Effects of caffeine, methylliberine, and theacrine on vigilance, marksmanship, and hemodynamic responses in tactical personnel: a double-blind, randomized, placebo-controlled trial

Abstract

Background: Tactical athletes require fast reaction times (RT) along with high levels of vigilance and marksmanship performance. Caffeine has been shown to improve these measures but also results in increased blood pressure and jitteriness. Research on other purine alkaloids, such as methylliberine and theacrine, has suggested they do not increase blood pressure or jitteriness to the same extent, but their impact on tactical performance is unknown.

Methods: A between-subjects, randomized, placebo-controlled design was used to test the effects of placebo (PLA), 300 mg caffeine (CAF), and a combination of 150 mg caffeine, 100 mg methylliberine, and 50 mg theacrine (CMT) on RT and marksmanship along with hemodynamic and arousal measures following a sustained vigilance task in tactical personnel (n = 48). Following consumption of the supplement, participants underwent a 150-min protocol consisting of two rounds. Each round began with leisurely reading followed by a 30-min vigilance task before beginning two trials of movement and marksmanship tasks. Hemodynamics and felt arousal were assessed throughout the protocol. Composite Z-scores were calculated for overall performance measures at each timepoint, and mixed-effects models were used to assess differences in RT, accuracy, and composite Z-scores along with hemodynamics and felt arousal. An α-level of 0.05 was used to determine statistical significance, and Cohen's d was used to quantify effect sizes.

Results: A Group-by-Time interaction for vigilance RT (P = 0.038) indicated improvements for both CAF and CMT from round 1 to round 2 (P < 0.01) while PLA did not change (P = 0.27). No Group main effects or Group-by-Time interactions were found for movement or marksmanship performance (P > 0.20). Group main effects for systolic (SBP; P = 0.001) and diastolic blood pressure (DBP; P = 0.028) indicated higher SBP in CAF (P = 0.003, d= 0.84) and CMT (P = 0.007, d= 0.79) compared to PLA but only higher DBP in CAF (P = 0.025, d= 0.74). No Group-by-Time interaction or Group main effect was found for felt arousal (P > 0.16).

Conclusions: These findings suggest similar benefits on RT during a vigilance task between CAF, containing 300 mg caffeine, and CMT above PLA, though CAF resulted in slightly less favorable hemodynamic changes. This study is the first to provide data showing similar efficacy of combined caffeine, methylliberine, and theacrine compared to double the caffeine dose consumed alone on vigilance RT but without a significant rise in DBP above PLA in tactical personnel.

Keywords: Sports nutrition; dietary supplements; reaction time; stimulants.

Figure 1.

Overview of the experimental testing protocol. BP = blood pressure, FA = felt arousal, HR = heart rate.

Figure 2.

Changes in (A) RT, (B) correct decisions, and (C) composite Z-score relative to PLA from round 1 to round 2 of the experimental testing protocol between groups during the vigilance task. ‘a’ denotes a Group-by-Time interaction; ‘c’ denotes a Time main effect.

Figure 3.

Changes in RT from round 1 to round 2 of the experimental testing protocol between groups during the movement task. ‘c’ denotes a Time main effect.

Figure 4.

Changes in (A) 15-meter RT, (B) 15-meter accuracy, (C) 15-meter composite Z-score, (D) 15-meter RT, (E) 15-meter accuracy, and (F) 15-meter composite Z-score from round 1 to round 2 of the experimental testing protocol between groups during the marksmanship task. ‘c’ denotes a Time main effect.

Figure 5.

Changes in (A) SBP, (B) DBP, and (C) heart rate throughout the experimental testing protocol between groups. ‘b’ denotes a Group main effect; ‘c’ denotes a Time main effect.

Figure 6.

Changes in felt arousal throughout the experimental testing protocol between groups. ‘c’ denotes a Time main effect.