Assessing the Contribution of Arm Swing to Countermovement Jump Height Using Three Different Measurement Methods in Physically Active Men

Background/Objectives: This study evaluated the reliability and validity of three methods to measure jump height during countermovement jumps performed with (CMJ_AS) and without (CMJ_NAS) arm swing: (1) an impulse–momentum method with force platforms (FP_imp), (2) a flight time method with force platforms (FP_time), and (3) an inertial measurement unit (PUSH Band 2.0; PUSH2). Methods: Eighteen physically active men performed CMJ_AS and CMJ_NAS on force platforms while wearing PUSH2 over two days. Besides jump height, we computed intraclass correlation coefficients (ICC) and the absolute and relative increases in jump height due to arm swing, compared to CMJ_NAS. Results: The reliability of intra-session, inter-session, and concurrent measures were good to excellent (intra-session ICC_2,1 = 0.957–0.979, inter-session ICC_2,1 = 0.806–0.990, concurrent ICC_3,1 = 0.940–0.973) for CMJ_AS and CMJ_NAS heights, in all three methods. The three methods showed high to very high reliability for both the absolute and relative indices of arm swing contribution (ICC_2,1 = 0.649–0.812). FP_time significantly overestimated CMJ_NAS height relative to FP_imp (<i>p</i> < 0.01). The absolute index of arm swing contribution was similar in FP_imp and FP_time (<i>p</i> = 0.38) but higher in PUSH2 (<i>p</i> < 0.01), indicating that arm swing amplified overestimation. Conclusions: All three methods demonstrated high reliability for jump height measurements, but FP_time and PUSH2 misestimated jump height depending on jump modalities. Caution is advised when assessing the absolute and relative contribution of arm swing, because errors in CMJ_NAS and CMJ_AS height measurements can affect these values and their interpretation.