## 4 Discussion  
The aim of this study was to identify and compare the effects of central and peripheral fatigue on angular kinematics and spatiotemporal parameters during running under the same methodology. Significant changes were observed after central and peripheral fatigue protocols in kinematics parameters. Central fatigue mainly increased the ankle, shank, and knee range of movement in the sagittal plane at initial contact, maximal knee flexion, and toe-off instants, increasing stand time. Our results supported the first hypothesis. In contrast, peripheral fatigue only induced small changes at the initial contact and toe-off instants, with no changes at MKF instant nor spatiotemporal parameters. Kinematics and spatiotemporal alterations were higher after central than peripheral fatigue, showing a reduction in the capability of dynamically stabilizing the knee after central fatigue, showing a more flexed ankle, knee, and thigh at MKF instant, and increasing the stance time by means of a reduction in swing percentage and a subsequent increment in propulsion time compared with peripheral fatigue. The second hypothesis also was supported by the results. Regarding the effect of fatigue (pre vs. post) on angular kinematics and spatiotemporal parameters, prolonged running fatigue protocols on a treadmill caused kinematics modifications in the stance phase without altering the swing phase. Central fatigue protocol resulted in a greater shank anterior oscillation at initial contact, while the thigh and knee flexion and ankle dorsiflexion were sustained. Previous studies suggested that after central fatigue, an anterior displacement of the center of mass [10] caused by an increased forward trunk lean (trunk flexion) [8; 10; 24] and pelvis anterior tilt [10] occurs. The authors suggest that the knee flexion was sustained [35], and in order to maintain balance after kinematics changes [35], the increased shank anterior oscillation could be an adaptation that would cause the initial contact to happen more forward. Coinciding with our results at initial contact, some studies showed that knee flexion was not altered in central fatigue [10; 24; 36]. Contrary, [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [37] described a decrease of knee flexion associated with the decline in pace runs, which is an aspect that did not happen in our study, since the speed was kept constant. Face reduction in a real situation was associated with the muscle breakdown blood markers related to fatigue [38]. The knee joint acts as a low-pass filter, so the more flexed the knee,the greater the shock attenuation [39; 40], but hitting the ground with a more extended knee can increase not only the forces received but also the risk of injury. However, other investigations reported a gradual increase in knee flexion as fatigue increase. The knee flexion/extension during the midstance phase is directly related to the severity of the received forces [7; 41]. Therefore, greater knee flexion acts as a corporal compensatory strategy to reduce the vGRF [6; 7; 41]. Our results showed a greater knee flexion in the maximum knee flexion instant or also called midstance after central fatigue protocol, reflecting the bodily protective mechanism to reduce external forces. It is important to mention that each knee angle increase means a 68 N reduction in vGRF approximately [41], but the metabolic cost also increases [6]. So, the oxygen consumption (VO2) could be increased by 25% for each 5-degree increase in midstance knee flexion angle [42]. In our study, both ankle dorsiflexion and rearfoot version were not affected by central fatigue during the stance phase. Similarly, other investigations reported the same behavior of the ankle dorsiflexion [24; 43]. However, an increased rearfoot version during fatigued running seems to be an accepted action in the literature [7; 24].