Purpose: To investigate the validity of reactive strength index (RSI), reactive strength ratio, and vertical stiffness (Kvert) from the countermovement rebound jump and 10/5 repeated jump test against these metrics from the drop jump. We also investigated relationships between RSI, reactive strength ratio, Kvert, and flight time to contraction time within and between protocols. Differences in variables and mechanical components in each jump and reliability of all measures were calculated. Methods: Twenty-six subjects (18 males, 8 females; age [range]: 20.17 [18.25-31.16] years, weight: 82.20 [64.90-110.30] kg) performed 2 sessions 7 days apart, each consisting of 2 trials of drop jump, countermovement rebound jump, and 10/5 repeated jump test. Bland-Altman limits of agreement were calculated to assess validity. Relationships between outcome metrics (eg, RSI) from different jump types were assessed using Pearson correlations. Differences between variables (eg, flight time) and mechanical components (eg, impulse) from different protocols were assessed using linear mixed models (Cohen d). Between-session reliability was determined as coefficient of variation and intraclass correlation coefficient. Results: Relationships between protocols for Kvert and RSI/reactive strength ratio were very weak to moderate (r = .11-.58), and ground contact time differed between protocols (d = 0.51-1.26). All variables demonstrated moderate to excellent reliability (coefficient of variation: 3.01-17.19%; intraclass correlation coefficient: .59-.97). Conclusions: Each outcome metric provides unique information, and differences in variables and mechanical components between protocols suggest distinct movement strategies. Protocols should not be used interchangeably.
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