Quantification of the Mechanical Response of the Plantar Fascia to Changes in Rearfoot Position

Background: Changes in the rearfoot calcaneal position affect the foot "arch structure" during the stance phase of gait and hence influence reactions in the plantar fascia thickness and stiffness during weight bearing. However, previous research has focused on the non-weight-bearing assessment of plantar fascia thickness (PFT) and stiffness (PFS) and has not linked these measurements to rearfoot position. Methods: This study aims to investigate if a change in the weight-bearing rearfoot position influences the PFT and PFS. A linear actuator-driven 3D-printed platform was utilised to reliably move the rearfoot through a range of frontal (F (4,12) = 19,585.8, p = 0.00) and sagittal plane angles (F (2,6) = 11,751.32, p = 0.00) whilst weight bearing. An ultrasound probe capable of shear wave elastography was incorporated into the platform for the closed-chain weight-bearing assessment of the PF. The PFT and PFS were collected for 13 (26 feet) participants (11 male, two female; age 35.62 ± 15.04; BMI: 30.31± 6.22 Kg/m²) from a convenience sample who met the inclusion criteria. Results: The data were subject to appropriate statistical, collective and cluster analysis. Individual participant data analysis showed a strong nonlinear correlation between PFT and PFS in the relaxed calcaneal position. The rearfoot sagittal plane cluster demonstrated an auxetic property in 54.3% of the group, where both the PFT and PFS increased. The frontal plane cluster demonstrated an auxetic property in 76% of the group, where the PFT increased as the PFS increased. Conclusions: The results suggest that the PF does have a specific response to changes in the rearfoot position for individuals, which, in some, can show an auxetic property.