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While having flat feet is no longer a bar to military service, many people still think they’re a problem. Why is that and what problems can they cause? Should I be worried if my feet become flat later in life?

At one time having flat feet meant exclusion from military service, and even now the presence of flat feet may be noted during a medical when joining the services.

Why is this?

Flat feet can be an indicator of poor biomechanics, which means your walking style. Because of the connectivity throughout the skeleton, problems with feet and walking can lead to problems elsewhere such as pain in the knees, hips, back and even neck.[1]

What are flat feet?

If you stand on a piece of absorbent paper with wet feet and then examine the imprint left, you will probably see that not all of the sole of the foot is in contact with the ground i.e. not all the arch comes out on the paper. 

If you compare your imprint to other people’s you will notice considerable variation as to how much of the arch comes out on the paper. 

In about 20% of the population there will be an extremely narrow margin of sole in contact with the paper along the outer border of the foot.  These are the people with ‘high arches’ or the supinators. But more of that later.[2]

If your paper imprint is complete i.e. shows all of the arch, or is nearly complete, then you have flat feet.  So a flat foot is simply one with a low arch. Very few people will have a completely flat foot i.e. a complete imprint.

Do flat feet cause problems?

Not always. You will notice that babies all have flat feet – with no visible arch at all.  They do have an anatomical arch, but it is filled with fat until later in development, when an arch gradually appears.

Some races of people are noted to naturally have a very low arch and in most populations arch height will vary from very low to very high with most people falling somewhere in the middle of the range.

 But this is referring to arch height at the end of your physical development – in your late teens/early 20s. 

If your arch height drops later in life, this is termed ‘acquired flat foot’ and is sometimes associated with problems, such as excessive pronation.[3]

What is excessive pronation?

Walking is a very complex process enabling us to pass our weight forward, over our feet - without falling over!  During this process, our feet flex and change shape to allow it to happen.  One change that occurs is called pronation and involves the foot rolling inward and lowering the arch closer to the ground.  This is soon followed up by the opposite motion – known as supination – where the foot rolls outwards and the arch moves away from the ground.  In some people their walking pattern is not ideal and they pronate at the wrong time or for too long. Excessive pronation usually causes overuse type injuries, occurring most frequently in runners. This is known as excessive pronation. Studies have proven the connection between an excessive pronation and the development of runner's knee (chondromalacia).[4

Problems associated with flat feet

What happens in one part of the skeleton automatically affects other parts of it. When the foot pronates, the weight-bearing bone in the shin (the tibia) rotates towards the other leg.  This stabilizes the knee joint to bear weight while taking a step, but when it happens too often or for too long – as in excessive pronation – it puts stress on the knee, causing pain.[4]

The twisting motion can also be passed via the thigh bone (which is part of the knee joint) to the hip joint, causing pain. The disruption to the normal pattern of walking can be transmitted up the body via the skeleton, to the back and neck, causing pain.

So is it better to be a supinator?

A supinated foot has a characteristically high arch and spends an excessive amount of time on the outer (lateral) rather than on the medial (arch) border of the foot (as in the case of pronators).  Supinated feet pronate very minimally. As they are the opposite of the flat-footed pronator, it might be reasonable to assume they avoid foot problems. But this is not the case. 

Pronation is an important component of normal walking as it enables the foot to absorb shock when coming into contact with the ground and taking the weight of the body. Because of a reduced ability to pronate, the supinated foot is less able to absorb shock and consequently can be more susceptible to injury.[5

One study of military recruits found that those with high arches sustained nearly four times as many stress fractures as their flat-footed colleagues.[6

What other problems can be associated with flat feet?

The arch is held up by similar engineering principles to those which keep bridges up – the use of a ‘keystone’ kept in place by the surrounding bones.  It is also maintained with tensioning applied by the tendon of the tibialis posterior muscle, which is found in the calf.  But trauma – such as falling off a kerb – or physical changes such as increasing age or weight, can cause this tendon to give way causing the arch to suddenly drop. The resulting change in foot function nearly always causes painful problems

Diabetes is associated with a neurological (affecting nerves) condition called Charcot’s Neuroarthropathy.  This leads to loss of feeling in the feet, and often painless but serious changes in foot structure including complete flattening of the arch.  Sometimes the arch bulges in the opposite direction leading to ulceration as the bulge is subjected to the pressure of walking.[7]

So is having flat feet really a problem?

If the arches have always been low, there is every chance that they will never cause any problems.  Even when associated with excessive pronation flat feet may function perfectly

But a reasonable percentage of flat feet which have other problems such as excessive pronation will encounter problems – in the feet and elsewhere in the skeleton.

Flat feet which occur later in life, especially suddenly, as a result of trauma or other medical conditions like diabetes, are almost certain to cause big problems.

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