## 2 Materials and Methods
### Surface Electromyography  
The sEMG signal was recorded with 16-bit accuracy at a sampling rate of 1500 Hz using the sEMG TeleMyot(tm) Direct Transmission System (DTS), Noraxon, Scottsdale, AZ, USA. The sEMG signal was processed using MyoResearch 3.14.38 software (Noraxon USA, Inc., Scottsdale, AZ, USA). Dual-electrodes for Noraxon sEMG (21 \(\times\) 40 mm) were used for this study.  
During the YBT-LQ, the sEMG signals were collected from the vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF), and semitendinosus (ST) muscles on the lower limb examined (standing limb).  
The VM and VL from the quadriceps and the BF and ST from the hamstrings were selected because they are key antagonist muscle groups that stabilize the knee. The quadriceps (VM and VL) control knee extension, while the hamstrings (BF and ST) manage knee flexion. Measuring their coactivation provides valuable insights into how these muscles work together to maintain knee stability during dynamic tasks, such as the YBT-LQ. The activation of the rectus femoris was not investigated due to controversies raised in the literature concerning crosstalk occurring during the assessment of this muscle [41].  
The area over the investigated muscle was prepared before the electrodes were placed on the skin. Hair in the area was removed using a disposable razor to improve the electrodes' adhesion. The skin at the electrode placement sites was cleansed by rubbing with a cotton swab soaked in 2% salicylic acid.  
The electrodes were placed following [PERSON] et al.'s (2014) study [42]. To collect VM muscle activation data, the dual-electrode was placed within the first 38 mm of the line on the distal portion of the muscle belly and oriented 50 deg with respect to the reference line between the medial side of the patella and the anterior superior iliac spine (Figure 1).  
The dual-electrode for collecting VL muscle activation data was placed within the first 43 mm of the line on the distal portion of the muscle belly and oriented 20 deg with respect to the reference line between the lateral side of the patella and the anterior superior iliac spine (Figure 2).  
Figure 1: The placement of the dual-electrode to collect the activation data of the vastus medialis muscle.  
The dual-electrode for collecting BF muscle activation data was placed between 72% and 100% of the line between the ischial tuberosity and the lateral side of the popliteal cavity (Figure 3).  
The dual-electrode for collecting ST activation data was placed between 74% and 100% of the line between the ischial tuberosity and the medial side of the popliteal cavity (Figure 4).  
To perform MVC normalization, maximum voluntary contractions (MVCs) were conducted for each targeted muscle before the YBT-LQ, following _Surface Electromyography for Non-Invasive Assessment of Muscles_ (SENIAM) project recommendations [42]. Participants were instructed to perform specific muscle contractions to elicit the maximum VM, VL, BF, and ST activation. For the VM and VL, participants performed an isometric knee extension against a fixed resistance while seated, ensuring that the hip and knee joints were at a 90-degree angle. Participants executed an isometric knee flexion for the BF and ST while lying prone with the knee flexed at 30 degrees. Each MVC was held for 3-5 s, and the highest stable value of the sEMG signal was recorded. This process was repeated three times for each muscle, with sufficient rest intervals between trials to avoid fatigue.