Manipulating athletes’ beliefs and expectations through deception has been shown to improve sport performance in artificial environments, but this has not been demonstrated systematically in real life competition where the effects could easily be masked. This study investigated the placebo effect of belief in a new performance-enhancing supplement on 5 km competitive time-trial performance. Fifteen competitive male endurance runners (mean±s: Age=33±5; Height=175±8.1 cm; Weight=71.0±7.2 kg, 5 km personal best: 19:13±1:13 minutes) volunteered to participate in four 5 km competitive time trials over a three week period (two experimental and two baseline). In experimental trials, participants ingested a placebo drink that was informed to improve 5 km endurance performance. Conditions were randomised between baseline and experimental. Magnitude inferences and 95% confidence intervals were used to determine if a positive expectancy could have a smallest worthwhile effect on 5 km endurance performance of 0.5%. Mean 5 km performance time-trial for the expectation of receiving a beneficial supplement was different from that of baseline (1195±79 s; 95% confidence interval 1152 to 1239 vs. 1216±81 s; 1171 to 1261 s, P<0.001). Relative to baseline, a very likely beneficial main effect of receiving a positive expectation (1.72±1.03, 95% CI 1.00±2.44%) was demonstrated. The results suggest that the placebo effect could be used in a real life competitive environment to improve 5 km running performance.
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