## 2 Materials and Methods
### Data Collection
#### 2.3.2 Surface Electromyography  
Electromyographic (EMG) data were reported according to the International Society of Electrophysiology and Kinesiology [24] as follows: passive disposable dual-surface Ag/AgCl circular electrodes, 27 mm \(\times\) 40 mm (width \(\times\) length), with low-impedance solid gel in the contact area (Noraxon, Inc.) were placed in the following muscles of each lower limb, vastus medialis (VM), vastus lateralis (VL), and biceps femoris (BF). The skin over the muscles was shaved and cleaned using an isopropyl alcohol solution, and allowed to vaporize so that the skin was dry before the electrodes were placed. The same researcher identified the muscles and placed the electrodes in the middle part of the muscle bellies on both lower limbs parallel to the direction of the muscle fiber, according to the recommendations of SENIAM [25]. The sensors, electrodes, and cables were slightly bandaged with Kinesio tape and elastic bands to prevent motion artifacts and cable movements. The six surface EMG signals were recorded using a telemetric EMG receiver (Ultium Desktop Receiver; Noraxon, Scottsdale, AZ, USA) at a sampling frequency of 2000 Hz. The signal was pre-amplified with an overall gain of 500, filtered with a high-pass filter set at 10 Hz,  
Figure 1: Drop landing test configuration.  
and low-pass filtered with a cut-off at 500 Hz. Each EMG channel had a common mode rejection ratio greater than 100 dB, and the input impedance was greater than 100 M\(\Omega\). The raw EMG signals were smoothed in the subsequent offline analysis using a root mean square (RMS) filter with a constant time of 50 ms [26]. The EMG RMS for each muscle during the drop-landing test was normalized to the maximal EMG RMS amplitude obtained from the respective muscles during isometric muscle voluntary contraction (MVIC). Muscle activation was recorded during muscle strength testing as described below. Mean EMG RMS amplitudes were obtained for the pre- and post-landing phases of each drop landing. The EMG RMS amplitude was obtained for a 25 ms window before and 70 ms after landing. The time windows for EMG amplitude analysis for the pre-landing and post-landing phases were chosen based on the relevance of these short durations of ACL injury [27]. The 25 ms pre-activation has been used to detect ACL injury risk in athlete populations [26; 28]. In addition, a time window of 70 ms for the post-landing phase was chosen because the ACL is injured during the first 70 ms after initial contact [27].  
Furthermore, two EMG sensors were placed over the tibial tuberosity of each lower limb and operated as inertial measurement units to detect the initial contact. The sampling rate of the accelerometer was set to 500 Hz. The IMU measures the accelerations along the x-, y-, and z-axes. Behavioral analysis of the IMU during drop landing was conducted in a pilot session before the pre-season measurements. The x-axes corresponding to the vertical axes and resultant accelerations were observed in relation to the force platform data. In the pilot session, the resultant acceleration data closely matched the force platform data, while the IMU data of the \(x\)-axis were presented with delay. At the resultant acceleration during the flight phase, the acceleration amplitude was close to 0 g, and when touching the floor, an initial peak close to 1 G was noted. Thus, to determine the landing in the present study, a cut-off point was set at 1 G of the resultant acceleration of the tibia. The quadriceps: hamstring (Q:H) EMG ratio pre-landing and post-landing was used in the analysis.