## 4 Discussion  
We hypothesized that compared with DL, the SL will show smaller knee and hip flexion angles and angular velocities as well as greater moment, power, VGRF, and loading rates. Meanwhile, the SL will show greater ankle plantar flexion angle and dorsiflexion angular velocity, and most of these differences were within the 30-50 milliseconds time frame of initial contact with the ground. Our results agree in part with the proposed hypothesis. SPM demonstrated that most differences occur in the first third of the landing phase. Our results are consistent with ACL injury mechanism in landing tasks, and SL may increase the probability of ACL injury risk. The wide ROM of the knee and hip sagittal plane are key to lower limb cushioning during landing [17]. Smaller knee flexion during landing can result in lower limb shock absorption, leaving the ACL at a higher risk of injury [40; 41; 42; 43]. Our results revealed that SL depicted a smaller knee flexion angle (about 13.8\({}^{\circ}\)) and hip flexion angle (about 10.8\({}^{\circ}\)) than DL during the landing phase, and SL also has resulted in a flexion angle of less than 45\({}^{\circ}\) within the 30-50 ms time frame of initial contact. [PERSON] and [PERSON] et al. both found that the quadriceps force will augment ACL loading, when the knee joint flexion angle is from 0\({}^{\circ}\) to 45\({}^{\circ}\)[18; 44]. Therefore, our current evidence pointed out that the quadriceps force of SL seems to increase the load on the ACL. Meanwhile, SL appears to be a hard landing with the smaller hip flexion angle, and the results of hard landing patterns are followed by an increase in knee and hip moments. Within the 30-50 ms time frame of the initial landing phase, we found that the knee moment of SL is about 2 Nm/kg greater than DL during the 17.74-21.82% (about 35-43 ms after initial ground contact) landing phase. Consistent with previous studies, smaller flexion angles are accompanied by larger moments, and this increases the risk of ACL injury [45]. At the same time, SL depicted a  
Figure 5: The Statistical Parametric Mapping (SPM) results between single-leg landing (SL) and double-leg landings (DL) tasks following a spike, depicting the mean vertical ground reaction force (VGRF) and standard deviation. Two dashed lines are the 30 and 50 ms time frame of initial contact. Grey shaded areas indicate that there are significant differences (\(p\) < 0.05) between SL and DL during the spike landing phase. greater negative knee joint power and hip joint power during the landing phase. Previous studies have shown that the case of the negative knee and hip joint power suggests that eccentric work is done by the extensor muscles of the knee and hip joint to consume the impact of energy [12]. With the increase of negative joint power, this means that the eccentricity to the joint extensor increases. The strain on the ACL will increase with the eccentricity to the joint extensor (quadriceps femoris and anterior tibia) then the risk of ACL injury will increase [13; 45]. The knee joint plays a significant role in absorbing shock during contact between the body and the ground [12]. During the landing task, the increased VGRF will also increase the contractile force of the quadriceps femoris, thus increasing the load on the ACL [13]. Previous studies have shown that higher VGRF and insufficient VGRF dissipation during the landing phase may also be a risk factor for ACL injury [12; 46]. Our results revealed that the SL depicted a significantly greater VGRF than DL during the 19.75-24.14% (38-47 ms after initial ground contact) landing phase, and SL also depicted greater peak VGRF (\(p=0.001\)) and loading rate of VGRF (\(p=0.001\)) than DL. [PERSON] and [PERSON] found that the maximum ACL tension occurred at the moment of the peak VGRF [44; 47]. A higher VGRF loading rate indicates a greater impact in a shorter period, thus increasing the risk of ACL injury with a high probability [3; 48; 49]. [PERSON] et al.