The purpose of this study was to determine the possible interaction between the effects of time-of-day (morning vs. afternoon) and caffeine ingestion on neuromuscular performance (i.e., dynamic and isometric strength as well as muscle power output) in the upper and lower body musculature. Specifically, we sought to investigate if the stimulant actions of caffeine could reverse the reductions in neuromuscular performance observed in the morning. In addition, we set to determine if acute ingestion of caffeine at a dose known to enhance endurance performance (i.e., 3 mg kg−1) would also increase muscle power output in highly resistance-trained athletes (i.e., AMPLAC vs. AMCAFF comparison). Our data supports that caffeine is an ergogenic aid for muscle power output in the upper and lower body musculature of resistance-trained individuals. Furthermore, our results suggest that caffeine ingestion in the morning restores neuromuscular performance (muscle strength and power output) of upper and lower muscle groups to levels found in the afternoon trial (i.e., AMCAFF vs. PMPLAC comparison, ). We consider that these two findings have practical applications during resistance exercise training and performance.
Using our dynamic measurement system attached to a barbell individually loaded to elicit bar displacements of 1 m s−1, we found that caffeine ingestion increased the morning SQ and BP muscle power output by 2.5–5.7% (AMCAFF vs. AMPLAC; ). Furthermore, morning caffeine ingestion improved the velocity against loads that maximize maximum strength adaptations by 5.3% and 4.6% in SQ and BP respectively (AMCAFF vs. AMPLAC; ). Thus, neuromuscular performance (i.e., maximum dynamic strength and muscle power output) improved in a range of 3–6% in the morning after caffeine ingestion, during the most common exercises used for resistance training (i.e., BP and SQ). We think that these strength and power output enhancements induced by 3 mg kg−1 of caffeine ingestion (a dose achieved with approximately 2.5 espresso coffees for a 75 kg athlete) have the potential to prevent the morning declines in sport performance, allowing athletes to train and compete at the level of the evening.
A recent meta-analysis defends the existence of an ergogenic effect of caffeine ingestion on maximal voluntary strength but only for knee extensors 
. In contrast, our data suggest that caffeine ingestion increased similarly upper and lower body morning dynamic maximum strength (, albeit from a close to significant finding in BP in panel B). Our subjects were young (~20 yr), but very experienced resistance-trained individuals (average 7 yrs of training) that used their upper and lower body during training and competition. Importantly, we found a caffeine effect only when the dynamic contraction was used since isometric leg or arm strength were not improved. Out of the 27 studies, included in the Warren and co-workers' meta-analysis, maximum strength was measured using isometric contractions in 21 of them (i.e., 78% of the entries). It is then possible that the lack of ergogenic effect of caffeine in the upper body strength found in the meta-analysis may be due to the large percentage of isometric contraction studies to evaluate the ergogenic effects of caffeine ingestion. Our data contends that acute caffeine ingestion, increases maximal voluntary strength and power output in the upper and lower muscle groups.
Several publications propose that the lower core temperature during the morning in comparison to the afternoon is one of the factors influencing the reduced morning performance. In fact, when body temperature is raised passively by resting in a hot environment 
, or actively by prolonged exercise (>15 min) muscle performance increased to levels near the afternoon trials 
. Knowing this literature, we attempted to eliminate the effect of core temperature by providing a standardized warm-up lasting 10 min that contained continuous running and calisthenics. Despite the warm-up, a tympanic temperature difference of 0.7° C between morning and afternoon persisted as has been described previously 
. Of note, the trials AMPLAC
had the same basal tympanic temperature () despite significantly different muscle performance (). Thus, although part of the differences between the morning and afternoon performance could be due to the differences in core temperature, the effects of caffeine in improving morning muscle performance seems to be predominant in comparison to the effects of core temperature. It is possible that the combination of raising morning core temperature to the afternoon trial levels plus caffeine ingestion could have resulted in larger gains in muscle strength and power. On the other hand, the amount and intensity of exercise required to increase core temperature 0.7°C in our 19°C dry-bulb environment, would have likely been fatiguing and energy depleting. Caffeine ingestion in the morning, a nutritional habit usual for many athletes, probably allows similar enhancement in muscle strength and power output than warming-up through strenuous exercise or passive warming.
Morning reductions in muscle contractility (i.e., the strength divided by the electromyographic activity) suggest that the morning declines in muscle strength are due to peripheral modifications and not to changes in central neural command 
. We attempt to measure if the effects of caffeine on improving morning performance were also due to a peripheral muscle factor. We electrically stimulated the right leg to contract by delivering the same individualized current intensity in all trials and found a 15% larger increase in force production during AMCAFF
vs. the AMPLAC
trial (). Our electrical stimulation directly depolarizes the motor units under the skin 
and thus the increase in isometric force 60 min after 3 mg kg−1
of caffeine ingestion is circumscribed to the muscle itself. Although an ergogenic effect of caffeine through maintaining central neural command has been suggested 
a peripheral effect of caffeine has not been discarded 
. Our electrical stimulation data suggest that caffeine improves neuromuscular performance by acting directly upon the muscles and is in agreement with the meta-analysis of Warren et al. 
, that found that electrically evoked strength was higher with caffeine ingestion when expressed as percent of maximal voluntary contraction.
After the battery of neuromuscular tests, subjects were required to perform a bout of 6 free-weight squat repetitions at 85% of 1 RM with the aim of markedly raising sympathetic nerve activity. We measure plasma norepinephrine concentration (NE) as our main index of whole body sympathetic activity since it is derived in more than 80% from the spillover of the terminal nerve endings of the motoneurons 
. An acute bout of resistance exercise has been shown to increase plasma concentration of NE 
. The magnitude of the increase in NE may be dependent upon the force of muscle contraction, amount of muscle stimulated and volume of resistance exercise 
. Despite using the same short-bout of resistance exercise in all three trials, we observed that plasma NE rose higher when caffeine was ingested (AMCAFF
; ), suggesting a facilitated sympathetic nerve activation. In contrast, plasma epinephrine concentration, which is mostly derived from the adrenal medulla, did not show a larger increase after caffeine ingestion in comparison to the other trials. Although caffeine ingestion has been consistently reported to raise plasma epinephrine during endurance exercise to fatigue 
, a similar increase during a single bout of resistance exercise not to failure is unreported. We did not obtain samples during recovery and thus we cannot discard a delayed increase in plasma epinephrine upon caffeine ingestion. Our electrical stimulation data in conjunction with the plasma NE values suggest that caffeine increases muscle strength and power output through a direct effect in the muscle.
We measured resting serum hormones to have an index of the anabolic-catabolic balance in relation to the performance measured (i.e., muscle strength and power output). Albeit measurements of only resting blood serum hormone concentration have their limitations, their levels have been extensively reported in resistance-training research 
. Like others 
, we observed higher levels of blood testosterone and cortisol in the morning than in the afternoon (). However, the ratio testosterone-to-cortisol (T/C) tended to be higher in the afternoon (ES
0.51) coinciding with the higher levels of muscle strength and power output. In contrast, Teo et al. 
, argue that the T/C ratio does not vary in accordance with the changes across the day in muscle strength and power output. We were aware that a moderately-high carbohydrate diet should be consumed to maintain validity of any observed changes in the ratio of T/C 
. Our subjects consumed 55% of their daily calories in the form of carbohydrates and in addition we provided a snack prior to every trial which included 68 g of carbohydrate. Additionally, we measure growth hormone and found a trend for increased levels in the afternoon, also suggesting (like with the T/C ratio) a more favorable anabolic state. In conclusion, our blood hormonal data suggest that in the afternoon, despite an absolute decrease in the concentration of free testosterone, the larger decrease in serum cortisol results in a more favorable anabolic state. The contribution of this hormonal milieu to the increased muscle performance observed in the afternoon is currently under investigation.
In summary, the acute ingestion of caffeine (3 mg kg−1) reverses the morning reductions in maximum dynamic strength and muscle power output (2.5–7.0%), increasing muscle performance to the levels found in the afternoon (PMPLAC trial). These caffeine ergogenic effects seem to occur only in dynamic and not in isometric muscle contractions, for both the upper and lower body actions, and are independent of body temperature. Our electrical stimulation data, in conjunction with the plasma norepinephrine concentration, suggest that caffeine increases muscle strength and power through an effect directly in the muscle. In conclusion, caffeine ingestion in the morning, an ergogenic aid of common use among athletes, avoids the morning reduction in muscle performance due to circadian rhythm.