## 2 Methods
### Experimental Task and the Parametric Prototype Handle  
The design and development of the handle shown in Figure 2 was based on a study of the ergonomic preferences of 135 surgeons during their period of training in laparoscopic techniques in the CCMIJU. Two of the handle design criteria were a large palmar grip surface and the combination of precision and turning ability. These criteria eliminated the possibility of a handle design with rings. The resulting handle design has been studied and further analyzed in the work of [PERSON] et al. [18]. In that study, a first approach was to define a grasper handle size that would best suit the surgeon's hand size, but without attempting to customize the design as this is the purpose of the paper at hand, considering designs based on four hand sizes (XS, S, M, and L). The results of that work are summarized in Table 2. It should be noted that, in addition to this study of [PERSON] et al. [18], other workers have proposed different sizes for surgical handles and hand tools, specifically by defining three sizes [5; 16; 39; 40; 41]. Nonetheless, given the precision required in surgery and the many hours that some operations last [36], together with the technological means available today such as additive manufacturing and the development of 3D parametric design software [42], a personalized solution would seem to be called for, with the handle being designed to have the size ergonomically best suited to the anthropometric dimensions of each surgeon's hand. Recent studies, such as that of [PERSON] et al. [43], show that the use of 3D printing can bring substantial benefits, such as customization, optimization and the manufacture of very complex geometries. The final design of the handle used in this study (Figure 2) allowed us to resolve a series of ergonomic problems that existed with other types of MIS handles which did not meet the design criteria obtained in ERGLAP. In this sense, it should be noted that, since the entire palm of the hand is now involved in gripping the handle, it is easier to achieve a neutral position of the wrist. This reduces fatigue and the risk of thenar lesions. Hence, the grip was designed in such a way that the pressure is distributed evenly between the palm and the fingers. Allowing greater contact of the handle with the whole hand avoids flexing the wrist, stretching tendons, distending the elbow, and limiting \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Hand Size** & **\%** & **Sex** & **Mean Diameter (mm)** \\ \hline \hline XS & 26\% & Both sexes & 29.4 \\ \hline S & 11.1\% & M & 32.9 \\  & 18.5\% & F & 31.7 \\ \hline M & 24.4\% & M & 36.4 \\  & 6.7\% & F & 33.1 \\ \hline L & 13.3\% & Both sexes & 37.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Optimal for each of the hand size categories. Figure 2: The laparoscopic tool handle developed in the ERGLAP project. shoulder movement. The length of the handle and its diameter were increased relative to the handles traditionally used in MIS, as this provides a better fit for different hand sizes, and the fingers have a larger contact surface thus reducing the pressure on localized areas, especially the thumb. Regarding the geometry of the handle design developed in the ERGOLAP project (Figure 3a), there are three significant parameters. These are identified as the \"inferior diameter\" (D\({}_{\text{I}}\)), \"median diameter\" (D\({}_{\text{M}}\)), and \"superior diameter\" (D\({}_{\text{S}}\)). The median diameter (D\({}_{\text{M}}\)) coincides with the diameter defined at the point of inflection of the curvature of the handle (Figure 3a). It also defines the position of the hand relative to the handle during grasping, since for all the surgeons this diameter coincides with the line defined between the third and fourth (Figure 3b). Similarly, D\({}_{\text{S}}\) and D\({}_{\text{I}}\) correspond to the narrowest and the broadest parts of the handle design, respectively.