Neuromuscular Responses to Incremental Caffeine Doses: Performance and Side Effects

Purpose: The purpose of this study was to determine the oral dose of caffeine needed to increase muscle force and power output during all-out single multijoint movements. Methods: Thirteen resistance-trained men underwent a battery of muscle strength and power tests in a randomized, double-blind, crossover design, under four different conditions: (a) placebo ingestion (PLAC) or with caffeine ingestion at doses of (b) 3 mg.kg(-1) body weight (CAFF(3mg)), (c) 6 mg.kg(-1) (CAFF(6mg)), and (d) 9 mg.kg(-1) (CAFF(9mg)). The muscle strength and power tests consisted in the measurement of bar displacement velocity and muscle power output during free-weight full-squat (SQ) and bench press (BP) exercises against four incremental loads (25%, 50%, 75%, and 90% one-repetition maximum [1RM]). Cycling peak power output was measured using a 4-s inertial load test. Caffeine side effects were evaluated at the end of each trial and 24 h later. Results: Mean propulsive velocity at light loads (25%-50% 1RM) increased significantly above PLAC for all caffeine doses (5.4%-8.5%, P = 0.039-0.003). At the medium load (75% 1RM), CAFF(3mg) did not improve SQ or BP muscle power or BP velocity. CAFF(9mg) was needed to enhance BP velocity and SQ power at the heaviest load (90% 1RM) and cycling peak power output (6.8%-11.7%, P = 0.03-0.05). The CAFF(9mg) trial drastically increased the frequency of the adverse side effects (15%-62%). Conclusions: The ergogenic dose of caffeine required to enhance neuromuscular performance during a single all-out contraction depends on the magnitude of load used. A dose of 3 mg.kg(-1) is enough to improve high-velocity muscle actions against low loads, whereas a higher caffeine dose (9 mg.kg(-1)) is necessary against high loads, despite the appearance of adverse side effects.