## 4 Discussion  
This study aimed to determine the immediate effects of wearing shoes with different Minimal Index (MI) on the patellofemoral joints and to explore whether directly changing shoes can reduce knee load and the risk of patellofemoral pain syndrome (PFPS). The conventional belief is that the cushioned structure of running shoes can reduce the impact force during running and protect the lower limbs. In this study, however, the MI 26% shoes caused no change in the impact force during the stance phase and the ankle/knee flexion angle at touchdown compared with those with the MI 86% shoes. Moreover, during the transition and propulsive phases of the stance phase (20-100%), wearing the MI 86% shoes can effectively reduce the patellofemoral contact force (PFCF) and patellofemoral contact stress (PFJS) by changing the parameters of knee loads, such as the knee flexion angle and knee extension moment. The mechanism of PFPS is that the patellofemoral joints are subjected to excessively high loads [14; 18; 19]. Therefore, the results of this study suggest that running in high-MI running shoes might be beneficial for reducing the risk of PFPS. This finding is consistent with those in previous studies [20; 21]. [PERSON] and [PERSON] [7; 8] reported that the body actively changed the lower limb strategy while wearing minimalist shoes, i.e., increased step rates, lowered foot inclination angle at foot strike and reduced knee maximum flexion angle and joint range of motion (ROM). Moreover, according to the equation for the knee joint angle and the patellofemoral joint contact area, a decreased knee ROM decreased the patellofemoral joint contact area during the stance phase of runners wearing minimalist shoes; this condition is adverse to the reduction of PFJS. By contrast, runners with high-MI shoes showed a considerably smaller knee extension moment during the propulsive phase, which substantially reduced PFCF and, eventually, the PFJS decreased by 10.4%. Furthermore, we explored the characteristics of PFCF and PFJS during the entire stance phase with minimalist shoes. Findings showed that in the 0-20% stance phase (braking phase), the curves were coincident on different shoe conditions (Figure 4), and the differences between the two shoes mostly occurred from 20% to 100% of the stance phase. First, the initial 20% of the stance phase is the braking phase. At this phase, the lower limbs were subjected to the impact of the ground, and the vGRF reached the FP (impact force). The results of the current study showed no significant difference in the impact or propulsive force (SP of vGRF) between the low- and high-MI shoes. The braking phase accounts for 1/4 of the entire stance phase, and this study suggests that wearing high-MI shoes is beneficial for decreasing knee load in the propulsive phase (20-100%). According to recent research supporting barefoot running, running shoes not only reduce the impact force and loading rate and filter high-frequency impact signals [22] but also provide different neural inputs to the lower limbs, resulting in different responses and changes in the lower limb strategy [23]. As shown in this study, the lower limbs actively changed the knee flexion angle and extension moment when the runner was