## 2 Materials and Methods  
A previously validated musculoskeletal model of a TKA patient was used in this study [17]. In brief, the model was built upon the Twente Lower Extremity Model 2.0 (TLEM 2.0) template [18] and integrated patient-specific data available from the fifth \"Grand Challenge Competition to Predict In Vivo Knee Loads\" dataset [19]. This lower-extremity model of the implanted side comprises 11 degrees of freedom in the TF and PF joints. Flexion and extension angle in the TF joint is motion capture driven, whereas the kinematics of the remaining degrees of freedom are solved quasi-statically, using the force-dependent kinematics (FDK) approach, as proposed by [PERSON] et al. [20]. This methodology enables the concurrent estimation of muscle, ligament, and joint contact forces. For this study, we introduced some changes with respect to the existing patient-specific model, using the AnyBody Modeling System v. 7.3.1 (AnyBody Technology A/S, Aalborg, Denmark); software for modeling and simulation analysis of the musculoskeletal apparatus [21]. The patellar tendon was modeled as three non-linear spring elements instead of a single rigid element connecting the patella and tibial tuberosity. Stiffness of the patellar tendon was determined by multiplying the median cross-sectional area from the patient-specific computed tomography (CT) with a modulus of elasticity reported in the literature [22]; likewise, stiffness and reference strain of the TF and PF ligament bundles were adopted from the existing musculoskeletal model configuration. In addition, we configured the model to incorporate a size 6 Triathlon (Stryker, Kalamazoo, MI, USA) cruciate-retaining implant (Figure 1), ensuring that the femoral anteroposterior anatomical dimension prevents anterior notching of the femoral cortex. This implant was chosen as it has shown good functional performance and ligamentous stability [23]. The femoral and tibial component geometry was positioned in such a way as to match tangentially the patient-specific distal femoral and proximal tibial bone cuts, applying the well-established mechanical alignment technique [24]. The patella was not resurfaced. Since the cartilage underside of the patella could not be determined from the patient-specific dataset, we generated a 3 mm offset on the dorsal facet of the patellar bone according to the mean patellar cartilage thickness estimation of [PERSON] et al. [25]. In this model, PE insert thickness was varied with 2 mm increments or decrements (\(-\)4 mm, \(-\)2 mm, \(+\)2 mm, \(+\)4 mm) compared to the reference case (9 mm), testing five thickness cases in total: 5 mm, 7 mm, 9 mm, 11 mm, and 13 mm (Figure 2). The effect of PE insert thickness was evaluated by simulating a chair-rising activity based on motion capture data available as part of the open access grand challenge competition dataset. Figure 1: Illustration of the patient-specific musculoskeletal force-dependent kinematics (FDK) knee model implanted with a Triathlon cruciate-retaining prosthesis. This full-body model includes the head, trunk, pelvis, and the lower extremity of the implanted side (thigh, shank, patella, talus, foot) (left). A close-up anterior view of the knee joint with the medial collateral ligament (MCL), lateral collateral ligament (LCL), posterior cruciate ligament (PCL), medial patellofemoral ligament (MPFL), lateral patellofemoral ligament (LPFL), and patellar tendon (PT) (right). To allow for an investigation of the model kinetic predictions on the TF joint, the primary outcome measures of the simulations included the medial collateral ligament (MCL), lateral collateral ligament (LCL), and posterior cruciate ligament (PCL) forces and strain, and the TF compressive force. To assess the biomechanical effects on the PF joint, the secondary simulation outcomes were the forces around the knee extensor mechanism, including the quadriceps muscle force, quadriceps tendon-to-femur force, and PF compressive force.