Caffeine Ingestion Reverses the Circadian Rhythm Effects on Neuromuscular Performance in Highly Resistance-Trained Men

Abstract

Purpose

To investigate whether caffeine ingestion counteracts the morning reduction in neuromuscular performance associated with the circadian rhythm pattern.

Methods

Twelve highly resistance-trained men underwent a battery of neuromuscular tests under three different conditions; i) morning (10:00 a.m.) with caffeine ingestion (i.e., 3 mg kg ⁻¹; AM _CAFF trial); ii) morning (10:00 a.m.) with placebo ingestion (AM _PLAC trial); and iii) afternoon (18:00 p.m.) with placebo ingestion (PM _PLAC trial). A randomized, double-blind, crossover, placebo controlled experimental design was used, with all subjects serving as their own controls. The neuromuscular test battery consisted in the measurement of bar displacement velocity during free-weight full-squat (SQ) and bench press (BP) exercises against loads that elicit maximum strength (75% 1RM load) and muscle power adaptations (1 m s ⁻¹ load). Isometric maximum voluntary contraction (MVC _LEG) and isometric electrically evoked strength of the right knee (EVOK _LEG) were measured to identify caffeine's action mechanisms. Steroid hormone levels (serum testosterone, cortisol and growth hormone) were evaluated at the beginning of each trial (PRE). In addition, plasma norepinephrine (NE) and epinephrine were measured PRE and at the end of each trial following a standardized intense (85% 1RM) 6 repetitions bout of SQ (POST).

Results

In the PM _PLAC trial, dynamic muscle strength and power output were significantly enhanced compared with AM _PLAC treatment (3.0%–7.5%; p≤0.05). During AM _CAFF trial, muscle strength and power output increased above AM _PLAC levels (4.6%–5.7%; p≤0.05) except for BP velocity with 1 m s ⁻¹ load (p = 0.06). During AM _CAFF, EVOK _LEG and NE (a surrogate of maximal muscle sympathetic nerve activation) were increased above AM _PLAC trial (14.6% and 96.8% respectively; p≤0.05).

Conclusions

These results indicate that caffeine ingestion reverses the morning neuromuscular declines in highly resistance-trained men, raising performance to the levels of the afternoon trial. Our electrical stimulation data, along with the NE values, suggest that caffeine increases neuromuscular performance having a direct effect in the muscle.