By T.J. Murphy

In Dr. Kelly Starrett’s NYT Bestseller, Ready to Run, one of the standards he advises triathletes and runners to follow is to migrate toward wearing flat shoes and avoiding shoes with raised heels. He also says there’s a lot of natural therapeutic power to spending time being barefoot and letting your foot be a foot.

So what’s the big deal about a good heel cushion? Isn’t that a good thing?

In fact, there was a day when triathletes typically extended the life of a traditional running shoe by using it as an everyday shoe after the cushioning had blown out. After the midsole on your Nike Air 180s was flat and the air pockets beaten lifeless, you’d wear the shoes to mow the lawn or to run errands or to just walk around in. Depending on the culture, you might even wear them to work.

It seemed to make sense that after a long run or hard interval workout it was good to change out of your lightweight trainers or racing flats and put on pair of heavy-duty trainers with a 12mm heel-to-toe drop (or more) to give the ankle complex a break.

And if you’ve ever suffered the debilitating effects of a sore Achilles tendon, you may have followed the advice to not only wear shoe’s with a substantial heel but to add a heel cushion to further decrease the stretch and stress on the heel cord.

Starrett’s key point is that the time we spend in shoes, especially shoes with significant heels, is time that the tissues of the lower leg shorten and mold into states of poor mobility. This time and the subsequent adaptations add up, Starrett says, and when you go out for a run, you’re asking the compromised muscles and connective tissues to handle not only a greater range of motion but also the duress of being springs with a significant load.

A 2014 study published in Medicine & Science in Sports & Exercise supports Starrett’s thesis. Researchers considered the common practices of runners in regards to heel-raised shoes: “Footwear remains a prime candidate for the prevention and rehabilitation of Achilles tendinopathy because it is thought to decrease tension in the tendon through elevation of the heel.”

Researchers used ultrasound technology and an instrumented treadmill to measure ground reaction forces and acoustic velocity in 12 male subjects. The participants were measured both with shoes and without, the running shoes having a 10mm heel offset. The results showed that with shoes there was an increase in step length, stance duration and peak vertical ground reaction force compared to barefoot walking.

Contrary to conventional expectations, the peak tension levels in the heel cord were significantly higher with running shoes.

Researchers concluded that, “Peak acoustic velocity in the Achilles tendon was higher with footwear, suggesting that standard running shoes with a 10-mm heel offset increase tensile load in the Achilles tendon.”

If you’ve been using shoes with high heel drops and want to convert to a practice of using shoes that are flat, Dr. Starrett urges you to make the transition slowly—don’t just buy a pair of Inov-8s and bury your motion control shoes in the front yard. Rather, start off spending a short amount of time each day walking around in flat shoes (and when you’re in the house, go barefoot as much as possible), and steadily increasing the amount over days and weeks to allow your feet, ankles and lower legs to progressively adapt to the demands.