Placebo effect of carbohydrates on muscle force production in male collegiate athletes: The role of misinformation and feedback-reinforced cues

A carbohydrate placebo (CHO-PLA) is a non-metabolic substance guised as carbohydrate. When information about the treatment was not disclosed, CHO-PLA enhanced strength performance through the sweetness cue, which psychologically strengthened participants' belief in its efficacy. However, the effects of CHO-PLA when participants are misinformed that they are consuming actual carbohydrates, and the role of additional cues (visual reinforcement), remain less understood. Active males (n = 45) were assigned to either misinformed (MIS), misinformed with feedback reinforced (MIS+R) or control groups. Isokinetic force production during knee extension and flexion was measured before (pre-test) and after (post-test) CHO-PLA ingestion. A fatiguing task involving a single set of repetitions to failure for knee extension and flexion, respectively was conducted a week later to assess rate of force decline. After CHO-PLA ingestion, the MIS and MIS+R groups showed a 5.7 and 6.9 % increase in knee extension strength, respectively (p < 0.05). For flexion, only the MIS+R group demonstrated improvement (4.9 %; p < 0.05). Additionally, the MIS+R group showed the smallest decline in force (rate constant) during the fatiguing task, which corresponded to the highest total work done. These findings suggest that force production is greater when feedback reinforcement is provided (MIS+R). The findings also indicate that the placebo effect is strongest when all three cues, sweetness, verbal information, and visual feedback are present. In summary, the observed improvements in force production appear to be behavioral; when participants were misinformed and reinforced about the treatment received, their belief in having consumed carbohydrate was strengthened, leading to enhanced strength performance.