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

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 x 5 g/day, 4 days), short-term caffeine intake (Caf, 5 mg x kg(-1) x 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 approximately 5% (P < 0.05). Conversely, Caf increased RT (+ approximately 10%, P < 0.05), in particular as RT increased because of fatigue. RT was not significantly changed by either Cr+Caf or ACaf. T(max) and CT were similar during all experimental conditions. Initial T(max) was approximately 20% of voluntary maximal isometric contraction force, which was not different between treatments. It is concluded that Caf intake (3 days) prolongs muscle RT and by this action overrides the shortening of RT due to creatine supplementation.