## 2 Materials and Methods
### Procedures  
#### 2.2.1 Assessment of Single-Leg Drop Landing  
Prior to testing, each participant performed a 5 min warm-up using a stationary bike at self-selected speeds [25]. After warm-up, three wearable IMUs (DSP Wireless 9-Axis Motion Sensor; SPORTS SENSING Co., Ltd., Fukuoka, Japan) were placed on the mid-lateral shank, mid-lateral thigh, and lumbar region (Figure 1). The mid-lateral shank sensor was placed at the midpoint of the line connecting the fibular head and lateral malleoli. The mid-lateral thigh sensor was attached at the midpoint of the line connecting the greater trochanters and the lateral epicondyles of the femurs. The lumbar sensor was attached at the midpoint of the line connecting the right and left iliac crests. Each IMU was secured using a Velcro strap with a sensor pocket. The participants then performed a single-leg drop landing task with bare feet (Figure 2). They were instructed to stand on one leg on the edge of a 30 cm high box with their arms folded in front of their chest and then drop off the box and land on the force plate (SS-FP40 AO-SY; SPORTS SENSING Co., Ltd., Fukuoka, Japan) with the same leg [22; 26]. The participants were required to maintain the landing posture for at least 3 s after landing and perform until three successful trials. They practiced several times to become used to the landing task. The order of measurement was randomized for uninvolved and involved limbs. The sampling rates were set to 200 Hz and 1000 Hz for the acceleration from the IMUs and force plate data, respectively. \begin{table}
\begin{tabular}{l c} \hline \hline
**Characteristics** & **Values** \\ \hline Age, years & 20.5 \(\pm\) 5.1 \\ Sex (male/female) & 12/14 \\ Height, cm & 166.1 \(\pm\) 7.9 \\ Body mass, kg & 67.0 \(\pm\) 10.8 \\ Preinjury modified Tegner activity score & 7.4 \(\pm\) 0.7 \\ Meniscus surgery at the same time as ACLR (yes/no) & 14/12 \\ Isokinetic quadriceps strength at 60 deg/s & \\ Involved limb, Nm/kg & 2.2 \(\pm\) 0.5 \\ Uninvolved limb, Nm/kg & 2.3 \(\pm\) 0.5 \\ LSI, \% & 89.4 \(\pm\) 12.1 \\ Isokinetic hamstring strength at 60 deg/s & \\ Involved limb, Nm/kg & 1.2 \(\pm\) 0.3 \\ Uninvolved limb, Nm/kg & 1.3 \(\pm\) 0.3 \\ LSI, \% & 92.0 \(\pm\) 11.2 \\ IKDC-SKF score & 88.4 \(\pm\) 8.8 \\ ACL-RSI score & 73.8 \(\pm\) 18.3 \\ \hline \hline \end{tabular} Data are shown as mean \(\pm\) standard deviation or number. ACLR, anterior cruciate ligament reconstruction; LSI, limb symmetry index; IKDC-SKF, International Knee Documentation Committee Subjective Knee Evaluation Form; ACL-RSI, anterior cruciate ligament-return to sport after injury.  
\end{table} 
Table 1: Participants’ characteristics. A custom-written MATLAB program (version R2019a; MathWorks, Inc., Natick, MA, USA) was used for data processing. The maximum value of the VGRF was calculated after the initial contact (when the vertical ground reaction force exceeded 10 N for the first time), and the average value of the three successful trials was used in the analysis. The triaxial accelerations in the \(x\)-, \(y\)-, and \(z\)-directions were combined into the resultant acceleration using the following formula:  
\[a_{r}=\sqrt{a_{x}^{2}+a_{y}^{2}+a_{z}^{2}}\]  
where \(a_{r}\) is the resultant acceleration, \(a_{x}\) is the acceleration in the \(x\) direction, \(a_{y}\) is the acceleration in the \(y\) direction, and \(a_{z}\) is the acceleration in the \(z\) direction. Because the direction of \(x\)-, \(y\)-, and \(z\)-axes relative to gravity changes due to the change in segment posture during landing, the resultant acceleration was calculated in this study. The average resultant acceleration of the three successful trials was used in the statistical analyses. The VGRF data were normalized to the participants' body weight (BW; body mass \(\times\) 9.81). The VGRF and acceleration were filtered using a fourth-order Butterworth low-pass filter with cut-off frequencies of 50 Hz [27] and 12 Hz [16; 17], respectively. The limb symmetry index  
Figure 1: Location of IMUs in the shank, thigh, and lumbar region.