Extended Data Fig. 1: Skin stiffness.

a, Comparison of skin stiffness in usually shod (blue; n = 46) and usually barefoot (red; n = 34) individuals. Data are mean + 1 s.d. Significant differences were calculated after log-transforming the data (two-tailed Welch’s two-sample t-test, *P = 0.03). No significant difference was found between usually shod and usually barefoot individuals at the first metatarsal head (P = 0.09; two-tailed Welch’s two-sample t-test). For test statistics, see Extended Data Table 3. b, c, Relationship between skin hardness and skin stiffness at the heel (b; n = 79) and the first metatarsal head (c; n = 79). Squares indicate men and circles indicate women. d, e, Relationship between callus thickness and skin stiffness at the heel (d; n = 74) and the metatarsal head (e; n = 74). r and P values are from Pearson product–moment correlation association tests. Lines represent linear regression model fits for the two variables.