## 4 Discussion  
This study compared the lower-extremity dynamics (joint kinematics, kinetics, and vertical GRF) and the PFJS between the landing with and without looking at the landing spot during a single-leg drop jump task. Although we observed several alterations in the frontal plane ankle and hip joint (Figure 2), the magnitudes of changes were negligeable (altered joint angles \(<\) 1.2\({}^{\circ}\), d \(<\) 0.35). Therefore, our hypothesis that no difference would be observed in movement dynamics and PFJS was mostly supported, which agreed with the results of previous reports in which visual input was manipulated. Specifically, blindfolded subjects in previous studies did not show alterations in vertical GRF [21] and preparatory muscle activities [22] during the double-leg landing relative to a condition with normal vision (not blindfolded). Moreover, landing while receiving less visual information (e.g., not looking at the ground with eyes open) showed no difference in vertical GRF and preparatory muscle activity [18] during a double-leg landing compared with landing conditions that received full visual information (e.g., looking at the ground). Our data therefore propose  
Figure 5: Vertical ground reaction force and patellofemoral joint stress (PFJS). Descriptive statistics for each condition (mean \(\pm\) 1 standard deviation error cloud) for vertical ground reaction force (**A**) and PFJS (**B**). Time was determined from initial contact (0%) to peak knee flexion (100%). The black line is the t-value of each time point for the vertical ground reaction force (**C**) and PFJS (**D**). The dotted redline is the threshold of significance. Likewise, no significant difference was observed in vertical ground reaction force and PFJS between the landing conditions. that less visual focus on the landing spot during the landing phase of a single-leg drop jump does not increase the risk of lower-extremity traumatic and overuse injuries. Furthermore, no difference observed in most dependent variables between the landing conditions explained the delimitation of this study. Hence, the landing height and the horizontal distance from the force platform were fixed at 30 cm and 3 cm, respectively. The lower-extremity joints also have to manage greater external forces as landing height increases [40]. Therefore, previous studies have suggested that the dropping height, which possibly increases injury risk during double-leg landing, was >60 cm [41]. A study also proposed that single-leg landings from a height of 30 cm or less does not exceed the threshold at which the normal integration of sensory input is altered [42]. Our participants were healthy populations that the amount of visual deficit would have easily been complemented by other systems (e.g., substitution of proprioception) [43], including a short-term learning effect [44]. Previously, only ten practice trials ended the neuromechanical compensatory movements during double-leg landing when participants were blindfolded [22]. Another study [21] reported that perceiving and understanding the laboratory environment (e.g., landing height, equipment, and testing procedures) before being blindfolded did not result in movement deviations. However, our participants were proposed to have learned movement patterns of the single-leg drop jump task through repetitive practice trials over three separate days. Therefore, the adaptation of the task-specific movement [16,22] and environmental perception [21] may also explain why we did not observe any movement differences between the landing conditions. As we discussed in the introduction, little data are available on movement alterations regarding the level of visual information during landing [18, 19, 20, 21, 22, 23]. Double-leg landing without receiving visual information (being blindfolded) resulted in decreased knee flexion and increased vertical GRF [19,20].