“If you want to run, run a mile. If you want to experience a different life, run a marathon...”

This was how international athletics legend, interval training trailblazer and quadruple Olympic champion Emil Zátopek saw the legendary 26.2-mile distance: as an opportunity to become a better version of yourself by experimenting with pushing past your limits. However, once your bib is pinned on and you’re covering the miles, it's not only your life that is set to change. No offence to Emil, but as you rack up the distance, your feet - the key for any runner - will also change. So, after a marathon your life will never be the same again - and nor will your feet.

Your stride shortens but speeds up

The most obvious change, and one that is sometimes brutal for marathon runners, is a shorter stride. Sidas Podiatry Centre Manager Marie Maligorne explains this phenomenon: “Physical stress tires the body and this leads to decreased muscle tone. Your stride becomes heavier and less dynamic. This makes it more robotic and less natural. Your brain tells you to run, but your legs no longer respond.”

Clément Scalliet, an osteopath and physiotherapist who works with several high-level athletes, continues: “Your stride shortens but your pace increases to compensate. At the start of a marathon, people tend to favour the front cycle, which absorbs impact and propels the body forward. However, little by little, they tend to abandon this bouncy yet energy-intensive midfoot approach for a heel-first style that is more economical, but also more traumatic for the bones and muscles.”

This transfers the load from the toes and forefoot to the heads of the metatarsals, leaving them fragile and at risk of a stress fracture. “Hitting the wall” at around the 20-mile mark is not only bad for the brain, it also damages the feet.

Your plantar arch falls

Once over the finish line, marathon runners will have a deservedly bigger ego, but will actually be smaller in size. This is due to two things (both temporary, don't worry): the back muscles contract with slower blood flow between the vertebral discs; and, above all, the plantar arch falls. A study by two American researchers has proved that the navicular bone at the top inner side of the foot falls half a centimetre on average after 26.2 miles.* “A finding that makes running marathons rather elitist,” says sports doctor Marion Delespierre.

She continues: “Less skilled runners more often fall victim to the harmful effects of this physical stress on the foot as their bodies are less well prepared and they spend more time on the tarmac.” The numbers speak for themselves: a champion’s trainers will hit the road 25,000 times during a marathon, whereas those of an intermediate runner will do so twice as many times.

*The effects of prolonged running on foot posture: a repeated measures study of half marathon runners using the foot posture index and navicular height

Emma Cowley and Jonathan Marsden. Journal of Foot and Ankle Research (2013)

Accentuated pronation

Fatigue not only shortens your stride. It also accentuates your running style. Your “style” means whether you are a pronator and your foot rolls to the inside as you step, or a supinator and your foot rolls to the outside. Clément Scalliet explains: “In simple terms, 80% of runners are pronators and 20% are supinators. As people cover the miles, their inherent tendency only comes to the fore and intensifies. This can lead to hallux valgus bunions for those whose feet roll to the inside, and quintus varus bunionettes for those whose feet roll to the outside.”

Longer feet

Podiatry expert Marie Maligorne confirms what long-distance athletes already know: “When you run a marathon, your feet will extend and gain between half a size and a whole size in length, or 4 to 8 mm. So, the recommendation to get yourself some shoes that are slightly longer with some margin for expansion is worth listening to.”

Marion Delespierre analyses this phenomenon: “When the body is under physical stress the feet swell as more blood flows to them. The influx is an important way to provide energy. This applies to all working muscles including the heart and lower limbs. However, it is at the expense of the digestive system, which lacks blood flow and this can lead to stomach problems.” So, to finish a marathon you need to be plucky and have plenty of blood flowing to both belly and feet!

One increasingly dominant foot

Bipeds all have one foot stronger than the other. Your dominant foot is the one you kick a ball with when on the football pitch and the one you naturally lead with when going downstairs. During a marathon, your stride puts a kind of pressure on your feet called plantar pressure. This pressure is slightly greater on your dominant foot for the first few miles.

However, as you cover the distance, your stride gradually becomes more unbalanced in the sense that your dominant foot works harder, while your weaker foot works less. As you approach the finish line, the pressure on your dominant foot is 2.5 times greater than that on your weaker foot.

*Long distance running increases plantar pressures beneath the metatarsal heads: a barefoot walking investigation of 200 marathon runners.

Fernholz F, Kibele C, Rosenbaum D. (2008)

Your foot temperature rises

You’ll certainly have some sweat on your brow when you finish a marathon, but you will also have sweaty feet. They are made to work incredibly hard as you run and sweat a great deal. Your foot temperature can rapidly increase due to increased blood flow, friction and - sometimes - the weather.

These hot, humid conditions, combined with fatigue and loosening laces, create the perfect environment for blisters to form, especially under the heads of the metatarsals and at the back of the heel.

Help your foot manage these changes with an insole

During a marathon, your feet change due to the physical stress they are under. There can be two consequences: you may have to abandon the run or, worse, endure post-race trauma that could lead to more serious injuries. To counteract this, the ideal solution is an insole.

Céline Vocanson, Head of R&D at Sidas and a marathon runner herself, explains:

- “First, the insole has a shell at the heel to hold, support and mould to the arch of the foot. This means muscle groups have less work to do and limits the risks of cramping and the plantar arch falling. The aim is to maintain your natural stride for as long as possible!”

- “Next, the insole works like a car bucket seat in that once the foot is comfortably in place, it won’t slide back and forth. The goal is to avoid overheating and blisters.”

- “Finally, the insole supports and optimises the absorbo-propulsive movement of a runner striding along. This is because there is gel or EVA at the heel for absorption and a more dynamic material at the forefoot.”