## 2 Methods
### Experimental Procedures  
A 10-camera Vicon motion capture system (Vicon Motion Systems, Oxford, UK) and Kistler force plates (Kistler Instruments, Winterthur, Switzerland) were used to synchronously collect kinematic and kinetic data during the forehand open-stance stroke. The Vicon system recorded lower-limb kinematics at 250 Hz, while the Kistler force plates captured ground reaction forces at 1000 Hz [31; 32]. A 49-marker model was used, covering the following anatomical landmarks: thoracic vertebrae 10, scapular-inferior angles, scapula-acromial edge, deltoid tuberosity, humerus-lateral epicondyle, radius-styloid process, ulna-styloid process, the basis of the forefinger, anterior superior iliac spine, iliac crest, posterior superior iliac spine, thigh triad 1-3, lateral epicondyle, medial epicondyle, fibular head, tibial tuberosity, calf triad, lateral prominence of the lateral malleolus, medial prominence of the medial malleolus, root bone, dorsal margin of the first metatarsal head, dorsal aspect of the second metatarsal head, and dorsal margin of the fifth metatarsal head [33; 34].  
A twelve-channel Delsys surface electromyography (sEMG) system (Delsys Inc., Natick, MA, USA) recorded muscle activity at 1000 Hz. The sEMG signals were synchronized with kinematic data via an A/D converter (Vicon Motion Systems, Oxford, UK) for subsequent analysis. Muscle activity was recorded from 12 muscles, including Rectus femoris (left LRF, right RRF), Biceps femoris (long head) (left LBF, right RBF), Medial gastrocnemius (left LMG, right RMG), Tibialis anterior (left LTA, right RTA), Erector spinae (left LES, right RES), and Gluteus maximus (left LGMAX, right RGMAX). Electrode placement followed the SENIAM (Surface Electromyography for the Non-Invasive Assessment of Muscles) guidelines [35], ensuring proper alignment with the long axis of the muscle fiber [36]. Prior to electrode placement, the skin was prepared by cleansing with alcohol wipes and removing hair to reduce impedance and optimize signal quality [37]. To minimize motion artifacts and enhance data reliability, electrodes were affixed using medical-grade adhesive and secured with elastic bands.  
Prior to testing, participants engaged in a 15 min warm-up routine comprising footwork exercises and practice swings to activate key muscle groups. Following this, they executed five high-intensity open-stance forehand strokes, focusing on striking the ball with the racket's sweet spot to refine force application and movement coordination.  
During formal testing, a professional coach tossed balls 1.5 m in front of the participants. The participants were required to hit the ball into a designated target area, a 3 \(\times\) 3 m\({}^{2}\), located 1 m from both the baseline and the singles sideline. A trial was considered successful if the shot landed within the designated area, with participants needing to complete 10 such successful attempts. A 15 s rest was maintained between trials to prevent fatigue-related muscle activity interference [38]. The testing process was supervised by a professional coach to ensure consistency and proper execution. Similar protocols involving coach-supervised skill execution have been used in tennis biomechanics research to ensure the ecological validity and technical accuracy of stroke performance [30]. None of the equipment used in the study interfered with or restricted the participants' movements (Figure 1a).  
Figure 1: Overview of the experimental framework. (**a**) Layout of the experimental setup. (**b**) Kinematic data processing and analysis using Visual 3D (C-Motion Inc., Germantown, MD, USA). (**c**) Defined phases of the forehand open-stance stroke. (**d**) Electromyographic (EMG) data were recorded from 12 muscles throughout the experiment.