Shoe geometry (rocker, drop and stack)
Evidence: moderate
The shape of the sole is the third design lever, alongside the foam and the plate. A thicker stack of compliant foam improves running economy; the curved rocker shifts work away from the toe joint and is an independent contributor to economy; heel-to-toe drop is mostly a comfort and habit choice with no reliable injury effect. The shape works with the foam and plate, not separately from them.
Most attention on modern shoes goes to the foam and the carbon plate, but the third element is the shape of the sole: how thick it is (stack height), how much higher the heel sits than the forefoot (drop), and how the sole curves up at the front (the rocker or toe spring). Geometry is the part the foam-plate-geometry “synergy” keeps pointing at, and on its own it explains a good deal of how a shoe rides.
Stack height
Stack height is the thickness of material under the foot. Thicker stacks have moved from a comfort feature to a performance one. In well-trained runners, a 40 mm advanced-footwear shoe improved running economy by about 2.4% over an entry-level shoe, and a 50 mm prototype improved it by a further 0.6 to 0.7% despite weighing more (Baumann et al. 2025). World Athletics caps road stack at 40 mm and track-spike stack at 20 mm, but that limit is a rule, not a ceiling on the physiology: economy kept improving past it.
The important nuance is that thickness only helps when it is thick compliant foam. Adding height in isolation lengthens the effective leg and tends to raise the metabolic cost; the gain comes from the extra depth of soft, resilient foam that can compress and return energy. Very high stacks also trade away stability and add mass, which is part of why the 50 mm shoe felt less comfortable than the 40 mm one (Baumann et al. 2025). A tall stack is also implicated in the unresolved navicular bone-stress question raised for plated shoes (Tenforde et al. 2023).
Heel-to-toe drop
Drop is the height difference between the heel and the forefoot, typically 0 to 12 mm. A low or zero drop shifts load toward the calf and Achilles and tends to encourage a flatter landing; a higher drop shifts it toward the knee. Strong opinions attach to drop, but the injury evidence does not support them. A randomised trial of 553 runners over six months found that drop did not change overall injury risk; the only signal was an interaction with running frequency, where low-drop shoes were associated with more injuries in regular runners and fewer in occasional ones (Malisoux et al. 2016). The practical reading is that drop is mostly a comfort and habit choice, that a large change in drop should be introduced gradually because it reloads the Achilles and calf, and that no single value is correct for everyone.
Rocker and toe spring
The rocker is the upward curve of the sole, and the toe spring is the curl at the very front. Both let the foot “roll” through the stance rather than having to bend at the big-toe joint and push off against a flat sole. This is the least-understood of the three levers and increasingly the one manufacturers tune.
The rocker matters for economy. When researchers individually optimised the longitudinal rocker profile of a running shoe, they increased the positive work done at the ankle and improved running economy, showing the curve is an independent contributor and not merely a side effect of the plate and stack (Tankink et al. 2024). It pairs with the plate: the plate stiffens the toe joint while the rocker provides the shape to roll over, which is why the plate’s effect cannot be separated cleanly from the geometry (Kobayashi et al. 2025).
The same rolling action offloads the forefoot, which is why rocker soles are used clinically. In people with osteoarthritis of the big-toe joint, rocker-sole footwear reduced the loading at that joint, lowering the peak bending moment and the pressure under the forefoot (Menz et al. 2016). That makes a pronounced rocker a sensible feature for runners managing a painful big-toe joint, forefoot or plantar fascia.
There is a trade-off worth stating. By taking work away from the foot, curved soles may, over time, let the foot’s own muscles do less. In walking, greater toe-spring curvature lowered the work required at the toes, and the authors raised the possibility that habitually offloading the foot this way contributes to weaker feet and conditions such as plantar fasciitis (Sichting & Holowka 2020). This is a walking study and a cautious hypothesis rather than a demonstrated harm, but it is the reasoned argument against treating maximal rocker as a free lunch, and it sits alongside the broader case for not letting shoes do all the work that foot and lower-leg strength should share.
How the three combine
Geometry is not a separate gadget bolted onto the foam and plate; it is what makes them work. The thick compliant foam supplies the energy return, the plate stiffens the toe joint and spreads the load so the foam compresses evenly, and the rocker supplies the shape to roll over without fighting the stiff forefoot. The current consensus on super-shoes is that the benefit is synergistic: no one element, geometry included, can be cleanly credited with the saving (Kobayashi et al. 2025; Healey & Hoogkamer 2021).
For a buyer, this means reading the whole package by feel rather than chasing one number. Stack height, drop and rocker shape the ride more than the marketing spec sheet suggests, the right shoe is still the one that feels right, and a big change in any of these dimensions, drop especially, should be introduced gradually.