All testing was performed in the Strength and Power Laboratory at the Jerzy Kukuczka Academy of Physical Education in Katowice. The experiment was performed following a randomized crossover, placebo-controlled, double-blind design, where each participant performed a familiarization session with a 1RM test on one day, and two different experimental sessions a week apart. The study participants were divided randomly into two groups: the group that ingested caffeine (G CAF ) and the placebo group (G CON ). Sixty minutes before each exercise protocol, CAF (5 mg/kg b.m.) or a placebo (all-purpose flour) were administered orally. CAF was provided in the form of standard capsules containing 300 mg of CAF, as well as those specifically prepared for the research, containing 50 and 5 mg doses of CAF. The placebo was provided in identical capsules as CAF. All CAF and placebo capsules were manufactured by Olimp Labs. Prior to the start of the study, an interview was conducted regarding caffeine intolerance, or any other physiological reactions to cafe intake. None of the subjects reported side effects in response to the use of CAF. The participants were instructed to follow their general nutrition and exercise protocols. All subjects completed questionnaires regarding health history, habitual caffeine intake from coffee, tea, soft drinks, chocolate, sport drinks, and caffeine-containing OTC drugs one week prior to testing. They were selected on the basis of their habitual caffeine consumption (< 200 mg/week) as determined by the questionnaire. In addition, subjects were asked to refrain from heavy exercise and alcohol consumption for 48 h before testing and to abstain from consuming caffeine-containing foods and beverages for 7 days prior to, and throughout, the study.

Study participants

Twenty healthy strength trained men volunteered for the study after completing an ethical consent form (age = 25.7 ± 2.2 years, body mass = 87.3 ± 7.7 kg, bench press 1RM = 102,3 ± 8.5 kg; data presented as mean ± standard deviation [SD]) with a least one year of resistance training experience (2.3 ± 0.63 years; mean ± standard deviation [SD]). All study participants were over 18 years old. The participants were allowed to withdraw from the experiment at any moment, and were free of any pathologies or injuries. The study protocol was approved by the Bioethics Committee for Scientific Research, at the Academy of Physical Education in Katowice, Poland according to the ethical standards of the Declaration of Helsinki, 1983.

Procedures

Familiarization session and one repetition maximum test

A familiarization session preceded the one repetition maximum testing. These two objectives were separated by a 15 min recovery period. The participants arrived at the laboratory at the same time of day as the upcoming experimental sessions (in the morning between 09:00 and 11:00 am) and cycled on an ergometer for 5 min at an intensity that resulted in a heart rate of around 130 bpm, followed by a general upper body warm-up. Next, the participants performed 15, 10, and 5 repetitions of the bench press exercise using 20, 40, and 60% of their estimated 1RM with a 2/0/X/0 cadence. The participants then executed single repetitions using a volitional cadence with a 5 min rest interval between successful trials. The load for each subsequent attempt was increased by 2.5 kg, and the process was repeated until failure. Hand placement on the barbell was individually selected with a grip width on the barbell of 150% individual biacromial distance (BAD) [46, 47]. BAD was determined by palpating and marking the acromion with a marker, and then measuring the distance between these points with a standard anthropometric tape [48]. The positioning of the hands was recorded to ensure consistent hand placement during all testing sessions. No bench press suits, weightlifting belts, or other supportive garments were permitted.

Experimental sessions

Two testing sessions were used for the experimental trials. The grip width was marked on the barbell with thin strips of athletic tape at the start of each session. The general and specific warm-up for the experimental sessions was identical to the one used for the familiarization session. After the warm up, participants started the main examinations and performed one set of the bench press to momentary failure with a load of 70%1RM. The eccentric and concentric phases were performed at maximal possible velocity (X/0/X/0). All repetitions were performed without bouncing the barbell off the chest, without intentionally pausing at the transition between the eccentric and concentric phases, and without raising the lower back off the bench. The interval between two stages of the experiment was 7 days. During the experimental trials the participants were encouraged to perform at maximal engagement according to the recommendations by Brown and Weir [49]. A linear position transducer system “Tendo Power Analyzer” (Tendo Sport Machines, Trencin, Slovakia) was used for the evaluation of bar velocity. The Tendo Power Analyzer is a reliable system for measuring movement velocity and power [50,51,52,53,54,55,56]. The system consists of a velocity sensor connected to the load by a kevlar cable which, through an interface, instantly transmits the vertical velocity of the bar to a specific software installed in the computer (Tendo Power Analyzer Software 5.0). The measurement was made independently in each repetition and automatically converted into the values of power (max, mean), concentric velocity (max, mean) and eccentric velocity (mean). All familiarization and experimental sessions were recorded by means of a Sony camera (Sony FDR191 AX53). Time under tension was obtained manually from the recorded data using slow speed playback (1/5 speed). In order to ensure the reliability of manual data collection, four independent persons made the data analysis from the Sony camera. There were no significant differences in the TUT [s] between the data collected by 4 evaluators. All participants completed the described testing protocol. The procedures were performed in identical environmental conditions, with an air temperature of 19.2 °C and humidity of 58% (Carl Roth hydrometer, Germany).

The following variables were registered:

TUT - time under tension [s]

REP - number of repetitions [n],

P MAX - maximal concentric power [W]

P MEAN - mean concentric power [W]

V MAX - maximal concentric velocity [m/s]

V MEAN - mean concentric velocity [m/s]

VE MEAN - mean eccentric velocity [m/s]

Statistical analysis