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:
▸ 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!