Calluses, skin thickened by rubbing against other surfaces, would seem to offer protection at the expense of sensitivity. But that isn’t what Harvard University human evolutionary biologist Daniel Lieberman has experienced when he runs barefoot in summer.
As Lieberman’s calluses get thicker, “it [doesn’t] hurt as much to run — I could step on acorns and other things,” he says. “But I never felt like I lost sensory perception.”
To explore why, Lieberman’s research group and colleagues in Germany and Africa measured the thickness of calluses on the feet of 81 adults in western Kenya. The participants, a mix of city and country dwellers, ranged from full-time shoe wearers to those who were usually or always barefoot.
The researchers measured how sensitive participants’ foot soles were with a small device that applied pressure to the skin. When participants felt a poke from the device, they pressed a button. The results of the experiment failed to find a relationship between increased callus thickness and reduced foot sensitivity, the researchers report online June 26 in Nature.
“People who had thicker calluses had no loss of sensitivity,” Lieberman says. And as human feet once were always bare and callused, it makes sense that “those calluses didn’t exert a cost in terms of our ability to sense the ground underneath us.”