# Duty Factor Reflects Lower Limb Kinematics of Running
## 4 Discussion
(1995), which showed that the eccentric action of the knee extensor muscles was playing a role to attenuate the impact when rearfoot striking. Additionally, [PERSON] et al. (2017) have shown that different FS patterns change the distribution of loadings in a runner's body. The authors concluded that a forefoot strike pattern was generally associated with a greater vertical maximum peak force but significantly lower loading rates in the ankle, knee, and hip joints, which is in line with the lower flexion and compression of the lower limb observed for DFlow than DFhigh. Similar studies were performed by other researchers and demonstrated that a forefoot strike pattern reduces knee loads whereas a rearfoot strike pattern reduces loads at the Achilles tendon ([PERSON] et al., 2017; [PERSON] et al., 2018; [PERSON] et al., 2018) and ankle joint ([PERSON] et al., 2018), again agreeing with less lower limb flexion for DFlow than DFhigh. Running speed affected most of the biomechanical parameters. \(t_{c}\) decreased while \(t_{f}\) increased with the increase of the running speed, leading to higher vertical oscillations of the COM during \(t_{f}\) but smaller ones during \(t_{c}\) for both DF groups. Similar observations were obtained by [PERSON] et al. (2019). These results suggest that the increase of the running speed promotes the DFlow temporal characteristics and COM displacements rather than the DFhigh ones, i.e., a faster running speed favors \(t_{f}\) and corresponding COM displacements. However, the hip and knee flexions and the ankle dorsiflexion were increased during \(t_{c}\) with an increase of the running speed for both DF groups. Moreover, a more rearfoot strike pattern was observed for both DFhigh and DFlow with increasing running speed. An increase of the FS angle was also observed for aerial and terrestrial runners with the increase of the running speed ([PERSON] et al., 2018). These results suggest more lower limb flexion with increasing running speed. Nevertheless, even if higher running speeds seem to produce a favor to \(t_{f}\) associated with more lower limb flexion during \(t_{c}\), differences between DFlow and DFhigh are present at all speeds, with DFlow showing a higher favor to \(t_{f}\) and a more extended lower limb during \(t_{c}\) than DFhigh. Overall, these findings suggest the presence of two different lower limb kinematic mechanisms, which are directly linked to the runner's DF. These two mechanisms could be related to two different impact attenuation strategies. The existence of different type of shock attenuation strategies was already proposed by [PERSON] (1998). Indeed, the author related these different strategies to the different forms of impact passive peak on the vertical ground reaction force ([PERSON] et al., 2018). A similar impact attenuation strategy than the DFhigh one was observed for \"Groucho\" running ([PERSON] et al., 2018), while the shock absorption for \"Pose\" running ([PERSON] et al., 2018) resembles the one of DFlow. These two different strategies seem to rely on different muscles, i.e., plantar flexor muscles for DFlow and dorsiflexor and quadriceps muscles for DFhigh. Altogether, it can be speculated that the impact attenuation strategy is also self-selected and is another part of the subconsciously driven self-optimization, a central element in the development of an economical and safe running gait ([PERSON] et al., 2017; [PERSON] et al., 2017; [PERSON] et al., 2017). A few limitations to the present study exist. The ground reaction forces were not measured, thus making it not possible to directly link the calculated running kinematics (temporal characteristics, COM displacement, and joint and segment angles of the running step) to the impact loadings on the lower limb. Furthermore, whole-body 3D kinematic data was collected in order to report how the body globally structures itself while running. However, this \"global\" choice limited the precision of our 3D measurements when considering segment angles (as depicted by the large standard deviations).