## 4 Discussion  
This study investigated the biomechanical and neuromuscular changes in the lower limbs during the landing phase of running under fatigue conditions. Using a methodological approach that included three-dimensional motion capture, force platform analysis, and surface electromyography, the kinematic, kinetic, and neuromuscular characteristics were analyzed across different phases of landing. The results showed significant post-fatigue changes in joint angles, joint moments, and muscle activation patterns, particularly in the knee and hip joints, as well as in the coactivation ratios of key muscle groups. These findings provide insights into the mechanisms of neuromuscular adaptation to fatigue and offer practical implications for training optimization and injury prevention in running. Exercise-induced fatigue leads to changes in the sagittal plane's kinematic characteristics of the lower limbs during running. Specifically, fatigue results in reduced hip and knee joint angles at the moment of landing, as well as increased joint range of motion throughout the landing phase. Increased flexion in the hip and knee joints can lower the body's center of gravity, thereby enhancing balance and stability during running. The reduced knee joint angle at the moment of landing, followed by increased flexion, extends the moment arm of the quadriceps, allowing for greater knee extension moments. This corresponds with the observed increase in peak knee joint moments in the study results. Other studies have suggested that increased knee flexion may serve as a compensatory mechanism post-fatigue to adequately absorb ground reaction forces, protect the joint, and maintain consistent power output [12]. [PERSON] [18] proposed that a reduced knee flexion angle could potentially decrease the effective mass of the body or lower limbs. During the landing phase, the ankle joint primarily transitions from dorsiflexion to plantar flexion. Experimental results indicate minimal changes in ankle joint angles pre- and post-fatigue, but ankle function decreases as dorsiflexion, dorsiflexion moments, and joint energy absorption diminish. Exercise-induced fatigue also alters the kinetic characteristics of the lower limbs during running. Fatigue leads to an increase in peak knee joint moments and vertical ground reaction forces (vGRFs). Previous studies have highlighted that the repetitive impact forces experienced by lower-limb joints during sustained running are a primary contributor to overuse injuries in the lower extremities [19]. The increase in peak knee joint moments may be associated with a greater flexion of the hip and knee joints at heel strike. For the hip joint, the increased flexion extends the hip forward, thereby enhancing hip extensor moments. Figure 6: Coactivation ratio at different landing phases pre- and post-fatigue. \({}^{\star}\) Indicates that the results are statistically significant. However, the present study found no significant differences in hip joint peak moments pre- and post-fatigue, which contrasts with some prior findings [20]. [PERSON] et al. [21] reported that while peak knee joint moments tend to increase post-fatigue, hip joint moments often decrease, potentially due to reduced ground reaction forces affecting hip moment generation. In this study, the lack of significant changes in hip joint moments may be attributed to differences in fatigue protocols. Unlike [PERSON] et al., who implemented a knee extensor-specific fatigue protocol, this study employed a whole-limb fatigue model, which may have led to different neuromuscular compensation strategies. Additionally, the timing of the measurements conducted immediately after fatigue induction might have influenced the observed outcomes, as fatigue progression could vary across different muscle groups. Similarly, no significant differences were observed in peak ankle joint moments pre- and post-fatigue, consistent with prior research. However, ankle function declines with reduced dorsiflexion, dorsiflexion moments, and joint energy absorption [22].