## 4 Discussion  
The aim of the present study was to investigate the distance between the MMA and the COM in young and older adults at different speeds during stepping. Consistent with our hypotheses, the results revealed that (1) older adults exhibited higher anteroposterior distances than their younger counterparts and (2) both anteroposterior and mediolateral distances increased with increasing speed during stepping. Our results also provide an answer to our second objective regarding the ability of the distances to discriminate the most unstable phases of the stepping. These results are further discussed in the following paragraphs. Regarding the spatiotemporal results, it is interesting to note that the forward speed of the stepping was not different between both groups at spontaneous speed, but it was slower in older adults than their young counterparts in the fast speed condition. This result is in line with the literature, which has already shown that there were no significant differences in spontaneous walking speeds up to the age of 80 [36]. Thus, it appears justified to compare the dynamics of both groups throughout the entire motion. Although there is no group effect for the total duration, the results highlight a significant group effect for the duration of the phases P1 (APAs) and P5 (retabilization phase). Compared to young adults, older adults decreased the duration of the first phase (\(p=0.003\)) and, on the contrary, increased the duration of the restabilization phase (\(p=0.003\)). This result confirms previous findings of the literature [37], which showed that the time required to stop the locomotion was longer in older adults. Conversely, one can note that speed effect is observed for all the spatiotemporal parameters, meaning that the volunteers correctly followed the instructions accelerating the stepping during the fast speed condition. The time evolution of the anteroposterior distance observed during stepping describes a profile consistent with that presented by [PERSON] et al. [15] for healthy people. The minimal distance (i.e., the largest negative distance) is observed at the end of the single support phase of the first step (i.e., the swing phase of the dominant leg\(-\)phase P2). This result is consistent with the fact that during the swing phase, the base of support is reduced, thus increasing the difficulty to control the instability. In addition, just before the contralateral heel strike (i.e., during the swing phase of the non-dominant leg\(-\)phase P4), the significant shifts in the COP and the COM, which progress between the two supporting feet during the step transition, further increase instability. In the context of stepping, a lower negative anteroposterior distance is observed during the second swing phase (P4). This result is probably related with the beginning of the deceleration phase and the decrease in the dynamics of the participants. Indeed, participants were instructed to stop just after phase P4 at phase P5. The negative values observed during P2 and P4 indicate that the MMA is behind the center of mass. Knowing that the external vertical force acts upwards, this means that, according to Equation (13), the derivative of the angular momentum is negative and generates global forward angular acceleration of the body. This angular acceleration is stopped at the time of the heel strike, which marks the onset of the double support phase. Concerning the maximum values of the anteroposterior distance, they are observed at the beginning of the swing phases (P2 and P4). At this time, the MMA shifts beyond