## 1 Introduction  
Running has become one of the most popular aerobic exercises worldwide due to its accessibility, minimal equipment requirements, and substantial health benefits [1]. With the vigorous development of global running events--including marathons, half marathons, and various short-distance road races--participation in running continues to rise. However, this growing engagement is accompanied by persistently high rates of lower-limb injuries. Epidemiological studies reveal that nearly half of runners globally experience lower-limb injuries at some point, adversely affecting both their performance and overall exercise experience, as well as placing additional strain on public health resources and healthcare costs [2,3]. Among the myriad factors contributing to running-related injuries, exercise-induced fatigue stands out as a critical determinant, exerting a profound impact on neuromuscular function and lower-limb biomechanical characteristics [4,5]. Understanding how fatigue modifies biomechanical and muscle activation patterns movement is essential for developing targeted training and rehabilitation strategies to mitigate injury risk and enhance performance. During the landing phase of running, when the foot makes contact with the ground, fatigue significantly alters lower-limb kinematics and kinetics, increasing mechanical stress on joints and soft tissues [6]. Ground reaction forces during landing can reach two to seven times body weight, and these forces are closely associated with injury incidence [7,8]. Previous studies have demonstrated that exercise-induced fatigue leads to increased joint flexion angles, higher impact forces, and elevated joint moments, which may contribute to impaired shock absorption and joint stability [7,9]. These alterations have been observed in multiple lower-limb joints, including the hip, knee, and ankle, where fatigue-induced changes in joint mechanics can result in compensatory movement patterns that further elevate injury risk [10]. However, most existing research has focused on the effects of localized muscle fatigue, investigating how fatigue in individual muscle groups, such as the hannstrings, quadriceps, and gastrocnemius, affects landing biomechanics [11]. Although these studies provide valuable insights into muscle-specific adaptations, they do not fully elucidate whether whole-limb fatigue alters lower-limb biomechanical characteristics and muscle activation patterns during running [11]. For runners, fatigue does not occur in isolated muscles but rather affects the lower limb as a whole, yet little is known about how global lower-limb fatigue influences biomechanical risk factors for injury [12]. Fatigue-induced changes in joint kinematics and kinetics are accompanied by alterations in muscle activation patterns, but the precise neuromuscular control strategies during the landing phase remain debated [13]. Some studies suggest that agonist-antagonist coactivation ratios remain stable under fatigue due to central neural mechanisms that regulate motor output [14], whereas others argue that antagonist activation decreases to compensate for reduced agonist force production, potentially compromising joint stability [15]. The lack of consensus in the literature highlights the need for further research to determine how whole-limb fatigue affects lower-limb biomechanics and neuromuscular control strategies during running. Therefore, the purpose of this study is to investigate the biomechanical and neuromuscular adaptations of the lower limbs during the landing phase of running under whole-limb fatigue conditions. Using three-dimensional motion capture, force plate analysis, and surface electromyography (sEMG), this study aims to quantify changes in joint kinematics, kinetics, and muscle activation patterns before and after fatigue. By analyzing neuromuscular regulation strategies, particularly agonist and antagonist muscle activation, this study seeks to provide novel insights into fatigue-induced injury mechanisms and inform injury prevention strategies for runners.