Associations Between Mechanical Properties of Plantar Fascia and Passive and Active Foot Arch Deformability

PURPOSE: The human foot has a unique arch-shaped structure that deforms and recoils in response to weight-bearing while storing and releasing elastic strain energy. This spring-like function of the foot arch is considered largely reliant on the mechanical properties of the passive structures, particularly the plantar fascia (PF); however, there is limited in vivo evidence to support this notion. Thus, this study aimed to examine the associations between mechanical properties of PF and foot arch deformability. METHODS: We performed two experiments on healthy young individuals (n = 12 in each experiment, age 19-30 years) to examine the associations between PF shear wave velocity (SWV), an index of tissue stiffness, and foot arch deformation under different loading conditions. To measure passive and active midfoot range of motion (ROM), a series of sagittal magnetic resonance images of the foot with passive ankle dorsiflexion and three-dimensional motion capture data during single-leg drop jumping were collected. RESULTS: The PF SWV values were 6.4±1.0 m/s and 6.8±0.7 m/s for each experiment. The midfoot ROMs during passive ankle dorsiflexion and single-leg drop jumping were 11.2±3.5˚ and 24.9±6.9˚, respectively. We found that PF SWV was significantly negatively correlated with both passive (ρ2 = 0.540) and active midfoot ROM (ρ2 = 0.541). CONCLUSIONS: This study demonstrated that the mechanical properties of PF were associated with both passive and active foot arch deformability, which provides first in vivo evidence that the individual variability in PF mechanical properties diversifies the foot arch deformability. Furthermore, the comparable explanatory power under passive and active conditions suggests that the mechanical properties of PF may serve as a key determinant of foot arch deformability across different loading scenarios.