## 2 Methods
### TKA Modelling  
A previously developed multibody model [26] was used to create 30 models representing the subjects' anthropometry (Figure 1). Within the OpenSim modelling environment, skin marker positions captured during the standing trials were used to scale the generic tibial and femoral bone geometries in the proximo-distal (PD) direction [33]. To validate this scaling method, the estimated bone lengths from the OpenSim linear scaling were compared to subject-specific bone lengths derived from frontal X-ray images of the lower limbs. Here, these subject-specific bone lengths were measured by calculating the distance between the proximal and distal joint centres. The knee centre was defined as the midpoint between the medial and lateral epicondyle, the ankle centre as the midpoint between the medial and lateral malleoli, and the hip centre as the centre of the femoral head. Specifically, for the six subjects with available X-ray images, the corresponding estimation errors were calculated and reported as percentages of bone length. Additionally, subject-specific implant dimensions were used to scale the bones in the medio-lateral (ML) and antero-posterior (AP) directions.  
The models had a 6 DoF joint defined between the tibial and femoral implant components positioned within their parent bones based on a mechanical alignment approach [34]. The ligaments were modelled as bundles of one-dimensional fibres, with 20 fibres dedicated to each of the PCL and MCL and 10 fibres to represent the LCL. Regarding the anatomic definition of knee ligament bundles [35; 36], the MCL was further divided into anterior (MCLa), intermediate (MCLi), and posterior (MCLp) portions. The PCL was divided into antero-lateral (PCLal) and postero-medial (PCLpm) portions, each represented by  
Figure 1: Workflow of the combined multibody modelling and video-fluoroscopy method used to assess ligament elongation patterns. Skin-marker motion capture data were used to scale the tibial and femoral bones in the proximo-distal direction (yellow lines) while scaling of the bones in the mediolateral (red lines) and anteroposterior (blue lines) directions was performed based on the subject-specific implant dimensions. Knee kinematics obtained from video-fluoroscopy were used to drive the 6-degree-of-freedom knee joint within the personalised knee models. Ligament fibres are shown in green.  
10 strands. Wrapping objects representing the bone surfaces were implemented to prevent penetration of the ligament fibres into the bone.  
Knee joint kinematics obtained from video-fluoroscopy were used as inputs to drive the 6 DoF tibofemoral joint within the multibody model. Elongations of the ligament fibres were calculated as changes in the ligament length (\(l\)) relative to the reference length (\(l_{0}\)) defined at heel strike during level walking (i.e., \((l-l_{0})/l_{0}\)). For each ligament bundle, an average elongation pattern was extracted from the length change patterns of the individual fibres within the bundle. The outcome patterns were presented either against the time normalised to the percentage of the gait cycle or against the corresponding knee flexion angle.