Lopez-Samanes, A., Garcia-Pallares, J., Ortega, J. F., Fernandez-Elias, V., & Mora-Rodriguez, R. (2012). Caffeine ingestion reverses the circadian rhythm effects on neuromuscular performance in highly resistance-trained men. Presentation 2793 at the 59th Annual Meeting of the American College of Sports Medicine, San Francisco, California; May 29-June 2, 2012.

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This study investigated whether caffeine ingestion counteracts the morning reduction in neuromuscular performance associated with circadian rhythm in highly resistance-trained men (N = 12). Ss underwent a battery of neuromuscular tests under three different conditions; i) morning (10:00 a.m.) with caffeine ingestion (i.e., 6 mg/kgBW); ii) morning (10:00 a.m.) with placebo ingestion; and iii) afternoon (18:00 p.m.) with placebo ingestion. The test battery consisted of bar displacement velocity during free-weight full-squat and bench press exercises against loads that elicit maximum strength (75% 1RM load) and muscle power adaptations (1 m/sec load). Maximum voluntary and electrically evoked isometric leg extension strengths were measured to identify caffeine’s action mechanisms. Plasma norepinephrine concentration was measured after a standardized free-weight full-squat intense bout (6 x 85% 1RM) as surrogate for maximal muscle sympathetic nerve activity since plasma norepinephrine concentration is mostly derived from the spillover of the terminal nerve endings of the motoneurons.

In the placebo afternoon trial, dynamic muscle strength and power output were significantly enhanced compared with the morning placebo condition. During the morning-caffeine trial, strength and power increased above morning-placebo levels except for bench press velocity with 1 m/sec load. During the morning caffeine trial, maximum voluntary and electrically evoked isometric leg extension strengths and plasma norepinephrine concentration were increased above the morning placebo trial.

Implication. Caffeine ingestion reverses morning decreases in muscle strength and power output, raising performance to levels of afternoon performances in the circadian rhythm. Electrical stimulation data, along with plasma norepinephrine concentration, suggest that caffeine increases neuromuscular performance via a direct effect in the muscle.

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