## 1 Introduction  
Long-distance running has many benefits, including reducing the risk of cardiovascular disease [1]. Although there is not sufficient literature demonstrating that injuries are inevitable for all runners, running-related musculoskeletal injuries are still very common. Previous studies showed that the incidence of injury per runner for 1000 h is 18.2-92.4% [2; 3; 4] or 6.8-59% [5]. Amongst them, more than 74% of long-distance runners adopt the heel strike pattern [6]. The repetitive impact forces occurred during touchdown can reach a magnitude ranging from 2- to 3-times body weight [7], and are considered as high risk factors of lower limb injury [6]. Long-term and high-intensity running inevitably induces neuromuscular fatigue, which further influences musculoskeletal system of the lower limbs. Generally, the movement control of the lower extremity decreases after fatigue, which is an important cause of running injury [8; 9]. However, conclusions on the relationship among fatigue, impact force, and lower extremity biomechanics during prolonged running remain uncertain [10]. [PERSON] [11] found that vertical ground reaction forces (vGRF) decreases with fatigue after multiple groups of repeated sprints are performed on a force treadmill. By contrast, [PERSON] et al. [12] observed that the impact peak and loading rate of the runners increase significantly after fatigue intervention. However, [PERSON] et al. [13] found no significant differences in vGRF after fatigue intervention induced by repeated sprints. Similarly, [PERSON] et al. [14] reported no changes in vGRF after nine well-trained runners performed an exhaustive exercise over 2000 m on an indoor track. Collectively, multifactorial causes underlie these different responses, and further studies beyond the analysis of the GRF level are warranted to provide insight on the joint mechanics strategies and the underlying stiffness characteristics occurring during running to fatigue. Most previous studies focused on the direct relationship between fatigue and injury, especially the biomechanical changes in the lower extremities before and after fatigue [15; 16; 17; 18; 19]. For instance, [PERSON] et al. [14] reported the biomechanical asymmetry (e.g., loading rate and knee/vertical stiffness) of the lower limbs before and after fatigue. However, the fatigued state was considered to be achieved only by increased rated perceived exertion (RPE) scores. [PERSON] et al. [17] compared the differences in running kinematics and shock absorption before and after fatigue. However, the influence of running fatigue on kinematics remains inconclusive, which might be partly due to the nonlinear adaptations in lower extremity biomechanics. [PERSON] et al. [18] reported an increased vertical force loading rate along with increased hip extension and ankle plantar flexion at incremental running speeds before and after two 20-min fatiguing treadmill runs. Nevertheless, they did not determine the threshold at which point the changes may occur. Thus, more investigations are necessary to examine fatigue-induced changes in lower extremity biomechanics not only in pre- and post-fatigue conditions but during the whole progress of a fatiguing run. In addition, none of the aforementioned studies provided a direct accessible evaluation of running fatigue. Fatigue is generally associated with lactic acid accumulation [20]. Blood lactate concentration (BLa) is an indicator of muscle metabolites and recovery [21]. In the current study, BLa was used to help determine fatigue as a consequence of an insufficient oxygen supply in the capillary blood [22]. The purpose of this study, therefore, was to determine the changes in lower extremity biomechanics, namely, vertical and anterior-posterior GRFs, loading rates, joint mechanics and stiffness, during treadmill running to fatigue. It was hypothesized that joint mechanics and stiffness with fatigue would be changed. Specifically, runners would increase their vGRFs and joint range of motion.