## 5 Usability Assessment Results  
At the beginning of the test, the users were trained in the overall procedure (how to wear the trackers, the myoelectric sensors, and the head-mounted display, and how to \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Task** & **Average Time [s]** & **Standard Deviation [s]** & **Level of Difficulty** \\ \hline Bottle & 8.4 & 2.4 & Level 1 \\ Ball & 7.2 & 3.2 & Level 1 \\ Plate & 6.8 & 2.4 & Level 1 \\ Handle & 6.0 & 4.0 & Level 2 \\ Slider & 19.2 & 4.0 & Level 2 \\ Buttons & 17.6 & 4.0 & Level 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Average and standard deviation time for the six tasks of VR-SHAP. Figure 11: An example of the second level of the VR-SHAP test: the rotation of a handle. The user locates the handle, grabs it, and then rotates it of 45\({}^{\circ}\). Figure 12: An example of the third level of the VR-SHAP test: the manipulation of the buttons. The user locates the buttons, grabs one at time, and then releases them inside a box. manage the virtual reality environment), both by means of oral instructions and graphical guidelines. Then, the users were asked to repeat the overall procedure five times, so as to gain in familiarity with the system. A preliminary usability analysis of the developed virtual environment was performed by means of a questionnaire collecting the degree of impact and the difficulties in understanding the guidelines, setup procedure, and scene interaction (also in terms of real-time aspects, excluding the classification performances), statistically executed on thirty-six users, evenly gender distributed, and 20-50 years of age (mean value 30 years, standard deviation 8.5 years). These users were asked to assign a score of 0 to 10 (from very bad/difficult to very good/easy condition). According to the results reported in Figure 13, the usability analysis reports a relatively high level of scores, testifying to an adequate implementation of the DT simulative model and communication interfaces. Moreover, users were asked to execute the VR-SHAP tasks following the procedure described in the previous section. The results in terms of the average time and standard deviation are reported in Table 1. These values are then used for evaluating the VR-SHAP test according to Equations (14) and (15). At the end of all of the tests, the users were asked to rate the level of difficulty encountered in performing the tasks and provide reasons for their ratings. As we could expect (Figure 14), the first-level tasks show a high easiness rating, due to there being fewer constraints for the implementation of the OAF method. Despite the higher difficulty of the third-level tasks (due to the reduced size of the objects to grasp), the easiness rating was higher than that of the second-level tasks. We argue that this can be related to the reduced number of degrees of freedom of the virtual prosthesis wrist which makes the reaching of large rotations more difficult. To score the clarity of guidelines and the general impact with virtual reality (related to the sensations experienced throughout the immersion in the virtual environment), users were asked to rate the (dis)comfort of their experience. According to the results (Figure 15), the guidelines were considered very clear (the application being as independent as possible from the operator), with the sickness directly correlated with the users' age. In general, the immersive experience was well tolerated by all the users without excessive discomfort. Figure 13: Usability analysis results about arm tracking (left) and ease in sensors’ positioning (right). Both the fidelity in tracking and the ease in wearing sensors scored high values, confirming the very good usability. Regarding the setup, a higher difficulty was reported in the positioning of the second tracker (that may interfere with the myoelectric sensors' mounting).