faster the runner, the more their performance improved with a stiffer running shoe. On the other end of the spectrum, the collected data points to slower, more recreational runners recording insignificant or no changes in RE when bending stiffness was increased.
It goes without saying that the velocity of a runner is not the only variable that can be taken into consideration, but the findings of this study point to two crucial conclusions: runners respond differently to increased bending stiffness from plates; and faster, high-level runners benefit from stiffer plates when recreational or slower runners don’t.
THE USE OF CARBON-PLATED SHOES ISN’T AN END IN ITSELF
If we consider that only a very small percentage of the running population finishes a marathon in under three hours, then only a small, relatively elite group of runners is actually reaping the benefits of stiff, carbon-plate running shoes. We can conclude that plates that increase bending stiffness are therefore best adapted to faster runners. That they are, in essence, designed for the best runners in the world, for the time they spend in their shoes and for the way they run in them.
Plates definitely work, but they don’t work the same way for everyone, and the notion of running performance is not universal. It’s specific to any given situation, measurable in each context and, in some cases, wholly subjective.
That’s why as a brand, Salomon believes the product solution to answer this quest for improved performance cannot be as wide-sweeping as one plated solution for all. Our current and future product developments aim to encapsulate this need for understanding, for inclusivity, and for a broader-reaching promise of finding the fastest version of yourself.
