Change-of-direction (COD) ability is a key determinant of soccer performance. This study evaluated the intra- and inter-day reliability and sensitivity of a single-radar device (SRD) for capturing phase-specific kinetics and kinematics during the 505 tests in 24 youth soccer players. Intra-day results showed high reliability for maximum velocity (Vmax, ICC = 0.67, CV = 1.51%) and moderate-to-high reliability for kinetic variables (P2_HBF avg and P3_HAP avg, ICC = 0.58-0.59). However, all intra-day metrics exhibited "Marginal" usefulness as Typical Error (TE) consistently exceeded the Smallest Worthwhile Change (SWC). In contrast, inter-day reliability was substantially higher, particularly for kinetic indicators such as braking impulse (P2_HBI avg, ICC = 0.90) and re-acceleration force (P3_HAF max, ICC = 0.91). Furthermore, inter-day assessment revealed "Good" usefulness (TE=SWC) for key metrics, including P2_HBIavg, P3_HAFavg/max, and re-acceleration distance/velocity. These results indicate that while intra-day stability is limited by movement variability, RRDs provide sufficient precision for individualized monitoring across testing days. Practitioners should prioritize inter-day kinetic profiles and incorporate familiarization to maximize diagnostic accuracy in youth soccer populations.
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