How Caffeine acts on the brain and body to improve physical performance.

INTERVENTION: This is a randomized, double‐blind, placebo‐controlled, crossover clinical trial to investigate the effects of caffeine mouth rinse on muscular endurance performance, maximum strength, corticospinal and muscular excitability, and brain activity. Experimental group: Nine men and nine women will receive four cups (25 milliliters) with 6% caffeine, 6% sodium benzoate, and deionized water. Control group: Nine men and nine women will receive four cups (25 milliliters) with 0.1% Theacrine, 0.5% potassium sorbate, and deionized water. The substances will be offered in a randomized manner over a period of 2 to 10 days. Participants will be instructed not to alter their diet throughout the experiment, not to perform intense physical exercise or consume alcoholic beverages in the 24 hours preceding the visit, not to ingest any product containing caffeine 8 hours before the test time, and to fast from food for 4 hours before the test. The study will consist of si Xvisits, the first two being for the familiarization of the participants with the test protocols, anthropometric measurements, and body composition, while the last four visits will be experimental in nature. The order of the experimental visits will be randomized and counterbalanced, and the participants' arrival order will define their sequence. Throughout the experimental visit, participants will rinse their mouths with the substance — without swallowing — for ten seconds at four different times: before the evaluation of corticospinal and muscular excitability; before the evaluation of resting brain activity; and before and during the physical test. In order to ensure the blinding of the substances, they will have similar odor, texture, and colors. Participants will be subjected to the following tests and protocols: maximal voluntary contraction test to measure maximum strength; isometric test at forty percent and eighty percent of the maximal voluntary contraction to measure muscular endurance through time to exhaustion; transcranial magnetic stimulation protocol to measure corticospinal excitability through motor evoked potential amplitude; muscular electrical stimulation protocol to assess muscular excitability through M‐wave amplitude; and an electroencephalography protocol to measure brain activity CONDITION: B01.050.150.900.649.313.988.400.112.400. Healthy Volunteers ; Humans PRIMARY OUTCOME: Physical performance is expected to be assessed through isometric tests at different intensities (40% and 80% of maximum voluntary contraction) until voluntary fatigue, comparing data collected before and after supplementation. It is also intended to measure the fatigue index through the maximum voluntary contraction test, using data obtained before and after the isometric tests. Additionally, corticospinal excitability will be evaluated via transcranial magnetic stimulation, while muscle excitability will be analyzed through electrical stimulation. Finally, muscle activation will be monitored by the electromyographic signal during the isometric tests, as well as brain activity, through electroencephalography INCLUSION CRITERIA: Healthy volunteers; Physically active; Age between 18 and 40 years old; Both genders. SECONDARY OUTCOME: M‐wave amplitude will be used to indicate muscle excitability. Thus, the efficacy of caffeine will be demonstrated by the increase in M‐wave amplitude after supplementation, and attenuation of M‐wave amplitude after the isometric test Maximum strength will be used to evaluate the fatigue index. Therefore, the efficacy of caffeine will be evaluated by an attenuation of the reduction in maximum strength when comparing pre‐ and post‐isometric test Motor evoked potential will be used to refer to corticospinal excitability. In this way, the increase in motor evoked potential will be used to indicate the efficacy of caffeine Muscle activation will be indicated by the root mean square of the electromyographic signal. Thus, the increase in the root mean square of the signal after caffeine use will be an indicator of the substance's efficacy Power spectral density will be used to evaluate brain activity. Therefore, the efficacy of caffeine will be determined by the increase in power spectral density Time to exhaustion will be used to assess physical performance. Thus, the efficacy of caffeine will be observed through a significant increase in time to exhaustion, to be measured in minutes and seconds through physical exertion tests (at 40% and 80% of maximum voluntary contraction) until the participant's voluntary fatigue ;