## 1 Introduction
[15] used this parameter to highlight the greater instability of locomotion in subjects with a transfemoral prosthesis. Gait initiation and stepping, which consist of a voluntary transition from a quiet standing position to a dynamic phase, are known to be a particularly critical phase of locomotion. Observations revealed that most of the falls in older adults occurred during these transitional tasks [4]. In particular, research has shown that anticipatory postural adjustments performed during the double support phase are crucial to guarantee and prepare the correct execution of the first step [16]. Indeed, at the time of the takeoff of the foot, a mediolateral gap is created between the COP located under the foot of the stance leg and the projection of the COM. This natural gap induces a fall of the body toward the swing leg side, which stops at the time of swing foot contact [17]. The authors showed, during the anticipatory postural adjustments, an initial shift in the COP laterally towards the swing leg that accelerates the COM toward the contralateral leg in order to decrease the amplitude of the lateral fall [17; 18]. Furthermore, they observed a forward displacement of the COM induced by the backward shift in the COP. The forward imbalance created by the anteroposterior gap between the COM and the COP, during the double support phase contributes to the generation of propulsive forces [19; 20]. Segmental movements resulting from these anteroposterior and lateral movements generate variation in angular momentum, which helps to initiate and continue the gait. In this context, some authors recently studied instability during stepping. Using the COP trajectory, [PERSON] and [PERSON] [21] demonstrated that action costs for maintaining postural stability are considered dominantly for planning the stepping. The margin of stability [22] was also analyzed during stepping in different situations, and the higher risk of falls in frail individuals such as older adults were highlighted. [PERSON] et al. [23] also studied the stepping instability in older adults using foot placement error, step duration and mediolateral center of pressure path. The results showed that compared to young adults, older adults exhibited a higher foot placement error, suggesting a reduction in the ability to control balance with age. However, while foot placement error increased with step execution velocity, the authors did not find an age-related difference at higher speeds. On the contrary, age-related changes in angular momentum were found in other studies [24; 25]. Compared to young adults, older adults exhibited higher values of angular momentum during stepping, emphasizing greater difficulties in the balance control during this task. It is noteworthy that these age-related changes were more pronounced in the sagittal plane. Furthermore, although both age groups increased angular momentum with increasing speed during stepping, the age-related changes previously observed at spontaneous speed were exacerbated at faster speeds [21]. Taken together, these results suggest that the greater variation in angular momentum with age and speed generates larger values of angular momentum derivatives and consequently contributes to increasing the distance between the COM and the MMA. However, to the best of our knowledge, no study has investigated instability during stepping using the distance between the COM and the MMA. This study aimed, therefore, to investigate the distance between the MMA and the COM in young and older adults at different speeds during stepping. We hypothesized that, due to the greater variation in sagittal angular momentum in older adults, the derivative of the angular momentum should increase. As a consequence, the anteroposterior distance between the MMA and the COM should also increase. Similarly, we hypothesized that increasing speed should generate a greater distance between the MMA and the COM due to a greater variation in angular momentum.