## 3 Results
### Sit-to-Stand-to-Sit Activity  
The sit-to-stand-to-sit activity was characterised by almost symmetrical ligament length change behaviour throughout the entire cycle. The LCL and MCLp elongated during the stand-up phase (LCL: from \(-\)15.8 \(\pm\) 2.2% up to \(-\)1.5 \(\pm\) 1.8% for GRAD and from \(-\)14.0 \(\pm\) 2.3% up to \(-\)1.2 \(\pm\) 2.4% for DUAL and MCLp: from \(-\)3.4 \(\pm\) 1.2% up to 0.6 \(\pm\) 0.5% for GRAD and from \(-\)4.0 \(\pm\) 1.6% up to 0.3 \(\pm\) 0.8% for DUAL), followed by a slackening during the sit-down period. Similar patterns could be seen for the PCL bundles, but with a rapid lengthening before knee extension and rapid shortening after knee flexion, resulting in a \"W\" shaped curve (Figure 6, Figures S3 and S4; Supplementary Information File). Conversely, the intermediate and anterior portions of the MCL slightly elongated before they continuously shortened and reached their minimum length at around the middle of the sit-to-stand-to-sit cycle (GRAD: 0.2 \(\pm\) 1.1% for MCLi and 0.3 \(\pm\) 1.0% for MCLa and DUAL: 0.1 \(\pm\) 1.1% for MCLi and 0.2 \(\pm\) 1.1% for MCLa). Figure 5: SPM plots indicating significant differences between GRAD and DUAL ligament elongation patterns during level gait against knee flexion angle. Group differences could only be evaluated in flexion ranges covered by data of all subjects. Red dashed line represents F value corresponding to the level of significance \(\alpha\) = 0.05. The gray shaded areas represent regions of statistical significance. PCLal and PCLpm: anterolateral and posteromedial bundles of the posterior cruciate ligament; aMCL, iMCL, and pMCL: anterior, intermediate, and posterior bundles of the medial collateral ligament; and LCL: lateral collateral ligament. The MCL bundles showed a bi-phasic pattern throughout the range of knee flexion (Figure 7). While the MCLa and MCLi indicated a considerable elongation during flexion for smaller flexion angles, the MCLp remained close to its reference length before all three bundles started slackening with higher flexion angles (Figure 7). The maximum elongation within the MCL was the largest for the anterior fibres (GRAD: 6.4 \(\pm\) 1.2% and DUAL: 6.4 \(\pm\) 1.0% for MCLa, GRAD: 3.0 \(\pm\) 0.8 and DUAL: 3.1 \(\pm\) 0.8% for MCLi, and GRAD: 0.9 \(\pm\) 0.4% and DUAL: 0.6 \(\pm\) 0.7% for MCLp). The two implant designs showed significantly different elongation patterns in the PCL and LCL bundles, almost throughout the entire range of knee flexion, with, in general, less relative elongation for the GRAD design (\(p\) < 0.009; Figures 8 and 9). However, significant differences in the MCL elongation between the implant designs were observed, mainly in the posterior fibres and at higher flexion angles (\(p\) = 0.043; above 68\({}^{\circ}\) for the pMCL). The GRAD design demonstrated a greater relative elongation compared to the DUAL design (Figures 6-9). Figure 6: Average elongation patterns throughout the complete cycle during sit-to-stand-to-sit activity. Solid lines represent inter-subject means and shadings represent \(\pm\) SDs. PCLal and PCLpm: anterolateral and posteromedial bundles of the posterior cruciate ligament; aMCL, iMCL, and pMCL: anterior, intermediate, and posterior bundles of the medial collateral ligament; and LCL: lateral collateral ligament. Figure 8: SPM plots indicating significant differences between GRAD and DUAL ligament elongation patterns along the complete cycle of the sit-to-stand-to-sit activity. Red dashed line represents F value corresponding to the level of significance \(\alpha\) = 0.05. The gray shaded areas represent regions of statistical significance. PCLal and PCLpm: anterolateral and posteromedial bundles of the posterior cruciate ligament; aMCL, iMCL, and pMCL: anterior, intermediate, and posterior bundles of the medial collateral ligament; and LCL: lateral collateral ligament. Figure 7: Average elongation patterns of the knee ligaments during sit-to-stand-to-sit activity throughout the range of knee flexion.