Optimum Insole Hardness for Attenuating Peak Plantar Pressure Under Simulated Loading Scenarios

Introduction

Plantar soft tissues (PSTs) under the heel and metatarsal head regions of the human foot play a key role in cushioning and shock absorption during walking (Ker et al., 1989; Scott et al., 2007; Yan et al., 2023). The PSTs act as a crucial interface between our body and the ground; its positioning relative to the rest of the body plays a pivotal role in maintaining stability, balance, and efficient gait patterns (Cleland, 2023). However, stiffened PSTs due to aging (Hsu et al., 1998; Kwan et al., 2010; Yan et al., 2023) and/or diabetes (Allan et al., 2022; Brady et al., 2023; Jan et al., 2013; Naemi et al., 2022; Zheng et al., 2000) may impair the cushioning capacity of the tissues, eventually developing into heel pain, metatarsalgia, and/or diabetic foot ulcers. Foot pain has consistently been connected to falls, causing various injuries in older adults (Menz & Lord, 1999; Mickle et al., 2010).

The therapeutic insole has been considered one potential tool for reducing the incidence of lower extremity injuries by offloading. Most existing therapeutic insoles have been designed with specialized geometry to fit the shape of the plantar surface and with varying material hardness to provide optimum cushioning properties. An investigation of the interaction between foot and footwear is mandatory if one is to select the optimum material for plantar offloading purposes by changing the material characteristics rather than the insole geometry.

Selection of cushioning materials for therapeutic footwear is mostly determined on the basis of the clinician’s experiences (Malki et al., 2023; Mandolini et al., 2017; Telfer et al., 2017). Charlotte Apps and her colleagues conducted comprehensive in vivo tests using an in-shoe plantar pressure system, aiming to meticulously compare the mechanical attributes of two distinct insole designs. They elucidated the material-induced impacts on both plantar pressure distribution and perceived comfort during both standard and weighted treadmill walking sessions. Their exhaustive study revealed that a softer, single-material insole exhibited superior efficacy in mitigating plantar pressure, thereby underscoring its potential benefits (Melia et al., 2021). Exploring the material properties of the PST is critical if we are to improve our understanding of the natural cushioning mechanism of the PST, and producing improved footwear with enhanced intrinsic mechanical property, the material properties of which are based on the PST. This may lead to new treatment options for foot pain, specifically that caused by fat pad atrophy, which is common in older adults (Im Yi et al., 2011; Mickle et al., 2011). One option could be insoles that are matched in stiffness with the PSTs. In order to quantify the material property of the PST for the purpose of developing biomimetic materials for footwear design, previous research endeavors have conducted in vivo tests with a tissue ultrasound palpation system (Kwan et al., 2010; Naemi et al., 2022; Tecse et al., 2023; Zheng et al., 2000) and in vitro tests using material compression testing machines (Grigoriadis et al., 2017; Miller-Young et al., 2002). These studies focused on testing the elastic modulus of PSTs, mostly in the heel region rather than in all plantar regions. However, the methods used and the parameters tested in previous studies are too complicated to be directly applied to footwear design and clinical assessment. Thus, current footwear design is usually performed without taking into consideration the individualized material property of the PSTs.