Figure 1.

Overview of the FootSim model mimicking the mechanotransduction process

We divided the foot sole into 13 regions with different mechanical properties and tactile innervation: toes 1–5, lateral/middle/medial metatarsal/arch, and lateral/medial heel (box 1, from the left). We incorporated different densities of specific afferent types (fast-adapting and slowly adapting type 1 and 2 – FA1, FA2, SA1, SA2) across regions of the foot sole based on empirically established innervation densities¹ (box 2). Stimuli are represented as spatiotemporal indentation profiles on the foot sole (box 3), creating an input to the mechanical model (box 4), where it is converted into quasi-static and dynamic stresses within the plantar skin at the locations of individual receptors.¹⁶ Quasi-static stress is associated with local vertical stress while dynamic component represents the pressure component propagated through the skin. Both stress components are passed through firing models (box 5) that simulate single afferent behavior. 11 parameters (1: low pass, 2:7: w₁-w₆, 8: saturation, 9: time constant, 10,11: post-spike inhibition parameters – slow and fast component, respectively) are fitted to replicate characteristics of individual afferent classes. As output, the FootSim model creates time-varying firing patterns for the desired afferent population (box 6). See also Figure S2.