## 4 Discussion  
To analyze the mechanism behind how humans fall during dynamic activities, it is relevant to find a way to quantify dynamic instability. The goal of this study is to determine whether the descriptor mentioned above may be a potential measure of dynamic instability. To address the matter, results in Table 2 show that the aforementioned descriptive parameters calculated on healthy subjects are statistically different than those calculated on subjects with amputation. The only exception is the average distance. Indeed, the average distance for people with amputation is close to the average distance of healthy subjects. This means that (1) if the average distance is a measure of instability, then the instability in amputee's gait is similar to that in healthy subject's gait; (2) however, other extracted parameters from distance curve were able to differentiate between subjects with amputation gait and healthy subject's gait. Considering healthy subjects, the profile of the distance is similar to the one found in the paper of [PERSON] et al. (Figure 3). The MMA is farthest away behind the subject's COM at the end of the swing phase (Figure 3: point A). At this time, the MMA is inclined toward the subject's COM. This contributes to a decrease in the maximum distance. Regarding the anterior-posterior distance, its profile is similar to the profile of the dimensionless spin along the mediolateral axis of the subject obtained in the paper of [PERSON] et al. During the swing phase, the subject's angular momentum like the anterior-posterior distance is negative and its magnitude is increasing. This gives the subject a tendency to rotate forward which is useful to the forward progression of the body. At heel strike, we can notice a drop in the distance. Thus, the MMA moves toward the COM and crosses it. It is now in front of the body (Figure 3: point B). However, the anterior-posterior distance at this point is much lower than at the end of the swing phase. Similarly, the positive angular momentum is much lower than the negative angular momentum through the gait cycle. This means that, while walking, the tendency to fall forward is much higher than the tendency to fall backward. One can note that the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{**Parameter**} & **A1** & **A2** & **A3** & **A4** & **A5** &
\begin{tabular}{c} **Healthy** \\ **(n = 14)** \\ \end{tabular} & \multicolumn{1}{c}{_p-Value**} \\ \hline \(\widehat{d_{\text{COM-A}}}\) & 47 & \(41\pm 2\) & \(38\pm 1\) & \(45\pm 3\) & 33 & \(37\pm 4.5\) & \textgreater{}0.05 \\ \hline \(\text{max}(d_{\text{COM-A}})\) before HS of SL (mm) & 124 & \(111\pm 5\) & \(108\pm 7\) & \(135\pm 3\) & 96 & \textless{}0.05 \\ \(\text{max}(d_{\text{COM-A}})\) before HS of PL (mm) & 54 & \(56\pm 3\) & \(62\pm 8\) & \(53\pm 0.0\) & 30 & \(79\pm 10\) & \textless{}0.05 \\ \hline \multicolumn{1}{c}{_Drop \(d_{\text{COM-A}}\) at HS of SL (mm)_} & 93 & \(106\pm 5\) & \(92\pm 8\) & \(129\pm 9\) & 84 & \textless{}0.05 \\ \multicolumn{1}{c}{_Drop \(d_{\text{COM-A}}\) at HS of PL (mm)_} & 41 & \(38\pm 18\) & \(50\pm 1\) & \(43\pm 2\) & 24 & \textless{}0.05 \\ \hline \(d_{AP}\) at HS of SL (mm) & 111 & \(107\pm 5\) & \(101\pm 5\) & \(124\pm 6\) & 90 & \textless{}0.05 \\ \(d_{AP}\) at HS of PL (mm) & 52 & \(53\pm 7\) & \(62\pm 9\) & \(53\pm 0.0\) & 29 & \textless{}0.05 \\ \hline \(d_{AP}\) at TO of SL (mm) & 40 & \(55\pm 3\) & \(37\pm 7\) & \(57\pm 1\) & 43 & \textless{}0.05 \\ \(d_{AP}\) at TO of PL (mm) & 1 & \(6\pm 9\) & \(7\pm 3\) & \(-1\pm 0.0\) & 19 & \textless{}0.05 \\ \hline Gait speed (m/s) & 1.13 & \(1.27\pm 0.0\) & \(1.14\pm 0.0\) & \(1.3\pm 0.1\) & 1.33 & \(1\pm 0.2\) & \textless{}0.05 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Calculated parameters among amputees (A1, A2, A3, and A4) and healthy subjects. HS: Heel Strike, TO: Toe Off, SL: Sound limb, PL: Prosthetic limb.