Opposite actions of caffeine and creatine on muscle relaxation time in humans

Exercise Physiology and Biomechanics Laboratory, Department of Kinesiology, Faculty of Physical Education and Physiotherapy, Katholieke Universiteit Leuven, Tervuursevest 101, B-3001 Leuven, Belgium The effect of creatine and caffeine supplementation on muscle torque generation and relaxation was investigated in healthy male volunteers. Maximal torque (T max ), contraction time (CT) from 0.25 to 0.75 of T max , and relaxation time (RT) from 0.75 to 0.25 of T max were measured during an exercise test consisting of 30 intermittent contractions of musculus quadriceps (2 s stimulation, 2 s rest) that were induced by electrical stimulation. According to a double-blind randomized crossover design, subjects ( n = 10) performed the exercise test before (pretest) and after (posttest) creatine supplementation (Cr, 4 × 5 g/day, 4 days), short-term caffeine intake (Caf, 5 mg · kg 1 · day 1 , 3 days), creatine supplementation + short-term caffeine intake (Cr+Caf), acute caffeine intake (ACaf, 5 mg/kg) or placebo. Compared with placebo, Cr shortened RT by ~5% ( P