Humans might have been born to run, but we definitely weren’t born to run on concrete. To cushion the blow from jogging on such hard surfaces, running shoes provide a barrier of springy foam between the road and our sensitive feet. But even as shoes have grown more advanced, the rate of running injuries hasn’t dipped for the last 40 years. That makes some researchers—and runners—suspect that the shoes might be doing more harm than good. Now, a new study tests that claim. When running, bare feet act like springs, absorbing the shock of striking the road, which they then use as energy to push off into the next stride. Shoe-doubters have claimed that overly bouncy running shoes interfere with that process, encouraging foot muscles to relax and eventually weaken. So scientists had 16 participants run, both barefoot and shod, on a treadmill outfitted with force sensors, while thin wires threaded under the skin of their feet tracked muscle activation. The running shoes did, in fact, interfere with the foot’s ability to act like a spring, decreasing how much the foot’s arch was able to compress when it hit the ground—whereas bare feet would have flattened out like a pancake, shod arches only got 75% of the way there. But in response, the foot didn’t relax, as many had suspected. Running shoes actually made those muscles work harder to keep the arch stable, the researchers report today in the Journal of the Royal Society Interface. So the evidence is in: Wearing shoes does change the physiology of running. Just not in the way scientists expected.