Dose-specific effects of caffeinated chewing gum on reactive blocking timing, jump mechanics, and neuromuscular responses after volleyball-specific fatigue in elite female volleyball players

Purpose To investigate the effects of caffeinated chewing gum at nominal target doses of 3 and 6 mg/kg on reactive blocking timing, block-jump mechanics, and neuromuscular responses in elite female volleyball players under post-fatigue conditions.Methods Fifteen elite female volleyball players completed a randomized, placebo-controlled crossover study implemented with double-blinding procedures. Participants consumed placebo (PLA), caffeinated chewing gum at nominal target doses of 3 mg/kg (CAF3) or 6 mg/kg (CAF6), and were tested after supplementation and warm-up at PRE and immediately after a volleyball-specific repeated blocking fatigue protocol (POST). Primary outcomes were reactive blocking timing-chain indices and block-jump (BJ) peak power. Secondary outcomes included additional BJ mechanical variables and general jump measures from countermovement jump (CMJ) and drop jump (DJ) tests. Exploratory outcomes included synchronized surface electromyography (sEMG) variables of lower-limb muscles. Participant blinding was assessed after each visit.Results Fatigue impaired jump-related performance across conditions (p < 0.05). For the primary outcomes, the descriptive decline in BJ peak power was 6.9% in CAF6 versus 13.9% in PLA; the adjusted POST comparison did not establish superiority of CAF6 over CAF3. For timing outcomes, overall sequence time increased by 18.4% in PLA, 6.4% in CAF3, and 3.0% in CAF6; CAF6 therefore showed the smallest descriptive deterioration in the integrated sequence measure, whereas CAF3 showed little change in the mean stationary and movement component times. CAF6 showed a reduction in mean stationary response time and the smallest increase in overall sequence time. For secondary outcomes, caffeine showed more favorable post-fatigue responses in selected CMJ/DJ measures, including CMJ mean propulsive power and DJ contact time (p < 0.05), and CAF6 showed greater preservation of selected stretch-shortening cycle-related mechanical measures, including DJ jump height and DJ vertical stiffness, than PLA (p < 0.05). In contrast, CMJ mRSI showed a divergent pattern and was not interpreted as evidence of caffeine-related attenuation of fatigue-induced decrements. Exploratory sEMG results showed smaller post-fatigue changes in selected frequency-domain and activation-timing variables under CAF6, particularly in measures related to extensor-muscle function. Participant masking was incomplete: perceived caffeine ingestion was reported in 0.0%, 53.3%, and 80.0% of PLA, CAF3, and CAF6 visits, respectively, and adverse effects were more frequent in CAF6.Conclusions The observed responses were outcome-specific: CAF6 showed a smaller decline in BJ peak power and the smallest increase in overall sequence time, whereas CAF3 showed little change in mean stationary and movement response times. These findings do not establish a generally preferable dose or a timing-stability advantage for CAF3. Incomplete participant masking limits attribution of the differences exclusively to caffeine dose.