Placebo effects and physical performance: central and peripheral mechanisms investigated through different experimental designs

Abstract

Double-blind randomized clinical trial (RCT) controlled with placebo is a standard methodological design for the investigation of the real effects of active pharmacological substances. In this design, participants are informed that both the active and placebo substances have a 50% chance of occurring, so that the participants' expectation regarding the ingested active substance is not controlled. This approach induces an important risk of bias, mainly when the active substance pharmacological effects are potentiated through the participants' positive expectation regarding the substance. Such risk of bias may be even greater in the presence of active substances-induced side effects, thus making it difficult to blind participants from the substance being ingested. One example is caffeine, a substance used to improve physical performance. One alternative to RCT is the use of designs that control for the participants' expectations, such as in the placebo perceived as caffeine design. However, this type of design is rarely used when investigating the mechanisms underlying active compounds, likely due to the lack of well-controlled evidence of its effectiveness to control the risk of bias of blinding participants. This study will compare the caffeine placebo effects on central and peripheral nervous system, muscle and torque between ECR and placebo perceived as caffeine designs. After familiarization and control sessions, participants (n = 18 men) will carry out 2 experimental sessions within each design (RCT and manipulated expectation) ingesting placebo in all sessions. A session ingesting caffeine perceived as caffeine will be the positive control. Measurements of cortical activation (EEG), corticospinal excitability (wave V), excitability (wave M) and muscle activation (EMG) and muscle torque will be obtained during maximal and submaximal voluntary contractions before and 45 minutes after the ingestion of the capsules. (AU)