The Use of Sternum and Sacrum Angles in the Assessment of Sitting Posture in Adolescents: A Cross-Sectional Comparison of Cohorts Assessed Before and After the COVID-19 Pandemic

<b>Background</b>: The COVID-19 pandemic has been associated with increased sedentary behavior in children, raising concerns about posture and spinal health. This study compared standardized measures of sitting spinal alignment in two independent cohorts assessed before (2017) and after (2024) the pandemic and examined correlations among alignment parameters across different sitting postures. <b>Methods</b>: This cross-sectional study included healthy children aged 9-13 years. The sternal angle, sacral angle, lumbar lordosis, thoracic kyphosis, trunk tilt, and lateral deviation were measured using a Saunders digital inclinometer and a DIERS Formetric 4D rasterstereographic system in passive, forced, and corrected sitting postures. <b>Results</b>: No statistically significant differences were observed between the 2017 and 2024 cohorts (<i>p</i> > 0.05). Within each cohort, significant posture-related differences were found for the sternal and sacral angles, lumbar lordosis, and lateral deviation (<i>p</i> < 0.05), while thoracic kyphosis and trunk inclination differed between passive and corrected sitting (<i>p</i> < 0.05). The sternal angle correlated moderately to strongly with thoracic kyphosis (<i>r</i> = 0.657-0.695, <i>p</i> < 0.001), and the sacral angle correlated with lumbar lordosis (<i>r</i> = 0.679-0.743, <i>p</i> < 0.001). <b>Conclusions</b>: Similar alignment parameters across time-separated cohorts suggest no major cohort-level shifts in standardized sitting posture; however, behavioral factors were not directly assessed. Strong correlations among sagittal angles emphasize the consistent geometric relationship between the sternum, sacrum, and spinal curvatures. A sternal angle of approximately 65° was consistently associated with physiologically favorable spinal alignment and may serve as a practical reference value for posture assessment and education.