The concept of ‘core stability’ of the foot

The concept of spinal ‘core stability’ is given a lot of prominence in the media, by coaches and by therapists and is allegedly an important concept for prevention of running injury, low back pain and postural related issues. It has gained widespread acceptance. Despite this, the definitive evidence supporting the concept is lacking; anecdotally a number of people I speak to who know more about it than I do, hate the term ‘core stability’; additionally I am picking up on more publications and blog posts arguing against the concept (eg here and here), so I am not totally convinced about it (but will still keep doing exercises for my core at the gym). The evidence supporting the concept is far from compelling. I only mention this to put into context a review of the concept of ‘core stability’ of the foot that just appeared in the British Journal of Sports Medicine:

The foot core system: a new paradigm for understanding intrinsic foot muscle function
Patrick O McKeon, Jay Hertel, Dennis Bramble, Irene Davis
Br J Sports Med doi:10.1136/bjsports-2013-092690
The foot is a complex structure with many articulations and multiple degrees of freedom that play an important role in static posture and dynamic activities. The evolutionary development of the arch of the foot was coincident with the greater demands placed on the foot as humans began to run. The movement and stability of the arch is controlled by intrinsic and extrinsic muscles. However, the intrinsic muscles are largely ignored by clinicians and researchers. As such, these muscles are seldom addressed in rehabilitation programmes. Interventions for foot-related problems are more often directed at externally supporting the foot rather than training these muscles to function as they are designed. In this paper, we propose a novel paradigm for understanding the function of the foot. We begin with an overview of the evolution of the human foot with a focus on the development of the arch. This is followed by a description of the foot intrinsic muscles and their relationship to the extrinsic muscles. We draw the parallels between the small muscles of the trunk region that make up the lumbopelvic core and the intrinsic foot muscles, introducing the concept of the foot core. We then integrate the concept of the foot core into the assessment and treatment of the foot. Finally, we call for an increased awareness of the importance of the foot core stability to normal foot and lower extremity function.

This was not research, but the proposal of a model or theory or framework or paradigm to direct research and interpret foot biomechanics and, hopefully, lead to better management of foot and lower limb problems. Basically, they propose:

core

▸ The foot core system is comprised of interacting subsystems that provide relevant sensory input and functional stability for accommodating to changing demands during both static and dynamic activities. The interaction of these subsystems is very similar to the lumbopelvic core system.
▸ The plantar intrinsic foot muscles within the active and neural subsystems play a critical role in the foot core system as local stabilisers and direct sensors of foot deformation.
▸ Assessment of the foot core system can provide clinical insight into the ability of the foot to cope with changing functional demands.
▸ Foot core training begins with targeting the plantar intrinsic muscles via the short foot exercise, similar to the abdominal drawing in manoeuvre, for enhancing the capacity and control of the foot core system.

I like the concept and it has considerable merit, but it does have some shortcomings. I do not have the space here to regurgitate all the details of the proposed model and the rationale behind it, so refer readers to the full paper in the BJSM. I will focus here on more of a critical appraisal of it, which may be difficult for some readers to understand in places without having the benefit of reading the full paper. As many well know, I have written a lot and lectured a lot on different models/theories of foot biomechanics in the last 30 or so years and have spent a lot of time going through them all (from the good ones to the crankpot insane ones) and looking for commonalities, in that if a lot of models or theories have something in common, then that just might be important. I also spend a lot of time looking for what they have that is different. While a lot of this could just be considered ‘academic wankery’; (ie an academic playing with himself), the driving force for this analysis has always been, what can we do better clinically based on this. Every model/theory of foot function has its fan boys¹ and this one will be no exception. Certain fan boys will blindly jump onto this model/theory as it fits their world view. However, one thing that fan boys have in common is their lack of critical thinking and appraisal skills which is why they open themselves to mockery and ridicule!

I wrote way back in 1999:

“A theoretical model is not necessarily right or wrong. It is valid insofar as its useful to inform clinical practice. They are offered as interpretations which can be validated by practical needs”

So is this proposed model of ‘core stability of the foot’ add anything useful that may have clinical applicability? As this is a blog post and not an academic publication, I will cover this in dot point form rather than in a coherent scholarly manner²:

  • the basis of the concept in the spine is that the local stabilizers are the muscles that provide a stable base, so that the primary mover muscles which are stronger and have bigger lever arms can create the gross movements. The proposed model suggests the same for the foot, in that “the arch is controlled with both the local stabilizers and the global movers of the foot, similar to the lumbopelvic core“. The local stabilizers are the small intrinsic muscles that cross the arch of the foot. The problem with applying this model to the foot is the timing of muscles firing during dynamic gait. The small intrinsic muscles (the alleged local stabilizers) do not actually start working until the later half of the stance phase after the extrinsic muscles (global movers) have already started working or firing. Just look at any chart that illustrates the timing of muscle activity during gait. So based on this, its not clear how the small intrinsic muscles can actually provide the local stabilization for the global movers to work, like they do in the spine. They may provide local stabilization during static stance when there is a perturbation that needs correcting for balance, but generally during static stance the intrinsic muscles are not active unless there is a perturbation. Some would probably consider this as a fatal flaw in the proposed model.
  • While the authors did cite many references, I can still think of a number that were not reviewed, some of which could be considered contradicting the model proposed. For example, Gray and Basmajian early work that showed the intrinsic muscles were more active in the flat pronated foot. If they are more active, then they must already be providing more ‘local stabilization’, that is obviously not working as they have a flat foot. Then their was the Lizis paper that showed no relationship between muscle power and arch height. And, there was this abstract that used the strengthening exercise that would be prescribed by the model and found a decrease in arch height. And, this abstract that showed that barefoot running did not increase the activation of the intrinsic muscles. The authors do appear to have been somewhat selective in the use of references to support what they are proposing. If this model/paradigm is to be useful, it is going to have to explain these inconsistences rather than just ignore them (ignoring inconsistency’s or failing to explain them has lead to the downfall of many a good theory!).
  • another inconsistency that the model can not explain is the ‘intrinsic minus’ foot that we see in the early stages of diabetic neuropathy. Here a weakness of the intrinsic muscles leads to higher arch of the foot and not a lower arch that the model would predict. A similar thing is seen in Charcot-Marie-Tooth disease in the very early stages when only the intrinsic are affected — they develop a high arch foot.
  • Even many of the references the authors did use that looked at intrinsic muscles function did so during static stance and not dynamic function. Given the issue raised in the first point above re the timing of the activity of these muscle during dynamic gait, just how valid are these studies?
  • There are many other things that can change the arch height unrelated to the muscles that have nothing to do with muscle strength. I addressed the issue of barefoot running increasing arch height here, and proposed a model by which it could happen totally unrelated to muscle strength.
  • Probably the most important dynamic determinant of the arch height is the windlass mechanism (reviewed here). Not once in this paper is the word even mentioned, let alone considered as part of the proposed model. I think that is a very big mistake.

When I first came across the title for the paper, my first thought was that this was a paper by Peter WB Oomens as this is what a lot of his work has been on, but it wasn’t and neither was any of his work acknowledged by the authors. However, most of this is in Dutch. There is quite a body of literature in Dutch and French on this concept, under the umbrella of ‘posturology’ that I have been following the best I can for some time now using Google translate which is not that good at getting some of the grammar right! Oomens’ most recent discussion of the concept in English is in the most recent edition of Current Pedorthics (for those that have access to it): Posturology as a Treatment: Intrinsic Muscles Instead of Rigid Foot Orthotics (not saying I necessarily agree with it for pretty much many of the same reasons above; but it does put the proposed model into a different context).

I will finish with Payne’s Law that I first proposed over 20 years ago:

The amount of passion involved in supporting a theory and the amount of emotional attachment to a theory is inversely proportional to the amount of evidence for that theory

…think about that….ring any bells?

Lets see what happens with this proposed core stability of the foot concept…..I do like it, but it has to be able to explain the inconsistencies above….and it does not matter how good a theory it is, the windlass mechanism is not going to go away and has to be incorporated into it.

As always: I go where the evidence takes me until convinced otherwise.

¹… and ‘girls’
²…and besides, it is also Saturday and want to take my girls swimming!

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7 Responses to The concept of ‘core stability’ of the foot

  1. Kevin A. Kirby, DPM March 22, 2014 at 5:53 am #

    Craig:

    I just read the paper and can’t say that I was very impressed with any of it as being “a new paradigm” for understanding the function of the intrinsic muscles of the foot. The only thing that is “new” about this paper is that they came up with a catchy new name “the Foot Core System” to describe the function of the smallest muscles of the lower extremity: the intrinsic muscles of the foot. To call this paper “a new paradigm” is quite a stretch, at best.

    The one positive aspect of this paper is that it did emphasize the intrinsic muscles of the foot which are often ignored and should be considered when biomechanically induced pathologies of the foot and lower extremity are being considered. However, that being said, there are many things that I don’t like about this paper.

    Contrary to what the authors imply in their paper, many authors have discussed the function of the intrinsic muscles of the foot over the past century and, from what I have read of their descriptions over the past three decades of my podiatric medical career, they seem nearly identical to the descriptions that McKeon et al give as descriptions of the function of these small muscles in this new paper. The plantar intrinsic muscles serve to help support the longitudinal arches of the foot and flex the digits at the metatarsophalangeal joints, something that has been said in the medical literature repeatedly over the years.

    However, something that has also been said by these other authors before McKeon et al proposed their “new paradigm” is that the intrinsic muscles of the foot are so small that they really should be considered as muscles that “assist” the functions of the larger extrinsic muscles of the foot (i.e. gastrocnemius, soleus, posterior tibial, flexor hallucis longus, flexor digitorum longus, peroneus brevis, peroneus longus, extensor halluxis longus, extensor digitorum brevis and peroneus tertius muscles) and have no real unique functions that the extrinsic muscles don’t already have. Collectively calling these muscles the “core of the foot” is like calling the palmar intrinsic muscles of the hand the “core of the hand”, or the facial muscles “the core of the face”. In other words, calling these small muscles “core muscles” sounds ridiculous to me and, in my opinion, do not in any way help us understand the function of the foot and lower extremity.

    Finally, the suggestion in the paper that foot orthoses need to only be worn for a short period of time before they should be removed does not, at all, coincide with my 30 years of treating foot and lower extremity injuries with premade and custom foot orthoses. The authors state “Current clinical guidelines include the use of foot orthotic devices for heel pain and plantar fasciitis, but lack any reference to strengthening of the foot. While temporary support may be needed during the acute phase of an injury, it should be replaced as soon as possible with a strengthening programme just as would be carried out for any other part of the body.”

    This statement makes it sound as if foot orthoses actually weaken feet which the scientific evidence does not show. Do the authors actually think that these little muscles of the foot can actually prevent the arches of the foot from collapsing by themselves without a foot orthosis in patients with significant flatfoot deformity? I certainly hope not. And to say that there is no reference to strengthening of the plantar intrinsic muscles in the literature for treatment of plantar fasciitis is simply wrong. Here are a just a few papers published years before McKeon’s paper where the concept of strengthening intrinsic foot muscles for plantar fasciitis is discussed.

    http://www.orthomaryland.net/files/plantar-fasciitis-hand-out.pdf

    MW Cornwall, TG McPoil Plantar fasciitis: etiology and treatment, – Journal of Orthopaedic & Sports Physica, 1999.

    T. J. Chandler, W. B. Kibler A Biomechanical Approach to the Prevention, Treatment and Rehabilitation of Plantar FasciitisSports Medicine,May 1993, Volume 15, Issue 5, pp 344-352.

    Roxas, Mario:Plantar Fasciitis: Diagnosis and Therapeutic Considerations.Alternative Medicine Review . Jun2005, Vol. 10 Issue 2, p83-93.

    Do optometrists, opthamalogists and other medical eye specialists tell their patients to only wear their prescription eye glasses a short period of time until their symptoms of headaches disappear when trying to read so that they can then “strengthen the core muscles of their eyes” to be able to read better? No. In much the same way, some people can quit wearing their orthoses for period of time and not have a return of symptoms. However, to think that all foot orthoses should be discontinued in patients because of some odd belief that foot orthoses “weaken feet” is based only on the unsupported conjectures of the barefoot and minimalist running shoe zealots, is not based on scientific research and is not consistent with the clinical observations of myself , and many other sports podiatrists, over the past half century regarding the foot strength of their patient who habitually wear foot orthoses.

    Is the “Foot Core System” a “a new paradigm for understanding intrinsic foot muscle function”. No. Is the “Foot Core System” a new name for something that has previously been published, sort of like making up the name “minimalist running shoes” to describe the racing flats that had existed for four decades before the “minimalist running shoes” was made up? Yes.

    If anyone wants to read some good research on the plantar intrinsic muscles, Luke Kelley’s research is fantastic and is really ground breaking stuff….and Luke doesn’t call them “core muscles of the foot” either!

    Kelly LA, Kuitunen S, Racinais S, et al. Recruitment of the plantar intrinsic foot muscles
    with increasing postural demand. Clin Biomech (Bristol, Avon) 2012;27:46–51.

    Intrinsic foot muscles have the capacity to control deformation of the longitudinal arch
    LA Kelly, AG Cresswell, S Racinais, R Whiteley, G Lichtwark
    Journal of The Royal Society Interface 11 (93), 20131188

    Dynamic function of the plantar intrinsic foot muscles during walking and running
    L Kelly, G Lichtwark, A Cresswell
    Journal of Science and Medicine in Sport 16, e4-e4

    Discharge properties of abductor hallucis before, during, and after an isometric fatigue task
    LA Kelly, S Racinais, AG Cresswell
    Journal of neurophysiology 110 (4), 891-898

    Do Foot Orthoses Alter Muscle Activation, Running Economy and Neuromuscular Fatigue During a 1-h Treadmill Run?: 843: June 1 2: 15 PM-2: 30 PM
    LA Kelly, O Girard, S Racinais
    Medicine & Science in Sports & Exercise 43 (5), 101

    Cheers,

    Kevin

  2. Dave Smith March 22, 2014 at 10:12 pm #

    Trying to keep things short and sweet for this format is a difficult task

    Stability: what do we mean when we use phrases like ‘maintain stability in the arch’

    Perhaps integrity is a better concept!?

    Integrity: the state of being whole, unified, sound in construction and without corruption, maintained like the original or perfect. Fit for purpose.

    The integrity of a structure or mechanism is not only defined by its physical state but also by an abstract idea of what that state should be I.E. An intended or optimal state and some acceptable range of deviation from that perfect or intended state.

    If the mechanism still performs to it’s intended purpose then it can be seen as maintaining its integrity
    The definition of integrity in this case could be, and in practice is, quite subjective

    A flat pes pancakus abducted foot is still a stable structure but has generally lost its integrity in terms if the mechanism

    Structure and Mechanism

    A structure is designed or has the ability to be stable to certain magnitudes of forces applied in known directions

    A mechanism is designed or is able to be predictably unstable to forces applied in known directions

    A mechanical structure is able to combine both these qualities
    In most cases mechanical structures also maintain some predefined structural integrity I.E it doesn’t just fall down or collapse in a heap
    The kinematic changes following kinetic changes are predictable within certain tolerances and boundaries that allow integrity to be maintained both physically and as a concept.

    • Dennis Kiper April 28, 2014 at 11:42 pm #

      “what do we mean when we use phrases like ‘maintain stability in the arch’

      Perhaps integrity is a better concept!?”

      I don’t know what others mean, but I mean that there is NO intrinsic pronation that occurs
      through heel off phase.

      Maintaining “integrity”, doesn’t mean anything to the stability IF there is “destabilization” through intrinsic pronation.

  3. Rodger Kram April 7, 2014 at 2:03 am #

    sorry, this is not directly related to this post, but have you seen this?

    http://www.acsm.org/docs/brochures/running-shoes.pdf

    talk about woo pretending to be science/medicine.

    • Craig Payne April 7, 2014 at 2:26 am #

      Thanks Roger. I had seen it and also seen comments in a few places congratulating ACSM on their evidence based approach …. go figure!

  4. Charlie Baycroft April 16, 2014 at 1:55 am #

    Hi Craig.

    I do tend to agree with Kevin that this is not a new paradigm but it does indicate that people are starting to understand that the muscles and their neural control are important to the function of the human foot and leg.
    Biomechanical function has been the prevailing paradigm since the 1960’s.
    I refer to the definition of a paradigm as proposed by Thomas Kuhn.
    Kuhn defines a scientific paradigm as: “paradigm as the underlying assumptions and intellectual structure upon which research and development in a field of inquiry is based.”

    Before Root’s Theory the prevailing paradigm was congentital or acquired flat foot due to “weak arches”. Root et al. applied new concepts of biomechanics to the diagnosis and treatment of lower extremity problems and that was indeed a breakthrough that has served Podiatrists (the community of practitioners) very well for the last 40 years. The biomechanical paradigm is the world view or accepted assumptions based on the application of mechanics to the structure and function of the human foot and leg.

    According to Kuhn, paradigms change or shift as science increases and improves our knowledge. This process eveolves from scientific findings that conflict with or contradict the prevailing paradigms. These are initially regarded as “anomalies and recent and ongoing research certainly supports the idea that biomechanics does not fully describe how the foot and leg function because it tends to ignore the muscles and the nervous system. These anomalies can lead to the proposal of new theories and over time these new theories or hypotheses can become accepted and form the basis for new paradigms.
    It was Kuhn’s opinion that new paradigms are generally more valid than the older ones.

    IMO, this is what is currently happening with our understanding of foot and leg function and the role of foot orthotic therapy. Research is starting to show the shortcomings of Root’s Theories and also that foot orthoses have effects on the synchronization and sequencing of muscle activity. The “old guard” who have a strong vested interest in the biomechanical paradigm are very resistant to change and tend to disregard the potential significance of literature that does not agree with their views. Some more open minded “authorities, younger people within the field and people from other disciplines are paying more attention to this new evidence and trying to propose theories that incorporate it. The paradigm is and will shift and in the future we will have a better understanding of lower extremity function and foot orthoses that is more consistent with the sum of the existing scientific evidence.

    The theory of the 3 subsystems that control stability was proposed by Panjabi 25 years ago in relation to the function of the spine and was very influential in changing the paradigm related to the diagnosis and treatment of back pain. The previous paradigm (that I learned in medical school) was actually more biomechanical and focussed on braces, splints and surgery to control “instability” (sound familiar?).
    Panjabi’s other important paper dealt with his theory of the neutral zone and this is worth considering in relation to Root’s neutral subtalar position. The application of this is to design foot orthoses to reduce the amplitude of subtalar oscillation during stance and thus reduce the energy expenditure and tissue stress related to gait.

    Of course we both value Kevin’s SALAR theory and the potential to apply forces through the foot that reduce the work of the muscles to supinate the STJ.

    As you say, the goal is to derive paradigms and practical procedures that are based on scientific evidence rather than professional wank and ego massage and these new paradigms ought to integrate the various theories and hypotheses that exist in relation to the evidence that supports them. This is the challenge for eventually utilizing the full potential of foot orthoses as a therapy to optimize the function of the lower extremity and possibly the musculoskeletal system in general. I believe that this could potentially have a very significant impact in the treatment and prevention of many musculoskeletal conditions that people recurrently and chronically suffer from.

    Another question that I have concerns whether or not our current “gold standard” RCT research methodology is the best way to study foot orthotic therapy. This method is well suited to the investigation of chemical therapy where the agent and the chemistry that it acts on are relatively well defined. The effect of a pill or capsule is related to the drug that it delivers to the body and IMO the effect of a foot orthosis is related to what the therapist designs into it and this is much different from a chemical. The inherent problem in interpreting and applying foot orthotic research is that no two patients, therapists or problems are the same. In chemical therapy the chemistry and the agent are the same (although individuals do have some differences in body chemistry).

    Foot and leg function are so different from person to person that gait has been proposed as a way to identify individuals (like fingerprints). The same feature in a foot orthotic has different effects on individual patients. Relief of symptoms can be due to realignment of forces and reduction of tissue stress rather than an overall improvement in function.

    I am proposing that foot orthotics should be thought of as a form of functional therapy rather than “insoles”. Functional therapy is (as Mert Root said) a clinical process and an art rather than a prescriptive event. Of course the practice of this art should be based on valid scientific principles and evidence as much as is possible at the time.

    • Dennis Kiper April 27, 2014 at 1:57 am #

      First, I’d like to ask if you are the founder of Formthotic?

      Charlie, I thought your post was very well written. I’m wondering if you are familiar with the fluid technology of the silicon orthotic?

      Either way, the ability for orthotic therapy and your post to be accurate, will necessitate a change in current technology.

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