## 4 General Discussion
Those results can probably be attributed to a suppression of sensory signals during unhindered running in order to avoid potential disturbances of the optimized spinal program controlling running motor pattern [71; 72]. In fact, studies have shown that locomotion without disturbances or obstacles is more independent of cortical control, the latter being mainly required when exact foot placement is needed, or when the system must react to external perturbations, notably by changing muscle activity to modify limb trajectories to step over obstacles [73]. This would also explain the lower impact of visual incongrurences on balance and locomotor patterns while treadmill running, as shown in Experiment 2. Our results contradict previous studies on walking, which showed an influence of an artificial optical flow on postural stability [31; 32; 33; 34] and on locomotor patterns. In particular, [PERSON] et al. [17] found that when walking overground, stride length decreases in presence of an artificial optical flow. [PERSON] et al. [15] found that when walking on a treadmill at constant speed, stride-cycle variability increases and the optical flow significantly modulates walking velocity. [PERSON] et al. [36] found that when walking at pre-determined speeds on a treadmill in VR, step width increases and step length decreases, whereas the variability of stride velocity and step width increases. All these parameters are known to characterize gait instability [51; 52], suggesting that the presence of VR during treadmill locomotion may lead to a more instable gait. In fact, a shorter step length and an increased step width are associated with a more conservative gait pattern, often adopted in case of instability or fear of falling. Other studies show that this influence on locomotor patterns extends also to the walk-run and run-walk transition, leading to changes in the transition speed based on the speed of the optical flow [38; 74]. Our results show that this influence on locomotor patterns cannot be extended to running individuals, and that for running at a self-selected speed, the presence and manipulation of the optical flow has no stabilizing or destabilizing effect. This could also be due to the fact that when visual cues are in conflict with other sensory information about self-motion, which is the case when manipulating optical flow speed without having the possibility to vary locomotor speed, visual information ceases to be dominant compared to other types of information [4; 14]. In fact, studies show that when participants are asked to walk at a constant speed, they tend to weight information from proprioceptive cues higher than cues derived from the visual system [15; 16; 17]. Our results also indicate that the simulated optical flow does not influence the process of accommodation to treadmill running, because no differences were found between the four optical flow conditions. This accommodation process has been shown to vary between individuals [53]. Healthy adults devoid of experience with treadmill running can familiarize themselves with the task after 6 min in a single session, and angular kinematics as well as spatiotemporal parameters are highly reliable after 2 min [75]. [PERSON] et al. [76] found that there was no significant difference in vertical force after 30 s of treadmill running. The differences we found between minute 1 and 3 could be linked to the new accommodation required after each pause in between the blocks. This would be in line with the fact that previous studies have shown that minimal amounts of treadmill training are necessary before full accommodation to treadmill locomotion [77]. Surprisingly, our results suggest that the presentation of an artificial optical flow can influence HR. Specifically, the results of Experiment 2 show a significant difference between the HR monitored in the noOF condition and the HR monitored in the VR conditions.