Background: Liquid-dissolved and encapsulated powder are two popular ways to consume caffeine for performance-enhancing effects. Caffeine in other delivery methods, such as chewing gums, orally dissolvable strips, gels, mouthwashes, energy drinks, and nasal sprays, is believed to be absorbed more quickly into the bloodstream. Inter-individual responses to caffeine's enhancing effects are recognized. The present study examined the inter-individual responses to the acute effects of encapsulated caffeine and caffeinated chewing gum on the lower-body isokinetic and isometric strength and power in strength-trained males.
Method: A randomized, cross-over, placebo-controlled study was conducted with fifteen strength-trained males (age: 25±4 years, height: 176±7 cm, weight: 75±11 kg, habitual caffeine intake: 66±15 mg·day⁻¹). Participants were randomly assigned to three conditions: i) caffeinated chewing gum (CG), ii) caffeine capsule (CC), and iii) starch capsule as a placebo (PLA). Participants consumed approximately 3 to 4.5 mg·kg⁻¹ of caffeine 60 minutes before testing. The washout period between conditions was one week. Participants performed the Sargent jump test, followed by a 5-minute active recovery (walking). Subsequently, isokinetic strength and power (60°/s and 180°/s) and isometric strength (45° and 60°) parameters were measured for knee extensor and flexor muscles. Data were analyzed using one-way repeated measures ANOVA and Bonferroni post hoc tests, with significance set at P ≤ 0.05. Responders to the caffeine conditions were identified using the smallest worthwhile change (SWC) analysis.
Results: In knee extensors, 1) average peak torque and power at 60°/s were higher in CC (P=0.045; +11.22% and P=0.038; +14.15%) and CG (P=0.044; +7.32% and P=0.015; +11.42%) compared to PLA with a co-response rate of 60% and 66%, 2) maximum voluntary isometric contraction at 45° (MVIC-45°) was higher in CC compared to PLA (P=0.031; +10.15% ), and 3) MVIC-60° was higher in CG compared to PLA (P=0.037; +10.15%) with a co-response rate of 60%. In knee flexors, 1) time to peak torque at 60°/s was higher in CG compared to PLA (P=0.011; +18.28%) with a co-response rate of 46%, 2) average rate of force development at 60°/s was higher in CC (P=0.007; +24.14%) and CG (P=0.050; +20.69%) compared to PLA with a co-response rate of 53%, and 3) average power at 180°/s was higher in CC compared to PLA (P=0.033; +18.08%) with a co-response rate of 46%. However, there were no differences between other strength indicators in the knee extensors and flexors between the different conditions. Vertical jump height (VJH) was higher in CC (P=0.001; +5.58%) and CG (P=0.001; +6.09%) compared to PLA, with a co-response rate of 53%.
Conclusion: Caffeine supplementation in CC and CG forms significantly enhanced lower-body strength, power, and vertical jump height in strength-trained males, with over ~ 50% of participants exceeding the SWC thresholds across key performance metrics. CC showed slightly higher responder rates for strength parameters, while CG excelled in time-dependent measures, supporting their use as effective and flexible ergogenic aids.