Fig. 7: Within-cluster subject variability of KS can be explained by variability of MS and muscle-tendon parameters.

To investigate the influence of limb biomechanical properties on the MS-KS relationship, simulated muscle-tendon parameters (Table 3) across four infantile stages were incorporated into the regression analysis of within-cluster subject variability (Fig. 5C). For each cluster, within-cluster subject variability for each muscle-tendon parameter was quantified by averaging its inter-subject SDs across the cluster’s dominant muscles (Fig. 4A). A–C At CH4, OptimalFibreLength, TendonSlackLength, and StrengthCoefficient were identified as significant predictors of MS-KS association. Data points for six clusters at CH4 are shown as dark blue circles (n = 6), with the fitted regression surface displayed in semi-transparent purple. D StrengthCoefficient demonstrated a significant effect on inter-subject KS variability across four infantile stages (n = 24), with the regression line shown in red. Observations from the four infantile stages are color-coded (CH1: red; CH2: green; CH3: light blue; CH4: dark blue), with each stage comprising six data points (six clusters from both limbs; Figs. 4A and S4). Regression coefficients and the p-values for the whole model and each factor are indicated at the top of each subplot.