## 4 Discussion  
The primary findings of this study were that (i) thoracic kinematics, contact pressure perception and performance changed across the familiarization protocol, and (ii) thoracic kinematics, contact pressure perception and performance reached a steady state after the third or fourth session of the familiarization protocol. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & **PRE-1** & **POST-1** & **POST-2** & **POST-3** & **POST-4** & **POST-5** & **POST-6** \\ \hline \multirow{3}{*}{Armpits} & STOOP * & 1.5 & 1 & 2 & 1.5 & 1.5 & 1 & 1 \\  & & (3.75) & (3.75) & (3) & (2.75) & (2) & (1.75) & (2.75) \\  & SQUAT & 1 & 1 & 1.5 & 1 & 1 & 1 & 1 \\  & & (4) & (3.25) & (2.75) & (1.75) & (1.75) & (1.75) & (2.5) \\ \hline \multirow{3}{*}{Trapezius} & STOOP * & 2 & 2 & 2 & 1.5 & 1.5 & 2 & 1 \\  & & (3.75) & (3.75) & (2) & (2) & (1) & (1) & (1) \\  & SQUAT & 2 & 2.5 & 2 & 1 & 2 & 2 & 1 \\  & & (2) & (3) & (1.75) & (2) & (1.75) & (1) & (1) \\ \hline \multirow{3}{*}{Back} & STOOP & 2 & 2 & 2.5 & 2 & 2 & 2 & 2 \\  & & (1) & (1) & (1.75) & (2) & (0.75) & (1.75) & (1.5) \\ \cline{1-1}  & & 1.5 & 2 & 1 & 1 & 1 & 1 & 1 \\ \cline{1-1}  & & (1) & (2) & (1) & (1) & (1) & (1) & (1) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Median (interquartile range) of armpits and trapezius pressure in STOOP and SQUAT in PRE-CON and over the familiarization protocol from PRE-1 to POST-6. * Statistical main effect of the familiarization protocol. Figure 6: Evolution of mean (SD) performance time in PRE-CON and over the familiarization protocol from PRE-1 to POST-6 for PRECI. * Significantly different from POST-1 to POST-6. \({}^{\circ}\) Significantly higher than POST-4 to POST-6. \(\Delta\) Significantly higher than POST-5 and POST-6. Back curvature, pressure perceptions, and performance changed over the familiarization protocol with the soft back-support exoskeleton prior to reaching a steady state. Indeed, thoracic flexion stabilized after session 3 in both STOOP and SQUAT. This might suggest that thoracic kinematics is changing to adapt to the exoskeleton constraints provided by the elastic bands along the back and buttocks and attached to the shoulders and to the knees. In addition, pressure perceptions in STOOP and SQUAT at knee and buttocks levels were also influenced by the familiarization protocol and reached a steady state after sessions 3 or 4, depending on the task and the pressure zone. Armpits and trapezius perceived pressures were also influenced by the familiarization protocol. These changes in perceived pressure of the exoskeleton might be a sign that participants felt increased comfort after several uses, which might speak in favor of a better acceptability of the exoskeleton. Interestingly, pressure regions that were most influenced by the familiarization protocol referred to the lower limb (buttocks and knees), while kinematic variations were located at the thoracic spine. Despite the variety of measurements, it appears that there was no evolution after session 4 for any variable. The performance time measured with PRECI stabilized after session 4 compared to their first use of the exoskeleton, suggesting that participants get more accurate after they get familiar with the exoskeleton. Overall, this suggests that a soft back-support exoskeleton familiarization model is characterized by a change in motor behavior, followed by its stabilization after the third or fourth familiarization session. Such changes, followed by a stabilization, have already been observed in the literature by [PERSON] et al. (2013) [14] and [PERSON] et al. (2019) [13] with walking exoskeletons. As a comparison, [PERSON] et al. (2019) [13] found that 3 sessions of 20 min induced a stabilization of metabolic cost changes when using a hip exoskeleton during walking, while [PERSON] et al. (2013) [14] found that metabolic cost stabilized after the first 18.5 min of walking with an ankle exoskeleton. Thus, the type of exoskeleton (back, hip, or ankle) and tasks (lifting or walking) appear to influence the familiarization time.