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Foot callus thickness does not trade off protection for tactile sensitivity during walking

Abstract

Until relatively recently, humans, similar to other animals, were habitually barefoot. Therefore, the soles of our feet were the only direct contact between the body and the ground when walking. There is indirect evidence that footwear such as sandals and moccasins were first invented within the past 40 thousand years1, the oldest recovered footwear dates to eight thousand years ago2 and inexpensive shoes with cushioned heels were not developed until the Industrial Revolution3. Because calluses—thickened and hardened areas of the epidermal layer of the skin—are the evolutionary solution to protecting the foot, we wondered whether they differ from shoes in maintaining tactile sensitivity during walking, especially at initial foot contact, to improve safety on surfaces that can be slippery, abrasive or otherwise injurious or uncomfortable. Here we show that, as expected, people from Kenya and the United States who frequently walk barefoot have thicker and harder calluses than those who typically use footwear. However, in contrast to shoes, callus thickness does not trade-off protection, measured as hardness and stiffness, for the ability to perceive tactile stimuli at frequencies experienced during walking. Additionally, unlike cushioned footwear, callus thickness does not affect how hard the feet strike the ground during walking, as indicated by impact forces. Along with providing protection and comfort at the cost of tactile sensitivity, cushioned footwear also lowers rates of loading at impact but increases force impulses, with unknown effects on the skeleton that merit future study.

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Data availability

All relevant processed data supporting the findings of this study are available as Source Data. Further data are available from the corresponding author upon reasonable request.

Code availability

All MATLAB and R code used to process and statistically analyse the data are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank M. Sang, J. Jemutai, N. Keter, the teachers and students of Pemja and AIC Chebisas School, C. Diggins, L. Kaufman, R. Doerfler, D. Schmidt, E. Schoeck, P. Schulze and M. Venkadesan. This research was funded by the American School of Prehistoric Research (Peabody Museum, Harvard University).

Peer reviewer information

Nature thanks Kristiaan D’Août and William L. Jungers for their contribution to the peer review of this work.

Author information

D.E.L., T.J.D., N.B.H., T.L.M., B.W. and C.Z. designed the experiments; all co-authors except C.Z. collected the data; N.B.H., D.E.L., T.L.M., B.W. and C.Z. analysed the data; D.E.L., T.J.D., N.B.H., T.L.M, B.W. and C.Z. wrote the paper.

Correspondence to Daniel E. Lieberman.

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Extended data figures and tables

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.

Extended Data Fig. 2 Skin material properties across regions.

a, Relationship between callus thickness and skin hardness. b, Relationship between callus thickness and skin stiffness. Data points indicate measurements from the heel (green circle) and first metatarsal head (light-blue square) in all participants. Dashed lines indicate the ordinary least squares regressions for the heel, and dash–dot lines indicate ordinary least squares regressions for the metatarsal head. Similar relationships between callus thickness and skin hardness are evident across regions of the foot, which indicates a consistent effect of callus thickness on skin hardness. By contrast, different relationships between callus thickness and skin stiffness occur in the two regions of the foot, which suggests that stiffness measurements are influenced by subdermal tissues.

Extended Data Fig. 3 Impact peak force results.

a, Relationship between callus thickness and impact peak force (BW) in usually barefoot (red; n = 29 individuals, n = 70 steps) and usually shod (blue; n = 28 individuals, n = 44 steps) Kenyan individuals. Box plots depict comparisons between usually barefoot and usually shod individuals. b, Relationship between callus thickness and impact peak force (BW) in US individuals (n = 22 individuals) when barefoot (yellow), wearing uncushioned shoes (orange) and wearing cushioned shoes (red). Likelihood ratio tests carried out on model variance from linear mixed-effects models presented in Extended Data Tables 5, 7 indicate that there is no significant relationship between callus thickness and impact peak force and no effect of footwear-use category or footwear condition (P > 0.05). For all box plots, boxes represent interquartile ranges, middle bars represent median values, whiskers extend to the most extreme data point ± 1.5× the interquartile range and more extreme data points are indicated by circles.

Extended Data Table 1 Kenya sample size information
Extended Data Table 2 Anthropometric, skin and sensitivity data of the Kenyan individuals
Extended Data Table 3 Skin mechanical properties in Kenyan participants
Extended Data Table 4 Vibration threshold model coefficients and statistical results
Extended Data Table 5 Impact force model coefficients and statistical results for the Kenyan cohort
Extended Data Table 6 Impact force results for the US cohort
Extended Data Table 7 Impact force model coefficients and statistical results for the US cohort

Supplementary information

Supplementary Information

This file contains Supplementary Methods 1-3, Supplementary Tables S1-S3 and a Supplementary Discussion.

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Source data

Source Data Fig. 1

Source Data Fig. 2

Source Data Fig. 3

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Fig. 1: Effects of callus thickness on skin hardness.
Fig. 2: Relationship between callus thickness and sensitivity.
Fig. 3: Relationship between callus thickness and the impact peak of the vertical ground reaction force during walking.
Extended Data Fig. 1: Skin stiffness.
Extended Data Fig. 2: Skin material properties across regions.
Extended Data Fig. 3: Impact peak force results.

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