Drop — the difference in sole height under the heel and under the forefoot — is usually the last line in a running shoe's spec sheet. Most mainstream running shoes have a drop of around 8–12 mm, while “natural” models sit at 0–4 mm. Until now the drop debate has mostly revolved around injuries. A new study is the first to cleanly measure how it affects energy cost.

What the researchers did

Renninger and Owen Beck of the University of Texas at Austin (USA) (Journal of Applied Physiology, July 2026) made custom shoes in which only the drop changed, while the model and mass stayed the same.

20 runners, at a speed of 3.5 m/s — a pace of about 4:46 per kilometre.
Four drop options: 0, 10, 20 and 30 mm.
They measured energy cost via gas exchange, ground reaction forces, joint angles and moments, and the length of the calf muscle fibres.

What they found

The higher the drop, the higher the energy cost. Going from 20 mm to 0 mm reduced metabolic power by 1.6%.
With a high drop the foot is more “toe-down”, and the fibres of the medial gastrocnemius work at a shorter length — at touchdown, at push-off and on average over ground contact. A shortened muscle produces force at a higher cost.
A high drop increased the load on the knee: peak and average knee joint moments went up.
The authors' conclusion is bold: runners who want to go faster should choose zero-drop shoes.

For scale: a 1.6% gain in economy is less than half the effect the first carbon-plated “super shoes” delivered compared with regular shoes. But this study compared 20 mm with 0, while in real life the choice is more often between 8–10 mm and 4–6 mm — and for that difference the effect is probably smaller. The abstract gives no separate figure for 10 → 0 mm.

The catch

Longer calf-muscle fibres mean a stretched Achilles. Zero drop is more economical precisely because the ankle works in greater dorsiflexion, with the calf muscle and Achilles tendon at a longer length. The same mechanics mean they take more load. The study did not look at this, but anyone who has switched abruptly to “minimalist” shoes knows how sore the calves get in the first week.

What a randomised trial showed. Laurent Malisoux and colleagues from the Luxembourg Institute of Health (American Journal of Sports Medicine, 2016) gave 553 recreational runners identical shoes with a 10, 6 or 0 mm drop and tracked injuries for six months.

Overall, injury risk did not differ.
But in regular runners (those who had run for at least six of the previous twelve months), a low drop increased injury risk — hazard ratio 1.67.
In occasional runners it did the opposite, reducing it (0.48).

The explanation is plausible: an experienced runner has spent years adapting to their usual drop, and an abrupt change shifts the load onto tissues that aren't ready for it. Talk about the Achilles after Berlin, where, according to her manager, Tigst Assefa ran the final kilometres with a ruptured tendon, has fuelled interest in footwear. But there are no data linking her injury to her shoes.

Limitations

Only 20 runners and one trial in each pair of shoes — an acute effect, with no adaptation.
One speed. At a recreational runner's marathon pace or during intervals the effect may differ.
Experimental shoes, not production models. A 30 mm drop is practically nonexistent on the market.
Injuries were not studied — the “choose 0 mm” conclusion rests on economy alone.

How to apply this

Don't change your drop before a race. A 1.6% gain — and with a real-world change of drop, most likely less — is pointless if your calves cramp at kilometre 30.
Transition gradually. Lower the drop in small steps, a few millimetres at a time, and introduce the new pair on short easy runs first. Rotating two pairs with different drops is a convenient approach, and rotation itself is linked to a lower injury risk — more in our article on running shoe rotation.
If you've had Achilles or calf problems, a low drop shouldn't be your first choice. Start with strengthening: how to load the Achilles.
If your knees hurt, the opposite applies: a moderate reduction in drop may offload the knee joint, since a high drop increases knee moments.
You can estimate what a few seconds per kilometre are worth over your race distance in the race planner.

The bottom line

In identical shoes with a 0, 10, 20 and 30 mm drop, running was more economical the lower the drop: 20 → 0 mm = −1.6% energy cost.
The mechanism: the calf muscle works at a more favourable length, and knee moments are lower than with a high drop.
The price is more work for the calves and Achilles. In a randomised trial in 553 recreational runners, a low drop raised injury risk in regular runners by a factor of 1.67.
You can lower your drop, but do it gradually and not before a race.

Sources: Renninger K., Beck O.N. “Shoe heel-toe drop affects running economy”. Journal of Applied Physiology, 2026;141(1):280–290. DOI: 10.1152/japplphysiol.00886.2025 · Malisoux L., Chambon N., Urhausen A., Theisen D. “Influence of the Heel-to-Toe Drop of Standard Cushioned Running Shoes on Injury Risk in Leisure-Time Runners: A Randomized Controlled Trial With 6-Month Follow-up”. American Journal of Sports Medicine, 2016;44(11):2933–2940. DOI: 10.1177/0363546516654690