## 4 Discussion
However, this speed effect is higher along the mediolateral axis (+85%) compared to the anteroposterior axis (+70%). These increases in range occur while phase durations decrease, with a mean value of 29%. It therefore corresponds to larger dynamics variations in shorter times. Along the anteroposterior axis, the largest variations of the distances are observed during the second swing phase P4, while the largest variations along the mediolateral axis occur during the double-stance phase P3. This result corroborates previous studies which revealed the large lateral displacement of the COM during this phase [39]. Finally, the analysis of the restabilization phase (phase P5) showed that the subjects anticipated the end of the stepping. Indeed, in this phase, the variations of the anteroposterior and mediolateral distances reach the lowest values until the end of the motion. However, in this phase, there is no decrease in duration with the speed condition. For the entire stepping, the differences between the maximal and the minimal values of the distances confirm our hypothesis about the speed effect, while a group effect is observed only along the anteroposterior direction. Moreover, these larger variations of the distance are globally realized with a shorter time (\(-13\%\)) at fast speed leading to greater dynamics variables of the subject and potentially higher instability. While our results demonstrate higher instability during the swing phases, it is interesting to note that these swing phases represent the largest part of the stepping time. Indeed, at spontaneous speed, the swing phase corresponds to approximatively 60% of the step composed of one swing phase and one double-stance phase. This result is reinforced at a fast speed, where the most unstable phase represents 70% of the step time without any group effect. Thus, it is an interesting result to note that the most unstable phases represent larger time percentage during the fast speed condition. Moreover, as mentioned above, our results show that age affects the duration of APAs and the restabilization phase, often referred to as \"corrective postural adjustments (CPA)\". Specifically, older adults exhibit a shorter APAs duration and, conversely, a longer restabilization duration compared to their young counterparts. It is an interesting result meaning that older adults are able to prepare their step more quickly, while they take longer to restabilize compared to young adults. These results are in line with the findings of the literature, which reveal that age modifies postural control strategies, with greater reliance on reactive versus anticipatory control mechanisms with aging [44]. This shift from anticipatory to reactive control with aging is thought to predispose the elderly to greater instability, making it more difficult for them to maintain balance during voluntary motor tasks or after an external perturbation [44]. This is corroborated by the present results, which show that the distances between COM and MMA increase with age. Nevertheless, it is important to note that some studies have reported an increase in APAs duration in people at high risk of falling, such as post-stroke people [45], compared to able-bodied adults. Taken together, these results suggest that the duration of APAs is a parameter that should be interpreted with caution, in particular when it is used to assess the instability of patients and therefore the risk of falls. In a clinical context, it therefore appears that the restabilization duration, or APC duration, as well as the distances between the COM and the MMA could be better criteria for quantifying instability. However, it remains evident that the restabilization time could not be used to monitor immediate fall risks, while the real-time analysis of the distances between the COM and the MMA could help to prevent falls if some thresholds can be established in the future.