## 4 Discussion  
The purpose of this study was to investigate and compare foot joint and segment kinematics between naturally RFS and FFS runners using an established and validated multi-segment kinematic model. In accordance with the first hypothesis, FFS runners landed with the forefoot first and showed an anticipated change in the Sha-Foo plantar/dorsiflexion in the stance phase compared to RFS runners. However, at mid-stance (15-40%), the Sha-Foo of FFS runners was more dorsiflexed, probably due to the larger external ankle dorsiflexion moment associated with this landing strategy [9]. The second hypothesis was also confirmed, as the first and second metatarsal bones in FFS runners was less inclined in relation to the ground (more dorsiflexed) at initial contact. With respect to the third hypothesis, RFS runners showed the fifth metatarsal bone in a upward orientation with respect to the ground and this orientation was combined with a greater adduction of the metatararus relative to the calcaneus (Cal-Met) at initial contact. These observations are consistent with the theoretical model of an oblique midtarsal joint where plantar flexion is associated with adduction [19; 20; 21] when running with an RFS pattern, which was not found in FFS runners. The larger ankle (i.e., Sha-Foo) plantarflexion at heel strike in FFS runners requires a higher eccentric activity of the calf muscles. This may lead to Achilles tendinopathy and muscle strain if these soft tissues are not adequately trained for these conditions [10; 22]. However, RFS runners are generally more susceptible to injuries due to the higher loads transmitted to the lower limb at heel strike [23; 24]. In agreement with [PERSON] et al. [9], the midfoot of FFS runners was less inverted with respect to the calcaneus (i.e., Cal-Mid joint) throughout the stance phase. In RFS runners, the larger inversion may play an important role in the so-called twisted osteoligamentous plate in the foot structure [25], resulting in an increased resistance to overall foot pronation during the loading phase of running, when this plate tends to untwist, thus providing the necessary protection for the tibialolar joint from high-impact forces during the stance phase [6; 26]. This larger inversion in RFS may also contribute to  
Figure 6: Sagittal-plane inclination of the first metatarsal bone to the ground (F2G), of the second metatararsal bone to the ground (S2G), and of the fifth metatararsal bone to the ground (V2G); transverse-plane divergence between first and second metatararsal bones (S2F) and between fifth and second metatararsal bones (S2V); and first metatararus and hallux angle (Met-Hal). Time was normalized across stance phase (0–100%) for forefoot strike (FFS) and rearfoot strike (RFS) patterns. Mean \(\pm\) standard error (SE) bands are reported (shaded regions). Below of each angle plot, there is a between footstrike pattern difference plot (FFS–RFS) containing the mean \(\pm\) 95% confidence interval (CI) bands (in red). Regions where those bands diverge from zero can be considered regions of significant differences. (Legend: FFS in blue, RFS in green). the conversion of the foot into a sort of rigid lever, which is necessary for effective propulsion during running [27; 28]. The calcaneus was more adducted with respect to the shank (i.e., Sha-Cal joint) in the FFS group during the whole stance phase. [PERSON] et al. [29] showed that calcaneus adduction may be beneficial to the ankle joint coupling, acting as a mechanism to control excessive tibial rotation. This was reported to be associated with patellofemoral pain and iliotibial band syndrome [30], as high relative rotation between the tibia and femur may alter the patella tracking on the distal femur [31]. The present results seem to suggest a biomechanical mechanism that might explain how a transition from RFS to FFS patterns in gait retraining could reduce patellofemoral pain in runners [32; 33] since FFS runners run with a more adducted calcaneus, thus improving ankle joint coupling and tibial rotation control.