For decades, avid runners and casual joggers have had their ups and downs with the running shoe. Some argue that the shoes’ spongy soles help us bound comfortably across our unforgiving urban landscapes of concrete and asphalt. Others, however, think the shoes simply run off with our body’s natural spring-like steps. During the last 40 years, skeptics are quick to point out that the rate of running injuries hasn’t stumbled.

Now, with a new study on the mechanics of running, researchers suggest that running shoes actually do a little of both—cushioning and altering our innate bounce. It just doesn't happen the way we may have expected, the researchers report in the Journal of the Royal Society Interface.

To track down the impact of running shoes, researchers at the University of Queensland outfitted 16 healthy volunteers with intramuscular electrodes that recorded the muscle activity in their feet. Then they had those wired volunteers run—both barefoot and shod—on a treadmill rigged with force sensors. The researchers paid particular attention to the muscles in their longitudinal arches, which have a natural spring-like action, bending as the foot lands and recoiling on the lift.

The researchers found that when the volunteers wore shoes, their arches didn’t bend as much—about 25 percent less than when the volunteers ran barefoot. This appears to support the idea that shoes effectively cushion each foot strike.

Shoe skeptics have argued that such cushy arch support means less muscle activity, leading to weakness, atrophy, and injuries. But, the researchers found the opposite—the arch muscles were actually working harder in shoes than they were barefoot.

To make sense of the data, the researchers drew on some results from earlier experiments. In those, the researchers stimulated muscles with electrical zaps and found that the muscles in the foot work as a unit, not individually. Putting it together with the runners’ data, the researchers hypothesized that with arch support from the shoe, the foot muscles had to work harder to match the extra stiffness in the system.

When the researchers sized up the data with different mechanical models, they found that the best fit was a scenario in which running shoes act as a spring that works in series with our feet. In other words, the shoes seemed to stiffen the foot’s natural springiness, rather than act like an independent shock absorber.

While this is far from the end of the road for running shoe research, the data does hint that running shoes may have benefits. Previous studies have found that maintaining leg stiffness during running is something our bodies innately try to do even when running on varied surfaces. This may be because our brains use leg stiffness to keep our balance and control our center of gravity as we dash around. Because running shoes provide some arch stiffness, foot muscles must work harder, boosting overall stiffness.

More research is needed to understand how this running shoe-altered foot action alters overall leg function and how it might change on different surfaces, like concrete, the authors say. Still, they conclude, “these findings highlight that the alterations in lower limb biomechanics observed when running in shoes are not a result of reduced or impaired neuromuscular function.”

Journal of the Royal Society Interface, 2016. DOI: 10.1098/rsif.2016.0174 (About DOIs).