Many people have a negative perception of flat feet and consult us to correct this problem. They believe that their flat feet are the cause of their injury. So what’s really going on?
More specifically, do we know at what point the foot becomes flat and what a flat foot is? Can plantar orthotics correct this “problem”? Do you know of people with flat feet, whose plantar orthotics were able to lift their arches and make their symptoms go away? A strange coincidence, no? Nevertheless, correlation does not necessarily mean that there is a cause and effect relationship! I can already hear you asking me: “What’s the connection?” The point is that there is no connection, but there is a correlation. Why should it be different for the plantar orthotics and flat feet, even if at first glance there seems to be a direct relationship? In fact, plantar orthotics are unlikely to achieve therapeutic results through kinematic changes (movements), but rather through kinetic changes (applied forces). There is therefore very little relationship between the change in the shape of the plantar arch (kinematics) and results!
I hope I am not saying anything new when I state that flat feet or pronation are not the cause of musculoskeletal injuries in runners and the general population. In fact, several excellent studies are quite clear on this subject [1-6]. Even the well-known researcher, Benno Nigg, PhD, who helped companies develop footwear that limits pronation and flat feet in the 1990s and 2000s, points out, in his most recent article, that pronation is a minimal risk factor for developing running injuries .
Nevertheless, I hope to provide some new information when I tell you that at the moment, we don’t really know what a flat foot is. In fact, it is currently very difficult to define at what point a foot becomes flat. It is a bit of a contradiction to use this term so commonly, while not being able to clearly define a flat foot [7, 8]. For instance, let's consider the Foot Posture Index (FPI-6), which is often used in studies to quantify foot type . This is a form to be filled out by a health care professional and includes six foot characteristics. The total of the six indices provides a result ranging from -12, a very supine foot (hollow) to +12, a very pronated foot (flat). Therefore, feet exist along a continuum between the two, in accordance with the distribution of a normal Gaussian curve [10, 11]. Hence, this system serves as a reference point to characterize the features of a foot and to classify it as hollow, normal or flat. For example, at +6 a foot is considered normal, but when you reach +7, you have a foot that is considered flat. Do you really believe that just one additional feature to add 1 point can actually increase your risk of injury? Or rather, do you think that the difference between having a normal foot and a flat foot is based on a single characteristic of your foot? That this characteristic is the deciding factor between being normal or having a condition (a flat foot). I highly doubt it! My clinical practice does not agree with that statement either.
As such, several retrospective studies and systematic reviews have examined the relationship between FPI-6, foot kinematics and musculoskeletal injuries. Indeed, as mentioned by Levinger, et al. , the asymptomatic population with a foot rated as flat according to FPI-6 exhibits greater eversion of the hindfoot and increased abduction of the forefoot than the population with a so-called normal foot. However, as I mentioned earlier, there is strong support that FPI-6, pronation and flat foot are not significant risk factors for developing injuries in the lower limbs [1-6]. I used FPI-6 as an example to illustrate my point, but the same is true for the height arch index , the foot impressions taken in sports stores, the arch height ratio  or navicular drift and drop . All of which have their own definition of when a foot is deemed to be flat. This increases the variability of findings with regard to defining what constitutes a flat foot.
Now that we know that we don't actually know what a flat foot is, let's talk about plantar orthotics and their impact on a flat foot. The etymology of the word orthotic originates from the prefix ortho, which comes from the Latin word orthos which means: correct; just; straight; normal. Plantar refers to everything beneath the sole of the foot. The combination of the words orthotics and plantar (plantar orthotics) would therefore imply restoring the foot to normal. This might explain why we are conditioned to believe that the plantar orthotics have a beneficial effect through their ability to position the foot in a correct, right, or normal position? However, based on mounting scientific evidence, I believe that the probability that plantar orthotics have a beneficial influence in realigning the lower limb (kinematics) is less than 1% . Several studies have shown that feet are “corrected” by 2 to 3 degrees by using plantar orthotics and that sometimes the opposite effect can be observed, i.e. the plantar orthotics may cause the foot to collapse (valgus/eversion) . On the other hand, a more plausible hypothesis is that the (kinetic) forces exerted on the lower limb can be modified through plantar orthotics. On the other hand, the effects observed in the studies are much more consistent when kinetics is considered. Moreover, even when a kinematic effect is absent, a kinetic effect can still be observed. This is because a foot can be in an identical position, but the forces applied by tissues are not identical. For example, if you put your foot on a block with your ankle at a 90-degree angle, you will feel some tension on the front side of your ankle (right foot with green arrows in figure 1). If you move your heel into the gap, the tension will gradually increase on your Achilles tendon as your heel moves deeper into the gap.
Similarly, plantar orthotics can change the way soft tissue applies tension and stress to your ankle, without necessarily changing the angle or movement of the ankle. By relieving tension and the forces acting on wounded tissue, it can heal. Plantar orthotics may act in the same way as resting, increasing cadence or decreasing the speed of the force of impact, i.e., it reduces mechanical stress on the injured tissue and allows it to heal. These factors have nothing to do with the shape of the foot or its degree of pronation. Mechanical stress quantification (MSQ) is a much more accurate concept. In fact, there is another tool that can be used to quantify mechanical stress. Indeed, at this time, the kinetic effect remains the best theory to explain the beneficial results of plantar orthotics for certain conditions. Future research will confirm or disprove this potential mechanism of action.
 R.O. Nielsen, I. Buist, E.T. Parner, E.A. Nohr, H. Sorensen, M. Lind, et al., Foot pronation is not associated with increased injury risk in novice runners wearing a neutral shoe: a 1-year prospective cohort study, Br J Sports Med 48(6) (2014) 440-7. https://www.ncbi.nlm.nih.gov/pubmed/23766439.
 G.J. Dowling, G.S. Murley, S.E. Munteanu, M.M. Smith, B.S. Neal, I.B. Griffiths, et al., Dynamic foot function as a risk factor for lower limb overuse injury: a systematic review, J Foot Ankle Res 7(1) (2014) 53. https://www.ncbi.nlm.nih.gov/pubmed/25598843.
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