Reliability, Concurrent Validity, and Side-to-Side Asymmetry of a Unilateral Rotational Medicine Ball Throw Test in Adolescents

Background/Objectives: This study examined the reliability, concurrent validity, agreement, and side-to-side asymmetry of a unilateral rotational medicine ball throw test in adolescents. Methods: Overall, 204 students (101 boys, 103 girls; 14.1 ± 1.3 years) completed two sessions of Eurofit frontal throws and rotational throws to both sides. Reliability, measurement error, concurrent validity, Bland–Altman agreement, and asymmetry were analyzed. Results: Reliability was good-to-excellent in boys (ICC = 0.90) and moderate-to-good in girls (ICC = 0.70–0.82), with the right-sided throw in girls below the prespecified thresholds (SEM = 0.34–0.44 m; MDC95 = 0.94–1.21 m). Performance improved from Day 1 to Day 2, suggesting a learning effect. The rotational throw was strongly associated with the Eurofit throw (r = 0.81). Bland–Altman analysis showed systematic and proportional differences (bias 0.53 m; 95% LoA −0.68 to 1.75 m). A significant group-level right–left difference was detected (mean asymmetry 7.49%), but the asymmetry index showed poor test–retest reliability (ICC = 0.22) and an inconsistent direction across sessions (κ = 0.14). Conclusions: The rotational throw is a related but not interchangeable complement to the Eurofit throw for whole-body rotational performance. Its asymmetry index describes group differences but is not suitable for individual monitoring. These findings are preliminary.