## 4 Discussion
The increased stiffness after slow walking may increase the potential for ischemic injury [30; 31]. Our previous study demonstrated that people with DM have impaired skin blood flow response to mechanical and thermal stresses [32; 33]. Additionally, when exercise intensity is sufficient, microvascular function would benefit from exercise to improve blood flow function. [PERSON] et al. demonstrated that walking speed at 3.6 mph would increase more skin blood flow compared to 1.8 mph [9]. Combined with the findings from this study, walking at 3.6 mph will have a higher increase in skin blood flow and lower stiffness of the foot compared to walking at 1.8 mph. The result of this study indirectly supports that weight-bearing exercise may not increase risk for foot ulcers because weight-bearing exercise (e.g., moderate-to-fast walking (brisk walking)) can improve skin blood flow and reduce stiffness of the plantar foot compared to slow walking (1.8 mph). The superficial layer (near the skin surface, Q1) showed lower stiffness, and the deep layer (near the bone, Q4) showed higher stiffness after walking. Our results are consistent with previous studies [34; 35; 36]. [PERSON] suggested that the viscoelasticity and strain rate effects of soft tissue are related to the structure of soft tissue [37]. For superficial pressure ulcers/injury, the causative factor is mainly friction and shear. For the deep pressure ulcers/injury, the causative factor is pressure [38]. Our results (softer superficial layer and stiffer deep layer) provide a reason for this clinical observation. Under pressure, the superficial layer has lower stiffness that can withstand pressure compared to the deep layer. This finding is supported by a previous study. [PERSON] et al. demonstrated that superficial soft tissue can withstand two times the prolonged loading of deep soft tissue [39]. Regarding the walking duration effect, there is no significant difference after walking 10 or 20 min. Although there is no difference, this provides evidence to walk up to 20 min because walking at various speeds for either 10 min or 20 min will not change plantar tissue stiffness. People at risk for foot ulcers may exercise up to 20 min without significantly increasing the stiffness of plantar tissue. According to the physical stress theory [26], these stresses caused by various walking durations are still within the safe threshold; therefore, plantar tissue can adapt to these stresses. This could also be explained by the interstitial fluid theory fluid flow within the plantar foot to absorb various plantar loads [40; 41]. [PERSON] et al. suggested that the measurement of strain, along with plantar pressure, can improve the understanding of the mechanical behavior of the plantar soft tissue [42]. Previous studies showed that faster walking speeds resulted in higher plantar pressure [43; 44]. However, stiffness is an indicator of the tendency for an element to return its original shape after applied force/stress/pressure. In this study, we used the strain ratio from the elastographic ultrasound to measure plantar tissue stiffness after walking at various intensities. Our results showed that in the superficial layer and the sub-superficial layer, the plantar tissue stiffness was significantly lower after 3.6 mph compared to 1.8 mph. The results are consistent with the force-relaxation curve of biological tissues (i.e., viscoelastic materials) [45]. This could also be explained by the increase in plantar skin temperature. [PERSON] et al. demonstrated that walking at 3.6 mph can significantly increase plantar skin blood flow compared to walking at 1.8 mph. An increase in skin blood flow is generally accompanied by an increase in skin temperature [9]. [PERSON] et al. demonstrated that soft tissue stiffness decreased when temperature increased [46; 47]. This could partly explain a lower stiffness of plantar tissue after walking at 3.6 mph compared to 1.8 mph because of a higher increase in plantar skin blood flow during walking at 3.6 mph [9]. There are limitations to this study.