Inside every running shoe is a set of muscles almost nobody trains. They are small, they attach only within the foot, and for most of the history of running advice they were treated as passive scaffolding for the arch. In 2015 a paper in the British Journal of Sports Medicine argued that this was backwards, and gave them a name that stuck.
The foot core
McKeon and colleagues proposed that the small muscles inside the foot — the plantar intrinsics — work the way the abdominal core does for the trunk. They are not the prime movers. They are the stabilisers that let the prime movers work, holding the arch's shape under load so that the bigger muscles of the calf and shin have a firm platform to push from.1
The paper's argument was that training programmes for the foot had been built around the wrong muscles. Strengthen the long muscles from the shin and you get a stronger pull on the toes. Strengthen the intrinsics and you get a more stable arch. Those are different things.
Why the usual exercise trains the wrong ones
The standard foot exercise is the towel curl: sit, put a towel under your foot, scrunch it toward you with your toes. It feels like foot work. The foot core argument is that it is mostly shin work — the long toe flexors from the lower leg do the curling, the toes claw, and the small arch muscles are bystanders.1
The alternative McKeon's group favoured is the short-foot exercise: keep the toes flat and un-curled, and draw the ball of the foot toward the heel so the arch lifts. It is harder to feel, and harder to do. That is roughly the point: it is the muscles you cannot easily feel that it is trying to reach.
This is a difference in selectivity, not a verdict that towel curls are harmful. But if the aim is the arch, position matters, and the position is toes flat and spread rather than clawed.
The trial that made the case for runners
In 2020 the concept was tested where it counts. Taddei and colleagues took 118 recreational runners and split them: half did eight weeks of foot-core strengthening — supervised, and built on exactly the short-foot, toes-spread principle above — and then kept it up for a year; half did a static stretching routine. Over twelve months the strengthening group was 2.4 times less likely to sustain a running-related injury.2 The more foot strength a runner gained, the longer it took them to get hurt.
Two details worth holding on to. The programme was bodyweight and equipment-light — nothing about it required a gym. And the comparison group was stretching, which is the thing most runners already do for their feet and which, in this trial, lost.
What it does not show
The foot core is often sold as a performance upgrade — a stiffer spring, a better toe-off, a faster you. We looked for that and did not find it. In a separate trial, competitive distance runners raised their toe flexor strength substantially, and their joint mechanics and running economy did not change.3 The case for training the foot core is injury, not speed, and we are not going to borrow the second to sell the first.
A small 2014 study is sometimes cited for balance gains after intrinsic strengthening.4 It exists, but it enrolled twelve men and a later meta-analysis rated its methods the weakest of the studies it pooled. We mention it because you will see it quoted, not because it carries the argument.
Training in the spread position
If the muscles you want are the ones that hold the toes apart and the arch up, then the position to train them in is toes apart, arch engaged. That is a hard position to reach unassisted, because years in shoes leave most people unable to spread their toes far on their own.
Toe-Rx is a tool for that: it holds the toes open, and you close them against it — abduction held, flexion worked. Whether the tool is necessary is an honest question. Taddei's runners did not use one. What it does is make the spread position available to feet that cannot get there, and give the closing movement something to push against. In the one trial that isolated this, wearing a separator passively changed nothing over eight weeks; adding the active movement changed every measure taken.5 The movement is the part that counts.
Ten minutes, seated, after a run rather than before, every other day. The full protocol, the sizing, and everything the evidence does and does not support is on 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
- McKeon PO, Hertel J, Bramble D, Davis I. The foot core system: a new paradigm for understanding intrinsic foot muscle function. British Journal of Sports Medicine. 2015;49(5):290. doi:10.1136/bjsports-2013-092690.
Confirm citation before publishing — see notes. - Taddei UT, Matias AB, Duarte M, Sacco ICN. Foot core training to prevent running-related injuries: a survival analysis of a single-blind, randomized controlled trial. The American Journal of Sports Medicine. 2020. doi:10.1177/0363546520969205
118 recreational runners, 12-month follow-up, static stretching control. - Day EM, Hahn ME. Increased toe-flexor muscle strength does not alter metatarsophalangeal and ankle joint mechanics or running economy. Journal of Sports Sciences. 2019;37(23):2702–2710. PMID: 31608832.
- Hashimoto T, Sakuraba K. Strength training for the intrinsic flexor muscles of the foot: effects on muscle strength, the foot arch, and dynamic parameters before and after the training. Journal of Physical Therapy Science. 2014;26(3):373–376. doi:10.1589/jpts.26.373. PMID: 24707086.
12 men, 8 weeks. Rated methodologically weakest in Wei et al. 2022 (PLOS ONE). - 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.
