Abstract

Objective: The cutaneous mechanoreceptors of the foot sole detect the changes in the application of mechanical loads on the plantar surface during gait and standing, and contribute to controlling the standing balance and postural reflexes in healthy subjects. A local thickening of the foot sole skin occurs in response to repetitive load application. We hypothesized that an elevated skin hardness of the foot sole could reduce its mechano sensitivity.
Methods: In healthy subjects, we quantified the sensation produced by different amplitudes of vibratory stimulations at two frequencies (25 and 150 Hz). The vibration threshold was determined on the 1st or 2nd, and 5th metatarsal heads, and the heel at each vibration frequency. The Stevens power function (Ψ=k.Φn) allowed to obtain regression equations between the estimate (Ψ) of the vibratory stimuli and their physical magnitude (Φ). Any increase in the absolute k value (all were negative) indicated a reduced sensitivity to the lowest loads. The n coefficient measured the global perception. The highest skin hardness (Shore) was measured on the 5th metatarsal head and the heel. In some subjects, superficial skin abrasion of the 5th metatarsal head was performed and the vibration sensitivity was tested again.
Results: The vibration threshold was significantly higher at the level of the 5th metatarsal head and the heel. The k value was significantly higher at the 25 and 150 Hz frequencies for the 5th metatarsal head, and only at 25 Hz for the heel. At both vibration frequencies, negative correlations were obtained between the k values and skin hardness. After skin abrasion, the n coefficient was significantly higher at both vibration frequencies.
Conclusion: Skin hardness affects the foot sole mechano sensitivity and could alter the control of posture during standing and walking. This indicates that foot care by podiatrist are relevant to improve posture control.

Keywords: Skin hardness; Foot sole; Sensitivity to vibration; Stevens power law; Healthy subjects