The present counterbalanced, double-blind, placebo-controlled trial examined the effects of two commercially available pre-workout supplements on concentric and eccentric force production during lower body resistance exercise. Both supplements contained 6 g of citrulline malate as the most prevalent ingredient. This compound has been reported to increase nitric oxide production [16, 17] and may decrease fatigue during exercise [18, 19], as well as increase training volume during resistance training . Other ingredients with varying degrees of performance-enhancing potential (creatine , betaine [21, 22], alpha-glyceryl phosphoryl choline [23, 24] and huperzine A) were also present in both supplements. A number of specific ingredients were only present in one of the two supplements. Perhaps the most notable differences were the presence of 300 mg of caffeine and 6 g of branched chain amino acids per serving in the caffeinated supplement and the absence of these ingredients in the non-caffeinated supplement. Caffeine has been reported to increase power output and exercise volume at doses ≥3–5 mg/kg body weight [25–30], similar to the dose administered in the present study. However, there is uncertainty whether caffeine’s effects on resistance exercise performance are meaningful, as well as evidence that there may be “responders” and “non-responders” to caffeine intake . The placebo in the present study contained only 5 kcal per serving and no active ingredients. Participants were not informed of the presence of a placebo condition and experienced similar increase in energy and focus and decrease in fatigue following acute supplement ingestion in all conditions. Despite this clear presence of the placebo effect and the lack of statistically different subjective responses between groups, effect size data indicate the possibility that the pre-workout supplements, particularly the caffeinated supplement, acutely enhanced some subjective measures to a slightly greater extent than the placebo. However, subjective responses during the exercise session were similar in all conditions: energy and focus were highest immediately after supplement or placebo ingestion and decreased progressively throughout the exercise session. Likewise, fatigue was lowest immediately post-ingestion, and fatigue and RPE increased progressively thereafter.
The incorporation of a 3-repetition maximal effort assessment prior to the exercise sets allowed for the examination of the effects of the supplements on maximal force production prior to fatiguing exercise. Despite the lack of statistically significant effects, examination of effect sizes and percent differences in force production, both of which were calculated relative to the placebo condition, allow for readily-interpretable metrics of possible supplement effects. These metrics revealed possible gender disparities between supplement conditions. Namely, force production in the NC condition was approximately 9% greater than placebo in males, but was 0 to 1.5% lower than placebo in females. In the C condition, force production in both males and females was 5 to 11% greater than placebo, although this was not statistically significant.
The results of the isokinetic exercise sets exhibited some similarities to the results of the max test, although differences were also apparent. Generally, significant differences in force production between conditions were not observed. In males, eccentric force production was lower during the later sets of the NC condition, as compared to the first set. This was not seen in the other conditions, although the apparently higher force production during set 1 in the NC condition likely influenced this result (i.e. high force production during the first set may have been at the expense of force production in later sets). In males, concentric force production was 5 to 20% higher during both supplement conditions as compared to placebo (d = 0.3 to 0.4 relative to placebo). However, in females, concentric force production appeared greater than placebo only during the first two sets in the C condition, and the overall effect of supplementation was minimal for both supplements (d = 0 to −0.1). In both sexes, neither supplement produced benefits relative to placebo in terms of eccentric force (d = −0.2 to +0.1).
It is important to emphasize that participants were not informed of the presence of a placebo condition in the present study. Additionally, due to the presence of three conditions, the counterbalanced and double-blind design, the similar appearance and presentation of the supplements and the similar subjective responses, we believe that participants remained unaware that one of the supplements was actually a placebo. Subsequently, since our effect sizes and percent differences for muscular performance were calculated relative to the placebo condition, they more closely reflect a true physiological effect than investigations in which participants are aware that they may be completing a placebo condition. Due to the strengths of study design, the present results may in fact underestimate the ergogenic effects of pre-workout supplements as they are typically utilized. An individual consuming a pre-workout supplement experiences not only any true physiological effects of the supplement, but also potentially powerful psychological effects. For individuals who believe in the effectiveness of pre-workout supplements, the alternative to consumption (i.e. abstaining from pre-workout ingestion) is unlikely to elicit the subjective increases in energy and decreases in fatigue elicited by the placebo in the present study. The real-world effect of a pre-workout supplement is the sum of psychological effects and any true physiological effects conferred by the active ingredients.
Due to the multi-ingredient nature of the pre-workout supplements employed in the present study, it is impossible to definitively state which compounds were responsible for the observed results. Additionally, very little is known about the specific interactions between different compounds contained in these supplements, despite some research support for the efficacy of individual ingredients. A consequence of these limitations is that results of separate investigations of pre-workout supplements may not be appropriately compared. While reports of improved exercise performance [4, 5, 7] or no benefits [6, 8–11] of pre-workouts have previously been presented, the differences in supplement formulations preclude appropriate comparisons.
Based on the nature of the mechanized squat machine used in the present study, eccentric and concentric force production should not be considered unrelated entities. Particularly due to the prolonged eccentric and concentric phases of each repetition (i.e. 4 s for each phase during each repetition), the effort expended in producing force during the concentric phase of a repetition likely had a direct impact on the ability to produce force during the eccentric phase of the subsequent repetition. Each set also affected subsequent sets such that increased force production in an earlier set potentially compromised force production in later sets. Additionally, although a resistance-trained population was utilized in the present study, none of the participants had prior experience with the mechanized squat device prior to familiarization. The device allowed for strict control of body positioning throughout each repetition, as well as the acquisition of a greater quantity of data than traditional strength testing. However, since the movement pattern of the device was fixed and the range of motion was prescribed by study investigators, the exercise should not be viewed as perfectly equivalent to a free-weight back squat, although it was designed to imitate one. In reality, each individual performs squats within the constraints of his or her anatomy and using his or her own unique combination of lifting form and cadence. Due to the controlled cadence of the device, as well as the inability of participants to alter the movement pattern, the effects of the dietary supplements on muscular performance during free-weight squats or other resistance exercises could potentially differ from the results observed in the present study.