(1) This study demonstrates the effect of the inner pressure of air insole on plantar pressure gradient (PPG) and plantar gradient angle (PGA).

Diabetes mellitus (DM) is a major public health concern worldwide, leading to morbidity, disability, and mortality (Lavery et al., 2003). The International Diabetes Federation has reported that the number of people with DM has tripled since 2000, rising from 151 million to 537 million in 2021. By 2045, the number of people with DM is predicted to reach 783 million (Sun et al., 2022). In 2015, USD 673 billion was spent on treating DM for adults alone, which climbed to almost USD 1,000 billion by 2021 and is predicted to increase to USD 1,045 billion in 2045 (Zheng et al., 2018; Sun et al., 2022). DM prevention is one of the most pressing concerns owing to its high cost (Van Netten et al., 2016). Social security and health systems must be prepared to address the adverse effects of DM.

Walking is the most common physical activity in activities of daily living (Wu et al., 2022). Several studies support the effectiveness of walking as an intervention for patients with DM (Wu et al., 2021; Wu et al., 2022). For a 10 min walk, a speed of 9 km/h walking significantly heightened the ratio of wavelet amplitudes across neurogenic, metabolic, respiratory, myogenic, and cardiac mechanisms in comparison to walking at 3 km/h (p < 0.05). Moreover, for a 20 min walk, walking at 6 km/h significantly increased the ratio of wavelet amplitudes related to metabolic, myogenic, respiratory, and cardiac factors compared to walking at 3 km/h (p < 0.05) (Wu et al., 2021). Following a 10 min walk, the PPP values beneath the first toe and the first metatarsal head were notably higher (p < 0.05) at 9 km/h (509.1 ± 314.2 kPa and 591.4 ± 302.4 kPa, respectively) compared to 3 km/h (275.4 ± 168.7 kPa and 369.4 ± 205.4 kPa, respectively) (Wu et al., 2022). After brisk walking, there was a 78.6% of their respondents decrease in blood sugar levels (p < 0.001) compared to control (Kasmad et al., 2022). Moreover, a recent study showed that walking briefly improved blood glucose profiles in people with DM, leading to sedentary lifestyles (Moghetti et al., 2020). However, repetitive high vertical or shear stresses on the foot may increase the risk of plantar skin breakdown in individuals with DM (Liao et al., 2019; Haris et al., 2021).

Recent studies pointed out that suitable insole hardness significantly decreases the peak plantar pressure (PPP) as well as decreases the risk of developing diabetic foot ulcers (DFUs) (Korada et al., 2020; Haris et al., 2021). An insole with proper hardness transfers high forces from the foot’s bony areas to nearby foot regions, significantly reducing 40% PPP and enhancing patient comfort (Jafarzadeh et al., 2021). Furthermore, appropriate insole hardness provides high resilience. This allows the forefoot to reuse mechanical energy during a push-off stance (Ahmed et al., 2020; Morita et al., 2021). Optimal insole hardness holds greater significance than the use of softer insoles. The effectiveness of softer insoles in preventing injuries such as metatarsal problems and blisters was more significant in PPP reduction. Researchers recommend further study since PPP reduction did not impact perceived footwear comfort (Melia et al., 2021). A stiffer insole reported decreased deformation across the plantar foot and increased PPP (Melia et al., 2021). Different densities may result in different hardness of the insoles (Teixeira et al., 2021). In the clinical setting, a suitable insole hardness might be applied to shoes to decrease PPP (Haris et al., 2021), either not too soft or too hard, further diminishing the risk of DFUs (Kim et al., 2015; Melia et al., 2021).

Cong et al. emphasized that the hardness of insoles is important (Cong et al., 2014). Air insoles might be a good solution to replace insole hardness to evaluate DFUs (Zhu et al., 2013; Kim et al., 2018; Teixeira et al., 2021). Kim et al. revealed that air insoles provide different degrees of hardness, significantly affecting walking speed and balance (Kim et al., 2015). The tremendous illustration of air insoles might be that the higher inner pressure would induce higher PPP owing to stiffer plantar soft tissues (Kim et al., 2018; Lung et al., 2020a). Previous studies have discussed inflatable devices with suitable air pressure for redistributing pressure (Arias et al., 2013; Yang et al., 2015). Considerable progress has been made in developing air insoles related to their hardness. The study revealed that air pressure-based techniques are emerging and bringing promising results (Chen et al., 2021). Our overarching objective is to enhance our comprehension of the impact of air insoles on the pathogenesis of DFUs.

Based on a previous study, it has been concluded that PPP may not be the only indicator to use when assessing the risk of DFUs (Lung et al., 2022). Moreover, Lavery et al. pointed out that relying solely on PPP is insufficient for accurately identifying high-risk DFUs (Lavery et al., 2003). In response, Mueller et al. introduced an additional valuable metric, the peak pressure gradient (PPG), which captures the spatial variation in plantar pressure around the PPP across adjacent sites of the foot surface. PPG offers insights into plantar pressure distribution and the potentially injurious internal stresses within the foot’s soft tissues (Mueller et al., 2005). Additionally, PPG may be responsible for skin breakdown if soft tissues are exposed to shear stress (Mueller et al., 2005; Jan et al., 2013; Lung et al., 2013; Lung et al., 2016). Therefore, PPG may be a reliable predictor of DFUs development.

Insole hardness may affect PPG (Cong et al., 2011; Cong et al., 2014). PPG is calculated without considering the time-varying PPP during gait, based on the pressure distributions during the overall contact time (Lung et al., 2020b). In the case of plantar region contact with the floor, successive maximal pressure gradients may be directed in different directions (Lung et al., 2016). PPP and PPG were significantly larger in the diabetic group compared with non-diabetics controls in the big toe and first metatarsal regions (Lung et al., 2016; Chen et al., 2021).

Variations in the gradient direction, called the pressure gradient angle (PGA), may lead to more complicated deformations of plantar soft tissues, despite the deformation location and magnitude remaining the same. With the use of the PGA, the directional angle of the PPG can be quantified instantly. Additionally, higher PGA reduces pressure concentration at one point of the plantar soft tissue, providing a new way to study the effects of PPP (Lung et al., 2016). Insoles with varying hardness can be used to investigate plantar stresses and deformations caused by dynamic plantar pressures. As a result of the hardness of the insole, PGA may be affected (Lung et al., 2016).

As Supredi et al. argued, a threshold value for the pressure gradient indicates foot ulcer risk (Supriadi et al., 2014). Therefore, measuring walking conditions in patients with DM, including different air insoles and different walking durations, is critical for proposing appropriate walking exercises and rehabilitation strategies. To the best of our knowledge, no studies have examined the impact of different air insoles while walking on PPG and PGA values in people with or without DM. As a first step toward identifying the effects of different air insoles and walking durations on PPG and PGA patterns, it is vital first to examine the responses of healthy people (Lung et al., 2013; Lung et al., 2016). The results of this study can be used to understand how DM affects the PPG and PGA patterns. The current study examined PPG and PGA patterns in non-diabetic individuals using different air insoles and walking durations.

In the interaction between the air insoles and walking duration on PPG and PGA, a MANOVA (three air insoles and two walking durations) showed that the air insole factor caused a significant main effects of PGA in T1 (p = 0.032) and M2 (p = 0.020). However, the walking duration factor did not have significant effects. It also did not have significant effects on the interaction between the air insole and walking duration.

This study aimed to analyze the effects of different air insoles and walking durations on PPG and PGA patterns in non-diabetics. In the interaction between the air insoles and walking duration on PPG and PGA, a MANOVA (three air insoles and two walking durations) showed that the air insole factor caused a significant main effect of PGA in T1 (p = 0.032) and M2 (p = 0.020). One-way ANOVA indicated that PPG was lower at the 80-mmHg air insole after 20 min of walking in the M1 region (Table 1; Figure 1B). PGA increased at 80 mmHg air insole (Table 1; Figure 1C). PGA decreased at 80 mmHg air insole after 20 min of walking in the T1 region (Table 1; Figure 1D). Paired t-test assessment showed that compared to 20 min walking duration, 10 min walking duration at 240 mmHg of air insole had the lowest PPG in the M1 and M2 regions (Table 2; Figure 5). The Pearson product-moment correlation analysis confirmed that PPP and PPG had significant positive correlations in all air insoles (Table 3; Figure 6). PGA had a significant negative correlation at 80 mmHg after 20 min of walking (r = −0.34, p < 0.036) (Table 3; Figure 6B).

The study indicated that the PPG value at 80 mmHg air insole was lower than that at 160 mmHg and 240 mmHg after 20 min of walking in the M1 region (Table 1; Figure 4A). According to a previous study, a lower hardness insole could decrease the pressure time integral during walking (Melia et al., 2021). Moreover, a previous study revealed that PPG over people with DM reported higher forefoot than rearfoot (Mueller et al., 2005). These findings are significant, aligning with previous studies indicating a markedly higher incidence of skin breakdown in the forefoot compared to the rearfoot of people with DM (Ababneh et al., 2020; Edmonds et al., 2021; Choi et al., 2022). We speculate that pressures exhibiting significant changes across adjacent areas on the foot’s surface, indicating a high PPG, may inflict more damage on plantar soft tissues than uniformly distributed high pressures across the foot. A relevant analogy well explained observed in underwater divers facing intense hydrostatic pressures, which, despite being high, do not lead to skin breakdown due to their uniform distribution across the skin surface. We propose that substantial pressure gradients contribute to skin breakdown by generating considerable shearing stresses within the soft tissues. People with DM had impaired skin blood flow response after high accumulated pressure time-integral walking (Duan et al., 2021). Low skin blood flow responses can induce neuropathy due to a high metabolic component, causing severe nerve damage (Jan et al., 2019). A lower PPG may be achieved by decreasing the pressure-time integral and redistributing the PPP equally (Lott et al., 2008; Pu et al., 2018; Duan et al., 2021). A review mentioned that a lower hardness insole significantly reduces the PPP and pressure-time integral (Ahmed et al., 2020). Jan et al. claimed that normal PPP and PPG are associated with the absence of alterations (i.e., shear stresses) in the viscoelastic properties of plantar soft tissues (Jan et al., 2013). The large PPG related to shear stress potentially damages the inner soft tissues between two adjacent sites on the plantar soft tissues (Lott et al., 2008). Decreasing the pressure–time integral might reduce PPG and minimize skin breakdown and ulcers.

Our results showed that PGA increased to 80 mmHg after 10 min of walking in the M2 region (Table 1; Figure 4B). A higher PGA indicates an instantaneously enhanced PPG in various directions, which reduces the risk of abnormal PPP. An analysis of PPG during walking can be achieved using PGA patterns to quantify the dynamic changes in PPP distributions (Lung et al., 2022). Lung et al. proposed that PGA increases when the pressure is distracted by a concert and decreases when it is concentrated in an indentation (Lung et al., 2013). A PPG derived from the surface pressure distribution can serve as a crucial variable to pinpoint the locations of maximum shear stress within the plantar foot’s subsurface tissue. The identification of this risk is crucial in determining the potential for skin breakdown (Zou et al., 2007). There could be two reasons for the lower hardness insoles increasing PGA. First, during 80 mmHg air insole walking, which has lower hardness properties, induces high resilience and allows the metatarsal joint to reuse mechanical energy during push-off and allows plantar soft tissue to continuously change the PPG direction, inducing a wider PGA (Lung et al., 2013; Ahmed et al., 2020; Morita et al., 2021). Second, the lower hardness of the air insole provides an unstable support base, resulting in an unstable contact surface between the air insole and the plantar soft tissue (Corbin et al., 2007; DeBusk et al., 2018). Unstable contact surface-induced dynamic situations (i.e., running, walking, and sports activities) might prompt a wider PGA (Fu et al., 2014). It is our recommendation that effective diabetic insoles should feature a wide PGA, which may not coincide with the point of PPP, in order to reduce foot ulcer risk.

However, the PGA narrowed in the 80 mmHg air insole after 20 min of walking in the T1 region (Figure 4B). The study indicated that walking for a 20 min walking duration with an 80 mmHg air insole might induce shear stress in T1. There is a potential risk of increased shear stress after vigorous activities and prolonged walking (Lung et al., 2022). The largest flexor muscle of T1, namely the flexor hallucis longus, increases with increasing plantarflexion torque levels (Péter et al., 2015). A less than 30° plantarflexion has been shown to increase the effective shear stress and force in soft tissues (Levy and Gefen, 2016). During normal gait, soles change the point of ground contact from the heel to the toe efficiently and successively (Sudo et al., 2006).

Additionally, plantarflexion between 10° and 30° provides anatomical torque, which agrees with clinical functions during walking, jumping, and hiking (Tang et al., 2022). Nevertheless, shear stresses might be triggered during longer walking by plantar flexion exceeding 30°, increasing and concentrating the PPG in T1 and further narrowing the PGA (Levy and Gefen, 2016; Lung et al., 2016). In people with DM, high PPG, lower PGA, and maximal shear stress offer insights into both plantar pressure distribution and the potentially harmful internal stresses within the soft tissues of the foot (Zou et al., 2007; Lott et al., 2008; Lung et al., 2013).

Regarding walking duration, the PPG value at 240 mmHg air insole after 20 min of walking in the M1 and M2 regions was higher than that after 10 min of walking (Table 2; Figure 5). People who exercise continuously and repetitively are more likely to have stiffer plantar soft tissues (Yum et al., 2019). This study implied that prolonged exercise on a stiffer air insole might induce fatigue that can increase plantar soft tissue stiffness (Hamatani et al., 2016). Jan et al. stated that DM is associated with an elevation in the viscoelasticity of plantar soft tissues, which could contribute to heightened PPP and PPG in the diabetic foot (Jan et al., 2013). Previous studies have shown that PPG is higher in stiffer soft tissues (Lung et al., 2016; Volmer-Thole and Lobmann, 2016).

Furthermore, soft tissue stiffness may cause bone loading and PPG to occur (Nagel et al., 2008; Lung et al., 2022). After walking for a longer period, local muscle fatigue might contribute to the load being centered on the metatarsal bone. People with DM should avoid using 240 mmHg air insoles because they may harm their plantar region due to elevated PPP.

There was a significant positive correlation between PPP and PPG for all air insoles (Table 3; Figure 6). As a result of this study, PPG can effectively assist PPP in the identification of high-risk DFUs (Lung et al., 2013; Lung et al., 2022). However, there was a negative correlation between PPP and PGA; the 80 mmHg air insole with lower hardness significantly decreased PPP and increased PGA (Table 3; Figure 6B). Insoles with lower hardness possess unstable materials, which result in dynamic positions (Chander et al., 2017). As discussed in previous paragraphs, lower hardness air insoles might induce high resilience and have an unstable support base that induces a wider PGA (DeBusk et al., 2018; Morita et al., 2021). However, it is good for PGA to have a wider angle to reduce the risk of skin breakdown (Lung et al., 2016; Chen et al., 2021). The problem of foot ulcer recurrence remains unresolved (Zhang et al., 2017). PPP alone is not an adequate diagnostic tool for identifying high-risk diabetic foot ulcers (Lung et al., 2022). Therefore, the PGA is a good diagnostic tool for resolving foot problems.

Despite its strengths, this study had some limitations. The first limitation is the small number of healthy volunteers in this study. Therefore, the current results should be interpreted with caution, as they tend to limit the statistical power of the analysis. The second limitation is the lack of bone loading evaluation in PPG and PGA for air insole walking studies. The study revealed that the bone loading rate had a negligible effect on the stiffness of the plantar soft tissue, indicating that further research may be required since the underlying bone was 10 times stiffer (Wearing et al., 2006). Third, this study assessed the risk of foot ulcers using the PPG and PGA values. Various factors contribute to the development of foot ulcers (i.e., low skin blood flow and high plantar foot temperature) (Pai and Ledoux, 2012; Beeckman et al., 2019). The combination of microclimate, PPG, and PGA assessment in future studies might significantly impact foot ulcer prevention.

This study determined that walking at 80 mmHg significantly lowered PPP and PPG and increased PGA in the metatarsal region compared to walking at 160 and 240 mmHg air insoles. Moreover, the 10 min walking duration had a significantly lower PPG than 20 min. In addition, the study showed that an 80 mmHg air insole significantly decreased PPP and increased PGA. Based on our methodology and results, individuals at high risk of DFUs are advised to utilize softer air insoles during at least 10 minu of walking. Additionally, particular attention should be given to the big toe during vigorous and extended walking sessions, as it may experience heightened shear stresses. Future research endeavors could explore optimizing PPP, PPG, and PGA outcomes using air insoles with pressures below 80 mmHg, aiming to reduce the risk of DFUs.