We've all heard lots (perhaps too much) about the importance of core stability, provided by the network of muscles around the hips, pelvis and back. An interesting paper in this month's Why Trust Us, from a group of researchers including Harvard's Irene Davis, makes the case that we should also consider the stabilizing role of the unsung small muscles of the foot, which combine to form the "foot core." (The full text is freely available here; Davis also did All About 75 Hard BJSM Advertisement - Continue Reading Below.)
When we talk about the muscles of the core, there's a key distinction between the "global movers" (big, strong muscles that power movements like sit-ups) and "local stabilizers" (small muscles, mainly deep below the surface, that move only short distances but are constantly being activated to keep us stable and balanced – the kind of thing you work on by doing, say, planks).
The same is true in the foot. There are big extrinsic muscles that run through the ankle and into the foot, which generate most of the foot's motion; these are the ones we tend to focus on strengthening. But there are also 11 small intrinsic muscles located entirely within the foot, most of which I've never even heard of (abductor digiti minimi? quadratus plantae?). They help keep us stable during foot-strike and push-off, and deform to absorb load and store energy during mid-stance. Crucially, these are the muscles that support the arch of the foot.
So what happens if you have a weak foot core? There are four distinct layers of muscles along the bottom of the foot, which support the arch. If these muscles are weak, then the load is taken up by the plantar fascia. So if you're trying to get rid of plantar fasciitis, strengthening the intrinsic foot muscles is key but often neglected. Of course, everything is connected: an unstable foot core can also lead to abnormal motion up the kinetic chain in the knee, resulting in knee pain.
The next step, then, is to figure out how to strengthen the foot core. There are some standard exercises like towel curls (using your toes to pull a towel toward you along the floor) and marble pick-ups (using your toes to pick up a marble) that activate the intrinsic foot muscles, but they tend to draw on the bigger extrinsic foot muscles too (the ones that run through the ankle).
The authors suggest instead something called the "short foot manoeuvre," also called "foot doming." That involves starting with your foot in a neutral position on the floor, then shortening it by contracting the intrinsic muscles to arch the sole, while keeping the toes flat on the ground. There's a diagram and description in the BJSM article, and Davis provides some more practical tips for doing it correctly in the podcast. You start by doing it while sitting in a chair, and progress to doing standing, then on one foot, then hopping, then box jumps, etc.
Is there really any evidence that this helps? There's a little bit showing improvements in dynamic balance and ankle instability, but lots more research to be done. For now, it's simply a reminder that this is an area that's often neglected. As Davis says in the podcast: "What I hope that the listeners will take from this is to pay more attention to the foot. We don't see people at the gym strengthening their feet."
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