Does Acute Caffeine Alter the Temporal Countermovement Jump Response Following Reactive Strength Index-Individualized Drop Jumps in Chinese Han Basketball Players with Different Habitual Caffeine Exposure? A Randomized, Double-Blind, Placebo-Controlled Crossover Trial

Background/Objectives: Whether caffeine provides additional benefit when superimposed on individualized plyometric conditioning remains unclear. We examined whether 3 mg·kg−1 caffeine altered the temporal countermovement jump (CMJ) response following reactive strength index (RSI)-derived individualized drop-jump (DJ) conditioning and explored moderation by habitual caffeine exposure. Methods: Thirty Chinese Han male basketball players (median age, 22.0 years [IQR, 21.0–27.75]) entered a randomized, double-blind, placebo-controlled crossover trial; 17 reported no caffeine exposure and 13 reported exposure during the preceding 4 weeks. After caffeine (CAF; 3 mg·kg−1) or placebo (PLA), a 60-min absorption period, and warm-up, participants performed a post-supplement, pre-conditioning CMJ followed by 3 × 5 DJs. CMJ was assessed at 15 s, 3, 6, 9, and 12 min. The primary outcome was the Condition × Time effect on CMJ. All 30 randomized participants contributed data; three did not complete Period 2 because readiness criteria were not met, but Period 1 data were retained (57 participant-period observations). Results: CMJ varied across time (p < 0.001), but Condition × Time was not significant (p = 0.305). CAF–PLA differences at 15 s, 3, 6, 9, and 12 min were 0.89 cm (95% CI, −0.11 to 1.88), 0.39 (−0.61 to 1.38), 0.21 (−0.79 to 1.21), 0.84 (−0.15 to 1.84), and 0.71 (−0.29 to 1.71), respectively (all Holm-adjusted p ≥ 0.399). Habitual caffeine exposure showed no global moderation continuously (p = 0.158) or categorically (p = 0.116). An isolated 9-min contrast at zero reported exposure favored CAF by 1.83 cm (95% CI, 0.74–2.92; Holm-adjusted p = 0.031) but was exploratory. RPE showed no Condition × Time interaction (p = 0.253); at 15 s, CAF–PLA was −0.60 points (95% CI, −1.09 to −0.12; Holm-adjusted p = 0.088). Conclusions: This study did not obtain clear evidence that 3 mg·kg−1 caffeine provided additional CMJ improvement following this specific RSI-derived individualized DJ protocol. Habitual caffeine exposure did not systematically moderate the temporal response; the isolated signal was exploratory.