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BackgroundCountermovement jump testing is widely used to infer lower-limb neuromuscular performance, readiness, fatigue, and adaptation in applied exercise physiology. However, commercially available force-plate, inertial-sensor, and smartphone-based systems use different signal sources and calculation methods, which may affect the physiological interpretation of jump-performance changes.ObjectiveTo evaluate the agreement and test–retest reliability of bilateral and single-leg countermovement jump outcomes obtained from force plates, an inertial measurement unit, and a smartphone application in male volleyball players.MethodsTwenty professional male volleyball players were enrolled. Bilateral and left/right single-leg countermovement jump trials were recorded concurrently using VALD ForceDecks, MyJump Lab 3, and Baiobit. VALD impulse–momentum-derived jump height was used as the reference outcome. Agreement analyses compared MyJump Lab 3 and Baiobit with VALD using bias, limits of agreement, ICC(2,1), SEM, and MDC95. Test–retest reliability was assessed between two sessions separated by 12 days. Sensitivity analyses examined VALD flight-time-derived height, MyJump flight time, best-trial values, and strict quality-control exclusions.ResultsMyJump Lab 3 systematically overestimated VALD impulse–momentum jump height by +1.88 to +4.16 cm. This discrepancy was markedly attenuated when VALD flight-time-derived height was used as the reference, while MyJump flight-time bias was negligible, indicating a calculation-method effect rather than temporal event-detection error. Baiobit showed smaller mean bias in some single-leg conditions but poorer individual-level agreement, with wider limits of agreement and less consistent absolute-agreement ICCs. VALD showed the strongest bilateral CMJ reliability (ICC = 0.929; MDC95 = 2.79 cm), whereas single-leg reliability was generally lower.ConclusionCommercially available jump-testing systems can lead to method-dependent differences in the interpretation of neuromuscular performance. Device-, task-, and metric-specific measurement error should be considered before using jump-height changes to infer readiness, fatigue, or adaptation in professional male volleyball players.