## 3 Results
### Stair Descent  
The elongation patterns of the LCL indicated consistent slackening of the ligament during the stance phase of the stair descent activity, followed by a quick lengthening in the swing phase (Figure 10, Figures S5 and S6; Supplementary Information File). The shortest LCL length occurred at push-off (65% GC), with 14.67 \(\pm\) 2.02% shortening for subjects with the GRAD implant and 13.34 \(\pm\) 1.16% for subjects with the DUAL implant. The MCL bundles underwent a rapid shortening before toe-off and a quick elongation after that, resulting in a local minimum at the transition from stance to swing. During the first half of the GC, the MCLa and MCLi lengths steadily increased (up to 5.1 \(\pm\) 1.0% and 2.6 \(\pm\) 0.7% for MCLa and MCLi in GRAD and up to 5.7 \(\pm\) 1.3% and 3.3 \(\pm\) 0.9% for MCLa and MCLi in DUAL), whereas the MCLp remained near isometric. The anterior and intermediate bundles of the MCL showed a considerable shortening during the second half of the swing phase (Figure 10, Figures S5 and S6; Supplementary Information File). Both PCL bundles started slackening at the beginning of the GC, followed by a lengthening within the second half of the stance phase. The anterolateral portion of the PCL reached maximum lengthenings of 6.0 \(\pm\) 3.4% (GRAD) and 6.4 \(\pm\) 2.9% (DUAL) at around 75% of the GC. Figure 9: SPM plots indicating significant differences between GRAD and DUAL ligament elongation patterns during sit-to-stand-to-sit activity 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 LCLL lateral collateral ligament. The elongation patterns differed significantly between the implant designs, with higher strains observed for the DUAL implant in comparison to the GRAD design. Significant differences were observed starting at 29\({}^{\circ}\) of knee flexion for the LCL, and within the mid-flexion range for the PCL and MCL (\(p<0.007\); MCLa: 33-56\({}^{\circ}\), MCLL: 27-59\({}^{\circ}\), MCLp: 26-56\({}^{\circ}\), PCLaI: 13-65\({}^{\circ}\), and PCLpm: 25-54\({}^{\circ}\)), as illustrated in Figures 10-13. Figure 11: SPM plots indicating significant differences between GRAD and DUAL ligament elongation patterns along the complete cycle of the stair descent. 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; andLCL: lateral collateral ligament. Figure 10: Average elongation patterns along the complete cycles of stair descent. Solid lines represent inter-subject means and shaded areas represent \(\pm\) SDs. The vertical dashed lines represent the average toe-off times. 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 dotted lines represent the average toe-off times. Figure 12: Average elongation patterns of the knee ligaments during stair descent throughout the range of knee flexion. Solid lines represent the group average across all subjects. Dashed lines represent the group average when data for more than 6 subjects were available. Shaded areas represent \(\pm\) SDs considering more than 6 subjects.