Footwear Flex in Trail Running

Contributed Perspective · Kate Harrison, PhD
Director – Human Performance Fit Lab at BOA Technology

Flex in Trail

Since 2016, flex has been a prominent feature in running shoes, most famously through the integration of stiff carbon fiber plates in the midsole. The combination of flex, foam, and their geometry has led to massive performance improvements in road running.¹ The men’s marathon world record has dropped by 3 minutes since the introduction of super shoes (or AFT - Advanced Footwear Technology - for footwear biomechanics geeks). The women’s world record has dropped by a whopping 5 minutes. Despite following the road run advancements, trail running shoes are having a harder time finding the perfect recipe that launches performance to a new level. That’s not entirely surprising. Trail running deals with different challenges. It’s harder to optimize shoes for the trails because of the irregular terrain, varying paces, and required protection from rocks and ankle rolls.

The Performance Benefits

Consider running economy, one of the most critical factors in running performance, we see extremely consistent performance improvements in stiff super shoes on smooth, flat terrain. In lab tests extending this research to sloped surfaces, it appears the shoes still provide some benefit on moderate grades, but that benefit essentially disappears on steep hills (e.g 15°, or ~27% grade).²⁻⁴ This is likely because stiffness in shoes contributes to energy storage and return in the midsole. When we land, we load the midsole, storing energy. Then, when we push off, the midsole returns a large chunk of that energy. On flat ground at steady paces, the balance of energy absorption and generation are virtually equal. Hills heavily skew this required balance. On steep uphills, the amount of force we absorb drops to almost nothing. There is only pushing upwards, hence the high muscular demand and burning. On downhills, we absorb huge amounts of energy, vastly exceeding the amount of energy the foam can store, and the need for energy generation is minimal. We really just need to control the fall down the hill.

Does this mean stiffness and carbon fiber aren’t important on trails? Heck no! With perhaps the exception of VKs or ascent races, trail runs include a variety of terrain, including flats and moderate slopes. However, it’s important to consider the demands of the event when designing footwear. We will likely experience the most success with a less stiff profile than we would use on roads.


The Stability Trade-off

Trail runners are no strangers to stability challenges, from wiping out on a sneaky root to rolling ankles on chunky rocks. By combining soft foam and stiff plates, super shoes can be less stable. When caught on the downstep in race photos, many athletes look like their inner ankles are going to scrape the ground. This observation is confirmed in the lab.⁵ Peak ankle rotation velocity, meaning how fast your ankle rolls when it hits the ground, is greater with a stiffer midsole. Why? A plate or stiffening agent functions like a lever. This can help to launch you forward (good), or to the side (probably not good). Luckily, not all stiffening agents are created equal. Consumers tend to evaluate shoes on a binary: plate vs. no plate. However, quite a lot of engineering goes into plate materials. For example, carbon fiber can be designed to give a plate cool properties like directional flex. Even cooler, the flex of the materials can vary depending on how FAST force is applied. Optimizing properties like these is especially important in trail running. Perhaps a good rule of thumb is that our plates should be just as technical as the terrain we run on.

The Impact on Trail Shoes?

After several years of experimenting, trail shoes are starting to catch up to their road counterparts! The long and short of it is, trail shoes should benefit from added stiffness – but the parameters matter. In most cases, they should be LESS stiff than what we see on roads due to slower speeds and greater slopes. Directional stiffness is also especially beneficial, as it can allow the stiffening agent to double as a rock plate. This prevents the terrain from digging into the foot while still allowing the absorption and return of energy with each landing. Geometry, such as split plates, can also go a long way to reduce the likelihood of nasty ankle sprains and still retain stiffness for performance in the forward direction.⁶ Lastly, with the variety of trails out there, it’s important to have a variety of shoes with different flex profiles on the market. Another excuse to try dozens of shoes to find the perfect one for your upcoming race or adventure – not that you needed one.

References

  1. Hoogkamer W, Kipp S, Frank JH, Farina EM, Luo G, Kram R. A Comparison of the Energetic Cost of Running in Marathon Racing Shoes. Sports Med. 2018;48(4):1009-1019. doi:10.1007/s40279-017-0811-2

  2. Perrin TP, Rossi J, Kerhervé HA, Millet GY. Increasing Shoe Longitudinal Bending Stiffness Is Not Beneficial to Reduce Energy Cost During Graded Running. Int J Sports Physiol Perform. 2023;18(2):209-212. doi:10.1123/ijspp.2022-0163

  3. Jaboulay C, Giandolini M. Effect of increased bending stiffness on running economy and joint biomechanics in uphill running and running on unstable terrain: is there any evidence for embedding carbon plate in trail running footwear? Footwear Science. Published online January 9, 2025:1-9. doi:10.1080/19424280.2024.2448653

  4. Whiting CS, Hoogkamer W, Kram R. Metabolic cost of level, uphill, and downhill running in highly cushioned shoes with carbon-fiber plates. J Sport Health Sci. 2022;11(3):303-308. doi:10.1016/j.jshs.2021.10.004

  5. Bruneau MM, Gaudette LW, Sirls E, et al. Biomechanics associated with bone stress injuries while using advanced footwear technology in elite distance runners. PM R. 2026;18 Suppl 2:S143-S150. doi:10.1002/pmrj.70153

  6. Teng J, Pan M, Han Y, Diaz-Cidoncha Garcia J, Yi Q, Shen S. Biomechanical insights into carbon plate geometry in running shoes in male runners: implications for sprint performance and ankle stability. Front Bioeng Biotechnol. 2025;13:1735951. doi:10.3389/fbioe.2025.1735951

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