Caffeine Expectancy Does Not Affect Interval Running Performance, Physiological Responses, or Running Kinematics in Trained Runners: A Randomized Controlled Crossover Trial

Objectives: This study aimed to analyze the placebo effect of caffeine expectancy on interval running performance in moderately trained athletes. Methods: Participants were randomly allocated to one of two treatment sequences (placebo–control or control–placebo) and completed two 5 × 1000-m training sessions on an outdoor athletics track under two conditions: (1) ingestion of a placebo capsule described as containing caffeine (3 mg·kg−1), and (2) a control condition (no ingestion). Running performance (primary outcome), pacing strategy, heart rate, kinematic variables, perceived exertion (RPE), and side effects were recorded. Results: Fourteen participants were randomized and all completed both study periods and were included in the analyses. No statistically significant differences were observed in total session time between conditions (1021.50 ± 114.01 vs. 1017.23 ± 109.08 s, control vs. placebo, respectively; p = 0.270; ES = 0.31). Likewise, no differences were found in pacing strategy, heart rate, rating of perceived exertion, or running kinematics (all p > 0.05). However, participants reported greater feelings of activeness and higher ratings of selected caffeine-related side effects, including increased urine production and sleep disturbances, under the placebo condition. Conclusions: Believing that caffeine had been consumed did not improve interval-running performance, pacing strategy, physiological responses, or running kinematics in trained runners. However, caffeine expectancy altered the perception of some expected caffeine-related side effects. These findings provide limited evidence for an ergogenic placebo effect of caffeine during interval training and highlight the need for further research to identify the conditions under which expectancy effects may influence exercise performance.