## 4 Discussion
These movement alterations also increased the risk of traumatic injuries because the vestibular and proprioceptive system did not completely compensate for visual information deficits (blocking vision) [20]. Alternatively, movement alterations were not observed when looking at the landing spot was manipulated during the single- [23] and double- [18] leg-landing periods. Hence, since our participants did not use a blindfold, they fully received visual information (central vision condition) or at least partially (peripheral vision condition) during the task. This result implies that the normal sensorimotor system would function as long as it received visual afferent information [45], thereby maintaining dynamic joint stability. Furthermore, we analyzed the whole landing phase (initial contact to peak knee flexion) of the task [26,33], which did not show any difference between landing conditions. Our data (time ranged between 162 ms and 169 ms, Table 2) also included the estimated time frame for ACL injuries (20 to 70 ms) [27, 28, 29] or ankle sprain (50 to 90 ms) [1]. Therefore, unless visual inputs were blocked, the level of visual information (visual focus) is not proposed to be a factor that alters the lower-extremity landing mechanics from a 30 cm height. As a single-leg drop landing can acutely cause femoral cartilage deformation [10], the amount of PFJS can be the potential source of overuse injury [10,46]. Likewise, our data showed that the level of visual information did not affect PFJS during the landing phase. Therefore, since the cumulative joint contact stress can lead to joint degeneration [46], the amount of PFJS estimates the risk of the patellofemoral conditions [30]. The peak PFJS in our participants during the task was also calculated as approximately 9 MPa (Figure 5), which is similar to that of descending walking [47]. Additionally, compared to other functional movements, this value is smaller than that of the body weight of two-legged squats (12 MPa) [48], but greater than that of single-leg squats (7 MPa) [49], forward lunging (6.0 MPa) [50], self-selected fast waking (3 MPa) [30], single-leg hopping (2 MPa) [51], and self-selected free walking (2.0 MPa) [30], whereas the amounts of peak PFJS during various movements have been reported [30,47, 48, 49, 50, 51]. Similarly, the specific amount or duration of (cumulative) PFJS that is proposed to lead to overuse injury is unknown [48,50,52]. Thus, our results demonstrated that the amount of PFJS related to overuse injury is not affected by the level of visual information (visual focus) during a task that requires single-leg dynamic stability. Notably, it is important to acknowledge the study limitations. Females have different landing mechanics, such as greater GRF [53], greater knee valgus [54], and less knee flexion [55]. Hence, comparing sex differences was not the purpose of our study, which was also not considered in the statistical analyses as in previous studies with a similar study design (crossover repeated measures) [20; 23]. Therefore, future studies should examine the interaction between the level of visual input and sex discrepancies in the risk of injury during a single-leg drop jump task. Second, PFJS in our study was calculated using the kinematic and kinetic variables in the sagittal plane [30]. Thus, since the patella moved in multiple planes, future estimation of PFJS should take account of variables in multiple planes using other methods (e.g., computational modeling using OpenSim) [56]. Lastly, parameters on landing conditions (e.g., visual focus, height, and movement after landing) were unpredictable in real athletic situations. An unanticipated single-leg-landing condition also showed greater ankle joint and impact kinetics [9; 57] than an anticipated landing condition. Therefore, care should be taken when applying the results of our study to unanticipated landing conditions.