## 4 Discussion  
This study analyzed the available data on the PPP effect at different walking speeds and various insole materials in healthy people and people with DM. Our results have important implications that the appropriate walking speed for healthy people was around 6 km/h. In contrast, an appropriate walking speed for DM people was around 4 km/h. Moreover, the forefoot pressure at 4 km/h walking speed in people with DM was nearly two times higher than that for non-DM people. The EVA was the most often used inside material for walking at the forefoot region (26%). EVA reduced the PPP by 37% on the DM forefoot region. Our results showed that 6 km/h was the appropriate walking speed for non-DM, with the lowest PPP in the forefoot, midfoot, and rearfoot regions (Figure 2). [PERSON] et al. demonstrated that when walking faster, the mean square error, one of the variability metrics of plantar pressures, increased considerably [75]. Elevated plantar pressures may accumulate tension abnormally over the plantar soft tissue [76]. Therefore, to reach the lowest PPP, DM people and non-DM may have different suitable walking speeds [31; 32; 77]. Non-DM appropriate walking speed was closely correlated with the neutral subtalar joint position, neither pronated nor supinated, which may reduce PPP [78]. Moreover, a walking speed above 6 km/h in non-DM has a high PPP on the forefoot region (Figure 2). [PERSON] et al. stated that ankle joint position changed into high pronation during faster walking [79]. The transformation into a pronated position at faster walking speeds might be due to muscle fatigue-inducing high pressure under the forefoot [28; 29]. In addition, a walking speed of less than 6 km/h showed increased PPP on the heel (Figure 2). One reason why the rearfoot region has a high PPP may be founded on the increased contact time of slow walking due to severe internal distortion between bone and soft tissue [80]. The study concluded that the forefoot PPP at 4 km/h walking speed was nearly two times (185%) higher in DM people than non-DM people (Figure 2). The high PPP at 4 km/h walking speed may have three reasons: increased contact time, ankle joint stiffness, and soft tissue stiffness. First, DM people lose ankle movement perception, owing to increasing step time to walk carefully to keep their balance at 4 km/h walking speed [27; 81]. Hence, DM people decreased their step length, gained slower walking speeds, and had higher PPP in the forefoot region than non-DM [27; 32]. Second, high PPP at 4 km/h walking speed on DM people was due to ankle joint stiffness (severe DM; 0.236 N\(\cdot\)m/deg compared with normal DM: \(-0.113\) N\(\cdot\)m/deg) [82]. Ankle plantar flexor (equinus) in DM becomes contracture, produced by stiff tissue causes the heel to rise earlier than normal, employing higher pressure on the forefoot [83]. Third, DM people have high PPP at 4 km/h walking speed due to tissue stiffness. [PERSON] et al. demonstrated that soft tissue stiffness in the forefoot of people with DM was around 1.5 times higher (155%) than non-DM [84]. Increases in diabetic soft tissue stiffness may be attributed to the accumulation of cross-linked collagen molecules, which are abundant in soft tissue [85]. Electron microscopy shows evidence of regular collagen alignment loss, increased collagen fibril density, and increased fibril adhesion in patients with DM [86; 87]. This study revealed that the most commonly used insole material in the forefoot was EVA (26%). EVA decreased the forefoot pressure by about 37% during walking in people with DM (Figure 3 and Table 5). The EVA insole, a foam-based material that provides high resilience, is excellent for reusing mechanical energy at the forefoot on the push-off phase during walking [44; 88; 89]. The typical rebound resilience of polyethylene foam ranged from about 30% to 40% [90]. In addition, the cushioning function of the insole provides extra resilience to the EVA insole material. [PERSON] et al.