What Long Runs Do to Your Arch, and What Training Changes

Run far enough and your arch drops. That much is measurable, and it has been measured. What tends to get left out of the telling is the second half of the finding — that the feet of people who run regularly drop a fraction as far, and recover in a fraction of the time.

What the study measured

In 2020, researchers at Waseda University had two groups run 10 km: ten recreational long-distance runners, and ten untrained men. Before and after, they measured the stiffness of the plantar fascia — the band of tissue under the foot that holds the arch up — at three points along its length, using shear wave imaging. They also scanned the foot in 3D to track the height of the navicular bone, a standard index of arch height.¹

The plantar fascia lost stiffness after the run, most of all at the heel end. The arch dropped. The two were tightly linked: the stiffness change at the heel end alone explained about 73% of the variation in how far the arch fell.

The half that usually gets left out

The two groups did not respond the same way.

• Untrained men: plantar fascia stiffness fell by 21.9%. Navicular height fell by 14.1%. The arch was still lowered 30 minutes later and had recovered by 60.
• Trained runners: stiffness fell by 4.0%. Navicular height fell by 6.3%. The arch had recovered within 30 minutes.

Same distance, same measurement, one-fifth the stiffness loss and half the recovery time. Whatever running regularly does to a foot, it makes the plantar fascia much better at handling exactly this load. The study's authors describe the effect as transient — a fatigue response, not an injury — and in trained feet it is a small one.

Distance matters

Ten kilometres is not a marathon, and the picture changes with distance. A review of the foot in long-distance running cites a navicular drop of around 5 mm after a half marathon, and a similar drop after a full marathon that was still present eight days later.² So the recovery-in-30-minutes finding belongs to 10 km in trained runners. Push to race distance and the arch takes longer to come back — which is one more reason the weeks after a marathon are for easy running, not for testing yourself.

What foot training changes

The 10 km study did not test any intervention, so it cannot tell you what to do about the drop. Other research can, up to a point.

A 2023 meta-analysis of 13 randomized trials found that training the intrinsic foot muscles significantly reduced navicular drop — the same measure the Waseda team used.³ In runners specifically, eight weeks of foot-core training reduced how much the medial longitudinal arch moved during the stance phase of running.⁴ And the same review that cited the Waseda study concluded that strengthening the intrinsic muscles "should be considered as a conservative treatment for runners thought to have weak intrinsics."²

Read together: long runs fatigue the fascia and lower the arch; trained feet resist that far better; and training the small muscles of the foot changes how much the arch collapses under load. The pieces fit. But we want to be exact about the gap: no study has trained runners' feet and then measured whether their post-run fascia fatigue got smaller. That is an inference from three separate findings, not a result. We think it is a reasonable inference. It is not proof.

Where Toe-Rx fits

Toe-Rx is a tool for the foot training in the paragraph above. It holds the toes open wider than you can unassisted and gives them something to close against — and in the one trial that isolated it, wearing a spacer alone changed nothing over eight weeks while adding the movement changed everything measured.⁵

It goes on after a long run, not before, and not because it "accelerates recovery" — we have looked for evidence that anything does that reliably and have not found it. It goes on afterwards because a long static toe hold is the wrong thing to do in the minutes before you need your feet to push off, and because afterwards you are done for the day and already barefoot. Ten minutes, sitting down, every other day. How to use it is here.

For what has actually been shown to reduce running injuries — a year-long randomized trial of foot strengthening in runners, and what it does and does not prove — see the runners' page.

*These statements have not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease.

References

1. Shiotani H, Mizokuchi T, Yamashita R, Naito M, Kawakami Y. Acute effects of long-distance running on mechanical and morphological properties of the human plantar fascia. Scandinavian Journal of Medicine & Science in Sports. 2020;30(8):1360–1368. doi:10.1111/sms.13690. PMID: 32306478. Read it (open access)
   10 runners, 10 untrained men, one 10 km run. Trained runners: −4.0% stiffness, −6.3% navicular height, recovered by 30 min. Untrained: −21.9%, −14.1%, recovered by 60 min.
2. Jastifer JR. Contemporary review: the foot and ankle in long-distance running. Foot & Ankle Orthopaedics. 2022. doi:10.1177/24730114221125455. Read it
   Narrative review. The half-marathon and marathon navicular-drop figures are cited from Cowley & Marsden and Fukano et al. via this review; confirm against the originals if you want them cited directly.
3. Jaffri AH, Koldenhoven R, Saliba S, Hertel J. Evidence for intrinsic foot muscle training in improving foot function: a systematic review and meta-analysis. Journal of Athletic Training. 2023;58(11–12):941–951. doi:10.4085/1062-6050-0162.22. PMID: 35724360. Read it
4. Matias AB, Watari R, Taddei UT, et al. Effects of foot-core training on foot-ankle kinematics and running kinetics in runners: secondary outcomes from a randomized controlled trial. Frontiers in Bioengineering and Biotechnology. 2022. doi:10.3389/fbioe.2022.890428
5. Kim MH, Yi CH, Weon JH, Cynn HS, Jung DY, Kwon OY. Effect of toe-spread-out exercise on hallux valgus angle and cross-sectional area of abductor hallucis muscle in subjects with hallux valgus. Journal of Physical Therapy Science. 2015;27(4):1019–1022. doi:10.1589/jpts.27.1019. Read it
   24 participants with hallux valgus, not runners. Both groups wore an orthosis; this shows what adding the movement did.