Resistance training and power are significant components of Conditioning Programs in competitive sports. The ability to generate high levels of power has been indicated as a determinant of success in sports that require an optimal ratio of strength to velocity when performing a motor activity [ Optimization of resistance training and the pattern of adaptive changes related to the development of muscular strength and power have been the focus of interest of scientists from different fields of studies. In addition to training, diet and supplementation also have a significant effect on adaptation and post-exercise responses. To date, few supplements have been shown to have a direct energetic effect on physical capacity. Among them are caffeine, creatine monohydrate, sodium bicarbonate, and beta aniline. Although studies have confirmed the energetic effects of caffeine in many aspects, much controversy remains about its effects on the power generated by the upper limbs. The most frequently consumed dose of caffeine during research with athletes ranges from 2 to 9 mg/kg body mass, ingested in the form of pills or capsules 30 to 90 min before exercise. Mechanisms responsible for energetic effects of caffeine are linked to the impact on various tissues, organs, and systems, of the human body. In the central nervous system (CNS), CAF acts through interactions with adenosine receptors that influence the release of noradrenaline, dopamine, acetylcholine and serotonin and, consequently, increase muscle tension. Increased muscle activation can lead to a higher energy demand during exercise, thus leading to a faster depletion of energy substrates in muscle cells. Caffeine can stimulate calcium release from the sarcoplasmic reticulum and can also inhibit its reuptake. Numerous studies have discussed the effects of caffeine intake on human physical fitness. However, in regards to strength and power performance, the results of caffeine supplementation are ambiguous. Studies have shown faster neuromuscular conduction, increased motor unit activation, and increased number of repetitions (REP) in the bench press following caffeine intake 60 min before exercise, compared with a placebo. Furthermore, Green et al. failed to demonstrate significant differences in the number of REP between the placebo and CAF groups during the bench press and for the first and second set of the leg press exercise. No ergogenic effect of CAF on REP was also documented in a study by Grgic and Mikulic. Considering the effects of CAF on the level of maximal strength, the results tend to be inconclusive. Goldstein et al. demonstrated a significant effect of CAF supplementation on strength in a group of women. On the contrary, Astorino et al., Beck et al., did not find such an effect in a group of experienced strength athletes. One should emphasize that previous research on CAF supplementation and the level of strength and power, as well as the number of performed repetitions, considered resistance exercises performed at volitional or maximal velocity (V) of the entire movement, without a precise control of movement tempo during the concentric (CON) and eccentric (ECC) phases. Movement tempo has been defined by seconds which correspond to individual movement cadences (ECC/pause/ CON/pause). Value X for the concentric movement represents maximal movement velocity. Changes in movement tempo during resistance training impacts exercise volume, the level of generated force, muscle power, and the rate of muscle hypertrophy. The number of REP performed at a specific tempo impacts total time under tension (TUT) in a particular set. TUT provides accurate information about the duration of resistance effort for a set and for the entire training session. Wilk et al. demonstrated significant differences in TUT and REP between the 2/0/2/0 and 5/0/3/0, as well as 6/0/4/0 tempos despite using the same external load and exercise to momentary muscular failure. That study demonstrated, that a greater number of performed REP was not synonymous with longer TUT. Therefore, TUT is an indicator of the work performed by the muscles in both, the ECC and CON phases of the movement. TUT determines how long the resistance effort lasts regardless of the number of REP performed. Previous studies regarding the ergogenic effects of caffeine on exercise volume, number of performed repetitions or the level of generated power did not consider exercise cadence, nor particular movement phases and TUT. This may explain the divergent results of studies that have used caffeine intake before resistance exercise. Furthermore, studies have failed to analyze the effect of CAF during controlled or variable movement tempo or cadence and the effect of these factors on strength, muscle power, and training volume. It has not been demonstrated whether the ergogenic effect of caffeine concerns the entire movement or particular phases of the movement (CON and ECC).
Therefore, the main objective of this study was to assess the effects of caffeine intake on time under tension, the number of performed repetitions, and to determine the influence of CAF on muscular power (P) and movement velocity (V) in the CON phase. An additional goal of the study was to demonstrate the effects of CAF supplementation on mean velocity in the ECC (VEMEAN) phase of the bench press movement.
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