###### Abstract  
While multiple criteria to quantify gait instability exist, some limitations hinder their computation during realistic walking conditions. A descriptor, computed as the distance between the center of mass of the body and the minimal moment axis (\(d_{\text{COM-\Delta}}\)), has been proposed recently. This present study aims at characterizing the behavior of the mentioned descriptor in a population at a higher risk of falls. Five individuals with transfemoral amputation and 14 healthy individuals were involved in an experiment composed of motion capture and force plates acquisition during overground walking at a self-selected speed. For both groups of participants, the profile of \(d_{\text{COM-\Delta}}\) was analyzed and descriptive parameters were calculated. The plot of \(d_{\text{COM-\Delta}}\) was different between groups and different relative to the leading limb considered (prosthetic or contralateral). All descriptive parameters calculated, except one, were statistically different between groups. As a conclusion, amputees seem to be able to limit the average of \(d_{\text{COM-\Delta}}\) in spite of a different evolution pattern. This is consistent with the ability of the subjects to maintain their dynamic balance. However, the extracted parameters showed the significant asymmetry of the gait profile between prosthetic and contralateral stances and highlighted the potential sources of imbalance. dynamic balance; minimal moment axis; transfemoral amputation; biomechanics; gait +
Footnote †: journal: Applied Soft  
support polygon and ZMP are not adapted to multi-contact locomotion or on uneven ground. Thus, a generalized stability criterion is yet to be established [5]. Several attempts have been done to generalize the concept of ZMP and predict falling risks. In this perspective, [PERSON] et al. [6] have defined a point called the extrapolated center of mass (XCOM). It considers the position of the COM together with its velocity and should also fall within the support polygon to achieve a stable gait. However, some cases exist where this point is outside the support polygon and no falling is observed. When considering gait on uneven terrain, a new difficulty appears regarding the definition of the support polygon. [PERSON] et al. have described a 3-dimensional convex hull built from two non-coplanar real support areas. The gait is said to be in balance if the ZMP falls inside a certain projection of this 3D convex hull [7]. Similarly, [PERSON] et al. have worked on defining a virtual surface and a virtual ZMP [8]. In contrast to the above-mentioned approaches which focus on monitoring a point with respect to the support polygon, [PERSON] et al. have approached the subject by studying the angular momentum during gait. They assumed that the minimization of the angular momentum at the COM is an aim of the nervous system [9]. Finally, [PERSON] et al. have modeled gait with a virtual pendulum model and have defined a virtual pivot point (VPP) where the resultant ground reaction forces intersect [10]. Additional research has been done by [PERSON] et al. to interpret whether it is an objective of the nervous system to make forces intersect at the VPP [11]. Recently, [PERSON] et al. proposed a new mechanical descriptor [5] based on an approach related to ground reaction forces and moments (GRF&M), and COM position. GRF&M can be represented by an external contact wrench (ECW). The central axis of the ECW, also called the minimum moment axis (MMA), is the axis along which the Euclidean norm of the moment is minimal and collinear to the resultant force [12]. The distance between the COM and the MMA was suggested as a locomotion descriptor. The effectiveness of this descriptor lies in its ability to describe the derivate of the angular momentum at the COM throughout the phases of the gait cycle, and in its capacity to be computed for generalized locomotion (uneven surfaces or multi contacts). Considering the derivate of the angular momentum at COM, as mentioned above, [PERSON] et al.