Caffeine potentiates low frequency skeletal muscle force in habitual and nonhabitual caffeine consumers

The mechanism of action underlying the ergogenic effect of caffeine is stil l unclear. Caffeine increases the force of muscular contraction during low- frequency stimulation by potentiating calcium release from the sarcoplasmic reticulum. Studies have also suggested an enhancement of lipid oxidation a nd glycogen sparing as potential mechanisms. Given that several studies hav e found an ergogenic effect of caffeine with no apparent metabolic effects, it is likely that a direct effect upon muscle is important. Twelve healthy male subjects were classified as habitual (n = 6) or nonhabitual (n = 6) c affeine consumers based on a 4-day diet record analysis, with a mean caffei ne consumption of 771 and 14 mg/day for each group, respectively. Subjects were randomly allocated to receive caffeine (6 mg/kg) and placebo (citrate) in a double-blind, cross-over fashion similar to 100 min before a 2-min te tanic stimulation of the common peroneal nerve in a custom-made dynamometer (2 trials each of 20 and 40 Hz). Tetanic torque was measured every 30 a du ring and at 1, 5, and 15 min after the stimulation protocol. Maximal volunt ary contraction strength and peak twitch torque were measured before and af ter the stimulation protocol. Caffeine potentiated the force of contraction during the final minute of the 20-Hz stimulation (P < 0.05) with no effect of habituation. There was no effect of caffeine on 40-Hz stimulation stren gth nor was there an effect on maximal voluntary contraction or peak twitch torque. These data support the hypothesis that some of the ergogenic effec t of caffeine in endurance exercise performance occurs directly at the skel etal muscle level.