## 4 Discussion
These differences provide a quantification of the credibility of each joint model: differences between M1 and M2 should in fact be negligible for a behavior consistent with joint anatomy. The results of the analysis presented in this paper show that similar joint models may lead to similar MSK output, and yet differ considerably in terms of estimate of the deformation of the articular tissues. The evaluation of anatomical consistency can thus be used to discriminate among different joint models and ultimately choosing the most suitable level of personalization of the kinematic chain within MSK models. Our results suggest that scaling of a generic KC provides acceptable results for the general characterization of a motor task, the pattern and the amplitude of all relevant quantities being indeed comparable with the other two models here considered. If a specific cohort has to be investigated and the analysis is extended to joint reactions, morphological fitting appears as a more reliable method, offering also a good compromise between accuracy and easiness in model definition. However, if a characterization of the single individual and its articular structures is needed, a level of personalization similar to the one obtained by congruence maximization is strongly recommended. Anatomical consistency may also help to extend the knowledge of joint models' sensitivity. SGJ and MFJ are both simple hinges, differing only for the location of the revolute axis with respect to the patient articulation. As previously reported in the literature, MSK models are quite insensitive to small variation in the definition of axis location of hinge joints [11]. Indeed, the two models led to similar joint kinematics (Figure S2), resulting in similar muscle lever arm and therefore in similar muscle forces. However, we showed that small differences in relative bone displacements may give rise to considerable ligament and cartilage contact forces: if these forces are considered all model outputs change considerably. This implies that, when investigating the articular response to load, a high kinematic accuracy needs to be achieved through an adequate model personalization. This may have important consequences for the design of prostheses and other medical devices, often relying on numerical estimation of joint ligament and contact forces. A highly anatomically consistent joint model would guarantee more credible and accurate MSK outputs, possibly opening the way to a full customization of the implant to the patient needs. In this perspective, MRI based personalization provided the best results in terms of anatomical consistency, showing the crucial role of an accurate representation of the patient articular surfaces. The role of rigid joints within MSK multibody approach deserves to be discussed. Real articulations are non-rigid and an accurate investigation of internal forces requires detailed models of ligaments and cartilage deformation under loads. These models are however computationally demanding. In addition, the number of parameters that must be determined and tuned grows with the model complexity. On the other hand, rigid joint models may provide accurate estimate when properly defined. [PERSON] and co-workers [36] showed indeed that a simple hinge predicted knee joint contact forces with the same fidelity of a 6-dof model with deformable ligaments for a prosthetic knee. Moreover, rigid body models can be used as preliminary step to inform deformable models, simplifying their definition and application. For example, the maximum congruence approach here used for the MCJ models may provide individual joint kinematics, as well as insertion and reference length for the isometric fibers, which are needed for investigation of the ligaments [37]. The same data can be used to design more complex joints such as rigid mechanism [38; 39], whose constraints can eventually be released to cope with deformable joints [40; 41]. This work has its limitations.