https://www.scientificamerican.com/article/your-thoughts-can-release-abilities-beyond-normal-limits/#

“Recent research on placebos gives us clues about the mechanisms by which our mental activity causes these effects. In a conceptual replication of earlier work, Antonella Pollo and colleagues asked people to lift a certain amount of weight before and after drinking caffeine at high doses. The liquid in fact contained no caffeine, but the weight was secretly reduced after people drank it. That way, people learned to associate the liquid with less fatigue. Later, when people lifted the original weight after drinking the liquid, they experienced less fatigue. It seems that a central neural governor of fatigue suppressed the fatigue response.”