Vertical Jump Performance, Arm Swing Benefit, and Body Size in Grades 3–5: A Cross-Sectional Study

Objectives. The study aimed to compare vertical jump performance across Grades 3–5, examine associations with body size, and secondarily evaluate arm swing benefits. Materials and Methods. This cross-sectional school study included 129 pupils from eastern Slovakia (3rd grade, n = 46; 4th grade, n = 50; 5th grade, n = 33). Data on sex, biological maturation, organized sport participation, and habitual physical activity were unavailable. The best of three valid Chronojump attempts was analyzed for countermovement jump without arm swing (CMJ), countermovement jump with free arm swing (CMJ FA), and squat jump (SJ). One-way ANOVA tested grade differences in these primary outcomes. Absolute and relative arm swing benefits were secondary exploratory outcomes. ANCOVA, joint models including height and mass, correlations, nonparametric sensitivity analyses, and grade × mass interactions were also examined. Results. Primary outcomes did not differ by grade (p = 0.19–0.59; eta squared = 0.008–0.026). Secondary exploratory analyses identified differences in absolute and relative arm swing benefit (both p = 0.002; eta squared is approximately 0.09), with higher values in the 5th grade than in the 3rd and 4th grades; nonparametric analyses supported these contrasts. Mass-adjusted ANCOVA revealed that mass was significant for all tests, and a grade effect emerged for CMJ FA (p = 0.013). In joint models, height was positively and mass negatively associated with each primary outcome, whereas grade effects and grade × mass interactions were nonsignificant. Conclusions. The findings indicate that primary jump outcomes showed no statistically detectable differences across adjacent grades. The larger secondary exploratory arm swing benefit in the 5th grade group does not demonstrate developmental change within pupils and may partly reflect unmeasured sex, maturation, or activity characteristics. Body size interpretation depended on model specification. School testing should emphasize standardized procedures and individual feedback rather than diagnostic labeling or ranking by grade.