## 2 Materials and Methods
### The Human Subjects Testing of MyFlex-\(\delta\)
#### 2.3.1 Procedure  
The functional evaluation of MyFlex-\(\delta\) for each of the participant was carried out in one day. Firstly, measurements were done with the participant using their own prosthesis, followed by the introduction and familiarization to the MyFlex-\(\delta\), and the measurements using the MyFlex-\(\delta\). The functional measurement as well as the participant experience were used to evaluate the MyFlex-\(\delta\) in comparison to their own prosthesis. Both pre-(using their own prosthesis) and post-measurements (using MyFlex-\(\delta\)) entailed the same measurement protocol. During the familiarization period, a prosthetic technician replaced the prosthetic foot of the participant with the MyFlex-\(\delta\) of the proper stiffness/weight category, with the participants wearing their own powered knee prostheses set for the characteristics and behavior of their foot prosthesis and not for Myflex-\(\delta\). The alignment reference line was aimed to be positioned 30 mm posterior to the middle of the foot, which is a common alignment recommendation for commercially available feet (Figure 3). After proper alignment, the participants were able to familiarize themselves with the MyFlex-\(\delta\) under supervision of a physiotherapist. Various activities were performed, e.g., sit-to-stand, level \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Participant 1** & **Participant 2** & **Participant 3** \\ \hline Age (years) & 63 & 46 & 61 \\ Gender & M & M & M \\ Years of amputation (years) & 13 & 14 & 10 \\ Height (cm) & 171 & 184 & 181 \\ Weight with prosthesis (kg) & 103.3 & 80.6 & 73.3 \\ Weight of own prosthesis (kg) & 3.7 & 3.1 & 4 \\ Leg length (cm) & 78 & 84 & 86 \\ Prosthetic knee & Genuin \({}^{a}\) & C-Leg 4 \({}^{a}\) & Rhee Knee \({}^{b}\) \\ Prosthetic foot & Trias \({}^{a}\) & Trias \({}^{a}\) & Proprio Foot \({}^{b}\) \\ MyFlex-\(\delta\) tested & 90, 100 kg & 70–90 kg & 60–80 kg \\ K-Level & K4 & K4 & K4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of the participants included in the MyFlex-\(\delta\) functional tests. \({}^{a}\) = [PERSON], \({}^{b}\) = [PERSON]. ground walking with (Figure 4a) and without support(Figure 4c), level-ground side walking (Figure 4b), walking on a treadmill, up- and downslope walking on a treadmill (Figure 4d), and walking on uneven terrain (Figure 4e). Introduction and familiarization with the MyFlex-\(\delta\) lasted around 4 h. Participants were able to try at least two different MyFlex-\(\delta\) stiffnesses and choose the most suitable one, according to their sensations and the physiotherapist's recommendations. The first participant, weighing 100 kg, had two prototypes of MyFlex-\(\delta\) tested, those theoretically optimized for 90 kg and 100 kg, preferring the latter. The second participant, whose weight was 80 kg, tested the MyFlex-\(\delta\)s optimized for 70 kg, 80 kg and 90 kg, choosing the one for 90 kg. Finally, the third chose the prosthesis for 70 kg after testing the MyFlex-\(\delta\)s for 60 kg, 70 kg and 80 kg. The measurement protocol consisted of gait analyses, where participants walked for 5 min on a treadmill (with force plates to measure the ground reaction force--Motekforce Link, Amsterdam/Culemborg, The Netherlands), fixed at a self-selected comfortable walking speed. To capture the participant's movements, reflective markers were placed on the participant's skin and prosthesis according to the 'Plug-In Gait Full body' marker model (Plug-in Gait Reference Guide, Vicon Motion Systems Limited, 2021). Three-dimensional movements were captured using the Vicon Nexus system (Vicon Motion Systems, Inc., Lake Forest, CA, USA, 100 Hz). In addition, participants wore a harness for safety (Figure 5). Throughout the whole day, participants were asked about their experience using the MyFlex-\(\delta\).