# Investigating plantar soft tissue elasticity modulation using air insoles during walking

## TL;DR

This study explores how air insole pressure affects foot tissue stiffness during walking, aiming to prevent diabetic foot ulcers.

## Contribution

The novel contribution is identifying optimal insole pressure and walking duration to reduce foot tissue stiffness linked to ulcer risk.

## Key findings

- 240 mmHg insole pressure significantly reduced stiffness in toe and heel regions after 10 minutes of walking.
- No significant differences in tissue stiffness were found between 10 and 20 minutes of walking for all insole pressures.
- Optimizing insole pressure and walking duration can help maintain foot health and reduce ulcer risk.

## Abstract

In the forefoot region, diabetic foot ulcer (DFU) is more likely to develop due to elevated plantar pressure and repetitive loading. This study investigated the effects of different forefoot insole pressures and walking durations on the elasticity of the plantar tissue to identify conditions that may help prevent DFU.

With thirteen healthy participants, we conducted a controlled walking exercise at a speed of 3.6 mph on a treadmill, comparing three different inner pressures of 80, 160, and 240 mmHg with two different walking durations of 10 and 20 min. We measured the elasticity of the first metatarsal head (M1) using a motor-driven indentation system with ultrasound post-walking exercise. We measured the effective Young’s modulus (E) for analysis, where E1 is taken from 5% of the initial thickness of the Toe region’s elasticity, E2 is taken from 10% of the initial thickness of the Heel region’s elasticity, and E3 is taken from 15% of the initial thickness of the Linear region’s elasticity. ANOVA and paired t-test analyses are used to find the relation between durations in the post-walking conditions.

The effect of the different insole hardnesses using ANOVA indicated that the 240 mmHg insole hardness significantly reduced the stiffness of M1 in E1 (P = 0.042) and E2 (P = 0.046), but not in E3 (P = 0.059). In the paired t-test, no significant differences were found between the 10 and 20 min walking durations for the three insole hardnesses.

The study concluded that after a 10-minute walking duration, using 240 mmHg insole hardness has lower stiffness than the recommended stiffness range. These findings highlight the importance of optimizing insole pressure and walking duration in maintaining foot health, and provide evidence to guide the design of functional air insoles to reduce the risk of DFU formation.

ClinicalTrials.gov Identifier: NCT06746597. Registered on 2024-12-09 (retrospectively registered).

1. A significant change was observed in the toe (initial) and heel (middle) regions of the J-shaped curve, reflecting variations in Young’s modulus across different insole pressures (80, 160, and 240 mmHg) and walking durations (10 and 20 min) at the first metatarsal head (M1).

2. The 240 mmHg insole hardness contributed to the lowest measurement of M1 in E1 (toe 5%) and E2 (heel 10%), but not in E3 (linear 15%) after walking for 10 min.

3. There was no significant difference in plantar tissue material property changes in M1 using 80 mmHg, 160 mmHg, and 240 mmHg inner pressure of the insole between the 10 and 20 min walking duration.