Associations Between Maximal Passive Knee Extension and Sagittal Plane Kinematic Patterns in Children with Spastic Cerebral Palsy: A Longitudinal Study

<b>Background/Objectives:</b> There is limited information on the interplay between passive joint motion and joint kinematics from three-dimensional gait analysis (3DGA) and its longitudinal evolution in cerebral palsy (CP). We aimed to associate clinical measurements and gait kinematics over time using a longitudinal study design. <b>Methods</b>: Ambulatory individuals with spastic CP, aged 4-18, who performed a minimum of two 3DGA at the Karolinska University Hospital between 2008 and 2025 were recruited. Primary outcomes were sagittal plane kinematics and maximum passive knee extension (pKE). Canonical correlation (R) with statistical parametric mapping was used to associate passive maximum knee extension with sagittal hip, knee, and ankle angles at two timepoints. <b>Results</b>: the 3DGA data of 31 children (age 4-17 years; mean age 10.4 +/- 2.9) with 22 bilateral (bCP, GMFCS I = 6; II = 13; III = 3) and 9 unilateral CP (uCP, GMFCS I = 8; II = 1) was included. For the whole and bCP groups, respectively, knee flexion/extension and pKE were correlated throughout stance (<i>p</i> < 0.001), with R between -0.47 and -0.57/-0.49 and -0.59 at T1 and between -0.46 and -0.72/-0.50 and -0.76 at T2. Hip flexion/extension and knee pKE were correlated from 17 to 62%/46-52% of the gait cycle (<i>p</i> < 0.001/<i>p</i> = 0.045) for the whole and bCP groups, respectively, with R between -0.41 and -0.57/-0.38 and -0.41 at T1 and from 15 to 64%/17 to 61% with R between -0.50 and -0.57/-0.42 and -0.57 at T2. <b>Conclusions</b>: Reported associations between structural knee properties and knee position during gait demonstrated progression over time, implying that a restricted range of motion may be driven by functional constraints. Combining knee contractures and their longitudinal development with 3DGA is a powerful approach for pre-intervention planning.