The key to staying injury-free, we've been told for years, can be found somewhere in the vicinity of the belly-button.
"When we talk about core stability, you immediately have visions of people training their abdominal muscles," says Dr. Patrick McKeon, an athletic therapist and professor of exercise science at Ithaca College in New York.
And it's true that training these muscles, along with hip and back stabilizers, helps keep the rest of the body aligned while running, jumping, walking down the street, or even just sitting in a chair. But that's only part of the equation needed for stability, according to McKeon.
"We also have a foot core," he says. "The muscles in the foot behave in the same way."
McKeon and colleagues from Harvard, the University of Virginia and the University of Utah presented their vision of a new "foot core paradigm" in the March issue of the British Journal of Sports Medicine, arguing that the stabilizing role played by a criss-crossing web of small muscles in the feet has been largely ignored by sports doctors and therapists. Their prescription: If your foot or lower leg hurts, start by strengthening the foot – or at least ensure that you're actually making full use of the muscles you've got.
The foot muscles we usually think of, if we think of them at all, are the extrinsic ones – big ones that run from the lower leg through the ankle into the foot, and actually move your foot. But there are also 11 small intrinsic muscles located entirely within the foot that are much more obscure. When's the last time you considered your quadratus plantae or adductor hallucis oblique?
Like the more famous abdominal core muscles, the intrinsic foot muscles aren't designed to power big movements; instead, they provide small corrections to keep you stable and balanced as you move. And crucially, they also serve as motion detectors, picking up subtle movements as they get stretched or compressed when your balance shifts.
"These muscles actually are dynamic deformation sensors in the foot that give us a lot of information about where the body is in space," McKeon says.
The foot's role in stability may seem obvious, but current clinical guidelines for conditions such as plantar fasciitis and heel pain make no mention of foot strengthening. That makes McKeon's paper an important corrective, says Matt Ferguson, CEO of Vancouver-based Progressive Health Innovations and co-inventor of a foot-strengthening device called the ankle-foot maximizer. "The sports-med community has been overly focused on the pelvis for the last decade," he says.
The problem is that the intrinsic foot muscles are not just weak, but essentially dormant in some people. If you're relying on external support – rigid shoes or orthotics, for example – to maintain your foot's arch, then you start using your intrinsic foot muscles less. Or if the intrinsic muscles are weak, their stabilizing role may be taken over by the larger extrinsic muscles, which leaves those larger muscles more susceptible to overuse injuries.
(That doesn't mean orthotics and supportive shoes are always a bad idea. Orthotics may provide the external support needed to allow a lingering injury such as plantar fasciitis to clear up, McKeon says. But in parallel, you should also be developing the intrinsic muscles so that the injury doesn't recur.)
The solution, then, is to learn to turn those intrinsic muscles on. "It's not a matter of developing a great deal of strength," McKeon says. "You're never going to develop this unbelievably big intrinsic foot muscle. Instead, it's more the idea of learning how to activate them and getting them to respond to deformation."
In their paper, McKeon and his colleagues recommend an exercise called "foot doming" (or, alternatively, the "short foot manoeuvre"), which involves arching the foot to shorten it while keeping the toes and ball of the foot flat on the ground. As you learn what it feels like to activate the intrinsic muscles, you progress from sitting to standing, standing on one leg, and eventually to squats and single-leg hops.
Other exercises that involve the intrinsic muscles include spreading the toes as wide as possible, squeezing them together, pulling a towel toward you on the floor by curling your toes, and picking up a marble with your toes.
That's fine in theory, but does the foot core really represent a breakthrough that will cure more injuries?
"These ideas have certainly been out there for as long as I have been a physio," says Chris Napier, a Vancouver-based physiotherapist who treats some of Canada's top long-distance runners. "Research always lags behind clinical practice, though."
Like McKeon and the other authors of the BJSM paper, Napier prescribes a progressive strengthening routine for the intrinsic foot muscles, including foot doming, for a wide variety of lower-leg injuries, such as shin splints and plantar fasciitis. But these muscles are often neglected, and the concept of a "foot core" offers a unifying paradigm that may help raise awareness of the foot's importance and encourage more studies, he says.
"Clearly, there needs to be more research in this area, but this is a step in the right direction," Ferguson says, "bad foot pun intended."
Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com. His latest book is Which Comes First, Cardio or Weights?