## 4 Discussion  
This paper aims to introduce a novel tool to help ergonomists in ergonomic risk assessments by automatically extracting angles from video acquisition, in a quicker way than the traditional one. Its overall systematic reliability and numerical accuracy are assessed by comparing the tool's performance in ergonomics evaluation with the one obtained by standard procedures, representing the gold standard in the context. Results suggest that it generally provides a good consistency in predicting the angles from the front camera and a slightly less accuracy with the lateral one, with a broader variability than the Vicon. However, in most cases, the average and median values are relatively close to the reference one. This apparent limitation should be analyzed in the light of the setup needed to obtain this data; in fact, by using only two cameras (instead of the nine ones the Vicon needs), we obtained reliable angles proper to compute a RULA score. Although it has greater accuracy than the proposed tool, the Vicon requires installing a vast number of cameras (at least six), precisely positioned in the space, to completely cover the work area and ensure the absence of occlusions. In addition, such a system requires calibration and forces workers to wear markers in precise positions. However, when \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**RULA RMSE (+SD)**} \\ \cline{2-5}  & \multicolumn{2}{c}{**RGB-MAS vs. Manual**} & \multicolumn{2}{c}{**Vicon vs. Manual**} \\ \cline{2-5}  & **Left** & **Right** & **Left** & **Right** \\ \hline T-Pose & 0.00 (0.58) & 1.00 (0.75) & 0.41 (0.37) & 0.41 (0.37) \\ Relaxed & 0.71 (0.00) & 2.45 (0.37) & 0.82 (0.37) & 0.82 (0.37) \\ Sit & 0.58 (0.75) & 1.41 (0.76) & 0.71 (0.69) & 0.82(0.58) \\ Reach & 1.35 (1.07) & 1.35 (0.82) & 1.78 (0.76) & 1.78 (0.76) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: RMSE (+SD) values obtained when respectively comparing RGB-MAS and the Vicon RULA with the manual one. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Median**} \\ \cline{2-7}  & \multicolumn{2}{c}{**Manual**} & \multicolumn{2}{c}{**RGB-MAS**} & \multicolumn{2}{c}{**Vicon**} \\ \cline{2-7}  & **Left** & **Right** & **Left** & **Right** & **Left** & **Right** \\ \hline T-Pose & 3.00 & 3.00 & 3.00 & 3.00 & 3.00 & 3.00 \\ Relaxed & 2.50 & 2.50 & 3.00 & 3.00 & 3.00 & 3.00 \\ Sit & 4.00 & 4.00 & 4.00 & 4.00 & 4.00 & 4.00 \\ Reach & 4.50 & 4.50 & 4.00 & 4.00 & 3.00 & 3.00 \\ Pickup & 5.50 & 5.50 & 6.00 & 6.00 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: RULA median scores for the three angle extraction methods and corresponding level of MSD risk (i.e., green = negligible risk; yellow = low risk; orange = medium risk). performing a manual ergonomic risk assessment in a real working environment, given the constraints typically presented, an ergonomist usually can collect videos from one or two cameras at most: the proposed RGB-MAS copes with this aspect providing predicted angles even from the blind side of the subject (like a human could do, but quicker), or when the subject results partially occluded. As proof of this, it is worth noticing that the pickup posture, initially considered for its tendency to introduce occlusion, was then discarded from the comparison just for the occlusion leading to the lack of data from the Vicon system, while no problems seemed to arise with the RGB-MAS. In addition, the RMSE values obtained comparing the RGB-MAS RULA scores with the manual one showed tighter variability than the same values resulting from the comparison between the RULA scores estimated through the Vicon and the manual analysis. This suggests that the RGB-MAS can be helpful to fruitfully support ergonomists to estimate the RULA score on a first exploratory evaluation. The proposed system can extract angles with a numerical accuracy comparable to one of the reference systems, at least in a controlled environment such as a laboratory.