# Ankle Joint Dynamic Stiffness in Long-Distance Runners: Effect of Foot Strike and Shoes Features
## 3 Results  
No main effect of group was found for \(\mathrm{K_{ankle}}\) (\(p=0.164\); \(\mathrm{\eta_{p}}^{2}=0.105\)), but the main effects for shoe type (\(p=0.008\); \(\mathrm{\eta_{p}}^{2}=0.272\)) and slope (\(p<0.001\); \(\mathrm{\eta_{p}}^{2}=0.889\)) were obtained (Table S1). Posthoc analysis revealed that \(\mathrm{K_{ankle}}\) was 12% higher in med-MI compared with high-MI shoes (\(p=0.007\); \(g=0.706\)). Table S2 and Figure 2 show mean and SD for \(\mathrm{K_{ankle}}\) in the three subphases of stance and among the three shoe conditions. Significant differences were found among all subphases: ERP-LRP (\(0.176\pm 0.01\); \(0.215\pm 0.01\) Nm/kg/\(\degr\cdot 100\)) \(p=0.001\); \(g=3.9\); ERP-DP (\(0.176\pm 0.01\); \(0.091\pm 0.01\) Nm/kg/\(\degr\cdot 100\)) \(p<0.001\); \(g=9.5\); LRP-DP (\(0.215\pm 0.01\); \(0.091\pm 0.01\) Nm/kg/\(\degr\cdot 100\)) \(p=0.001\); \(g=12.4\). Overall \(\mathrm{K_{ankle}}\) was highest when wearing med-MI shoes (although not statistically different from low-MI shoes; \(p=0.246\)); \(\mathrm{K_{ankle}}\) was highest during the late rising phase (LRP) and lowest during the unloading phase (DP). Runners in high-MI shoes exhibited a lower stiffness (more compliant ankle) during the impact phase (ERP) and late rising phase (LRP); during the unloading phase (DP), low-MI shoes allowed the most compliant ankle. There was a _Shoe_ by _Slope_ interaction effect (\(p=0.008\); \(\mathrm{\eta_{p}}^{2}=0.221\); Table S1) for \(\mathrm{K_{ankle}}\) (Figure 2, Table S2). Pairwise multiple comparisons showed that during the impact phase (ERP), \(\mathrm{K_{ankle}}\) in high-MI shoes was lower compared with that in both low-MI and med-MI shoes (\(-15\)%, \(p=0.013\), \(g=1.2\); and \(-16\)%, \(p=0.003\), \(g=1.25\), respectively). During the late rising phase (LRP), \(\mathrm{K_{ankle}}\) was the highest in med-MI shoes (\(0.227\pm 0.01\) Nm/kg/\(\degr\cdot 100\)), but only statistically different from  
Figure 2: Mean and SD values for ankle joint dynamic stiffness of FFS and RFS for the three phases of stance, in the three shoe conditions. ERP early rising phase, LRP late rising phase, DP descending phase. Shoes conditions are termed as low-MI (LOW), medium-MI (MED) and high-MI (HIGH). MI: minimalist indexes. FFS: forefoot strikers. RFS: rearfoot strikershigh-MI shoes (+12%, \(p=0.011,g=1.58\)). During the unloading phase (DP), differences between shoes were only significant for low-MI compared with med-MI shoes (\(-6\%\), \(p=0.009\), \(g=0.5\)). Figure 3 compares mean moment-angle loops for RFS and FFS. While curves are similar in low-MI shoes, (Figure 3, top) the base (ankle range of motion) is shifted toward the left for FFS. This is also true for medium-MI (Figure 3, middle), and high-MI shoes (Figure 3, bottom). Figure 3: Ankle moment–angle plot. Group mean profiles comparison for low-MI, medium-MI and high-MI shoes. Insets report linear regression lines among early rising phase (ERP), late rising phase (LRP) and descending phase (DP). Overall, runners exhibiting high K\({}_{\rm ankle}\) during the late rising phase (LRP) also have high K\({}_{\rm ankle}\) during the unloading phase (DP) (Table 1). For FFS, the correlation between K\({}_{\rm ankle}\) in the impact phase (ERP) and in late rising phase (LRP) increased with shoes' MI, with the highest correlation (r\({}_{\rm s}\) = 0.95; \(p\) \(<\) 0.01) in high-MI shoes. A similar trend was reported for correlations between K\({}_{\rm ankle}\) in impact phase (ERP) and in unloading (DP), and between K\({}_{\rm ankle}\) in late rising phase (LRP) and in unloading (DP), with highest values in the high-MI condition (r\({}_{\rm s}\) = 0.84, \(p\)\(<\) 0.01; r\({}_{\rm s}\) = 0.89, \(p\)\(<\) 0.01, respectively). Values were only significant in high-MI shoe conditions; this means that FFS in high-MI shoes with high K\({}_{\rm ankle}\) during impact phase will also have high K\({}_{\rm ankle}\) during the loading and unloading phases.