# Duty Factor Reflects Lower Limb Kinematics of Running
## 4 Discussion
Indeed, the body, and especially the foot, is applying a higher force to the ground during the ground contact phase of the running stride than when standing in a static upright stance, leading to more compression in the rear (DFhigh) or fore (DFlow) part of the shoe insole and thus providing a non-zero angle. In addition, DFhigh depicted a significantly larger forefoot angle at TO than DFlow, which was associated with a significantly larger \(|\Delta\theta_{p}|\) (Table 7). These findings further suggest that DFhigh promotes forward propulsion by pushing more with the toes during \(t_{p}\). DFlow exhibited significantly more plantar flexion at FS and more dorsiflexion during \(t_{a}\) and \(t_{b}\) than DFhigh (Table 5). A visual observation of Figure 2C depicts that a plantar flexion takes place at the very first instants of the stance for DFhigh, right before the usual dorsiflexion. These results suggest that DFlow relies on the midfoot to forefoot and the eccentric action of the plantar-flexor muscles to attenuate the impact. On the contrary, DFhigh seems to additionally rely on the eccentric action of the dorsiflexor muscles, as already pointed out by [PERSON] et al. (2019) for rearfoot strikers. The findings of this study mirror previous observations of rearfoot and forefoot strikers. Indeed, [PERSON] et al. (1998) observed that rearfoot strikers were absorbing most of the impact shock using the heel and skeletal tissue whereas forefoot strikers used an eccentric action of the plantar flexor muscles to absorb the shock [25; 38; 39]. Additionally, a rearfoot strike pattern was shown to cause a significant impact force on the heel, whereas peak plantar pressure was centered at the forefoot when forefoot striking [41]. Similarly, [PERSON] et al. (1985) mentioned that for forefoot strikers, instead of being primarily an energy producer, the ankle takes the additional task of absorbing energy during \(t_{b}\) and [PERSON] et al. (2007) pointed out the fact that shorter \(t_{c}\) and increased plantar flexion angles require a greater amount of energy to be absorbed at the ankle [64]. When focusing on the entire lower limb, a larger range of hip extension and a greater increase in the range of motion of hip extension with increasing running speed during \(t_{p}\) was observed for DFhigh than for DFlow (Table 4). Besides, DFhigh demonstrated more hip flexion at FS, IA, and MS (Table 3) but a similar range of motion during \(t_{a}\) and \(t_{b}\) than DFlow. In addition, the pelvis showed a higher range of retroversion during \(t_{a}\) and \(t_{b}\) and no change of retroversion with increasing running speed (Table 6) despite a statistically higher anteversion at FS for DFhigh than DFlow. The lower vertical kinetic energy at impact (shorter \(t_{d}\) and smaller \(\Delta z_{\text{COM},d}\)) and thus a softer (less stiff) lower limb allows a higher pelvis mobility in DFhigh than DFlow and to rotate in the three anatomical planes (see Supplementary Materials) while DFlow requires, due to the higher vertical kinetic energy at FS, a stiffer lower limb than DFhigh, which suggests limiting the rotation in the sagittal plane. Furthermore, less knee flexion was observed for DFlow than DFhigh at MS but a similar one at FS, which led to a larger range of motion of the knee joint angle during \(t_{b}\) and \(t_{p}\) for DFhigh than DFlow (Table 4). Therefore, a larger leg compression and decompression was observed for DFhigh than DFlow despite the similar knee joint angle at FS. In addition, with increasing running speed, DFhigh depicted a larger increase of knee flexion at IA than DFlow and a larger increase in the range of motion of the knee joint during \(t_{a}\) and \(t_{p}\) (Table 4). These findings are potentially representing a more pronounced eccentric action of the quadriceps muscle for DFhigh than DFlow during \(t_{b}\). These results are in line with the observations of [PERSON] et al.