## 4 Discussion  
The results of this study demonstrated that the walking speeds (1.8, 3.6 and 5.4 mph) significantly affected plantar tissue stiffness and the walking durations (10 and 20 min) did not. Our results indicate that plantar tissue stiffness after walking at 3.6 mph for 20 min was significantly lower compared to walking at 1.8 mph for 20 min. This finding is significant because moderate-to-fast walking speed (3.6 mph) can decrease plantar tissue stiffness compared to slow walking speed (1.8 mph). On the other hand, slow walking speed can increase plantar tissue stiffness that may cause a higher risk of foot ulcers [28]. This study suggests people at risk for foot ulcers walk at a preferred or fast speed (3.6 mph) rather than walk slowly (1.8 mph). [PERSON] and [PERSON] proposed the physical stress theory to provide a conceptual framework for guiding the prescription of physical activity to improve health in various physiological and pathological conditions [29]. An appropriate intensity of exercise can provide needed physical stress to maintain tissue health; when the intensity is either too low or too high, tissue injury may occur. However, there are no definitive values of the appropriate stress for various tissues. The authors indicate a need to establish thresholds for various conditions. In this study, we demonstrated that walking at \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & \multicolumn{2}{c}{**Duration**} & \multicolumn{2}{c}{**Paired \(t\)-Test**} \\ \hline
**Speed** & **Layer** & **10 min (Mean \(\pm\) SE)** & **20 min (Mean \(\pm\) SE)** & **_p Value_** & **_p Value_** \\ \hline
1.8 mph & Q-All & 56.9 \(\pm\) 1.0 & 55.1 \(\pm\) 1.0 & 56.4 \(\pm\) 0.8 & 0.363 & 0.170 & 0.696 & 0.321 \\  & Q1 & 39.4 \(\pm\) 1.8 & 35.6 \(\pm\) 1.6 & 37.9 \(\pm\) 1.6 & 0.293 & 0.123 & 0.556 & 0.331 \\  & Q2 & 39.4 \(\pm\) 1.4 & 36.8 \(\pm\) 1.4 & 38.3 \(\pm\) 0.9 & 0.374 & 0.165 & 0.542 & 0.428 \\  & Q3 & 65.1 \(\pm\) 1.3 & 65.5 \(\pm\) 1.8 & 46.2 \(\pm\) 1.2 & 0.741 & 0.444 & 0.663 & 0.740 \\  & Q4 & 80.8 \(\pm\) 1.3 & 80.5 \(\pm\) 1.4 & 82.3 \(\pm\) 1.4 & 0.603 & 0.864 & 0.442 & 0.349 \\
20 min & Q-All & 56.8 \(\pm\) 0.8 & 53.6 \(\pm\) 0.9 & 55.3 \(\pm\) 1.0 & 0.057 & 0.017 \({}^{*}\) & 0.251 & 0.191 \\  & Q1 & 40.4 \(\pm\) 1.3 & 34.0 \(\pm\) 1.5 & 35.5 \(\pm\) 1.4 & 0.007 \({}^{**}\) & 0.003 \({}^{**}\) & 0.018 \({}^{*}\) & 0.449 \\  & Q2 & 41.4 \(\pm\) 1.4 & 35.3 \(\pm\) 1.0 & 37.9 \(\pm\) 1.0 & 0.003 \({}^{**}\) & 0.001 \({}^{**}\) & 0.040 & 0.113 \\  & Q3 & 63.2 \(\pm\) 1.4 & 62.3 \(\pm\) 2.1 & 63.4 \(\pm\) 1.8 & 0.901 & 0.732 & 0.931 & 0.668 \\  & Q4 & 79.2 \(\pm\) 1.4 & 80.0 \(\pm\) 1.5 & 81.4 \(\pm\) 1.2 & 0.467 & 0.684 & 0.229 & 0.420 \\ \hline \hline \end{tabular} Note: The plantar tissue stiffness (Q-All) under the first metatarsal head consisting of four layers, including the outer layer (Q1, near skin surface), sub-outer layer (Q2), sub-inner layer (Q3) and inner layer (Q4, near bone surface); \({}^{*}\), \(p\) \(<\) 0.05; \({}^{**}\), \(p\) \(<\) 0.001.  
\end{table} 
Table 1: Effect of walking speeds on plantar tissue stiffness (Q-All) and evenly divided four layers (from superficial (skin) to deep (bone), Q1, Q2, Q3 and Q4). \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & & **Duration** & **Paired \(t\)-Test** \\ \hline
**Speed** & **Layer** & **10 min (Mean \(\pm\) SE)** & **20 min (Mean \(\pm\) SE)** & **_p Value_** \\ \hline
1.8 mph & Q-All & 56.9 \(\pm\) 1.0 & 56.8 \(\pm\) 0.8 & 0.874 \\  & Q1 & 39.4 \(\pm\) 1.8 & 40.4 \(\pm\) 1.3 & 0.283 \\  & Q2 & 39.4 \(\pm\) 1.4 & 41.4 \(\pm\) 1.4 & 0.149 \\  & Q3 &  
1.8 mph for 20 min resulted in higher stiffness of plantar tissue compared to walking at 3.6 mph for 20 min. This implies that walking speed at 3.6 mph is more appropriate in people at risk for foot ulcers, especially people with DM. Our finding also suggests that slow walking speed (1.8 mph for 20 min) may not be a good strategy for people at risk for foot ulcers.