## 2 Methods
### Experimental Procedure
#### 2.2.2 Experimental Protocol  
1. Pre-test preparation: Participants were tested individually to ensure consistency in experimental conditions and minimize fatigue effects. After recording anthropometric data (e.g., height, weight, leg length), participants were familiarized with the laboratory environment and experimental equipment. Participants wore tight-fitting shorts and heart rate monitors and were instructed to remove upper body hair at electrode placement sites. Skin preparation included abrasion with fine sandpaper and cleaning with alcohol wipes to reduce impedance.
2. Warm-up: A general warm-up consisting of joint mobilization (30 min) and treadmill running at 8 km/h for 5-10 min was performed, followed by a 3 min rest. sEMG electrodes and motion capture markers were then applied.
3. Fatigue protocol: A treadmill-based constant-speed running fatigue protocol was implemented to induce exercise-induced fatigue, based on the protocol proposed by [PERSON] et al. [17]. Participants ran at a constant speed of 14 km/h until exhaustion, defined by meeting all three criteria: 1. Heart rate reaching \(\geq\) 90% of age-predicted maximum heart rate. 2. Participant self-reported exhaustion using a Rating of Perceived Exertion (RPE) \(\geq\) 19. 3. Participant's voluntary termination of the task.  
Note: During the fatigue protocol, gait data were not recorded. The termination of the fatigue test was based solely on heart rate, RPE, and voluntary exhaustion criteria.
4. Biomechanical and sEMG data collection: 1. Pre-fatigue measurement: After the warm-up, participants performed a 10 m straight-line running trial at a self-selected speed (8-12 km/h), based on their habitual training pace. To control speed, a metronome was set to match the step cadence of each participant during their self-selected trial.  
A 20 m acceleration zone was provided before the 10 m test zone to ensure participants reached a steady state running speed before data collection.  
The force platform was placed in the center of the laboratory, and a 10 m deceleration zone was designated after the test zone to allow natural stopping without affecting running mechanics (Figure 1).  
Figure 1: Schematic of the experimental environment.  
Participants were instructed to step naturally onto the force platform without adjusting their stride, ensuring that their dominant right foot landed in the center.  
Surface electromyography (sEMG) signals were recorded using bipolar Ag/AgCl electrodes placed over the rectus femoris, biceps femoris, tibialis anterior, and gastrocnemius according to SENIAM guidelines. To minimize cross-talk, electrodes were positioned along the muscle belly in the direction of the muscle fibers, maintaining an inter-electrode distance of 20 mm. Before placement, the skin was abraded and cleaned with alcohol to reduce impedance. Muscle activation sites were identified through manual palpation and confirmed by voluntary contractions. To ensure reproducibility and reduce variability, the same researcher applied all electrodes, and placement was standardized across participants using anatomical landmarks.  
Successful trials were defined as full-foot contact within the platform boundaries. Three valid trials were recorded with a 10 s rest interval between runs.  
Three trials were recorded with a 10 s rest interval. Kinematic data (lower-limb joint angles) and kinetic data (GRF) were captured simultaneously with EMG signals from the RF, BF, TA, and GAS muscles.  
1. Post-fatigue measurement: Immediately following the fatigue intervention, participants repeated the running trials under the same conditions as the pre-fatigue test. The same cadence (set by the metronome in the pre-fatigue test) was used to regulate running speed, ensuring a consistent running pattern before and after fatigue. All biomechanical and sEMG data were collected using the same protocol.