Individualization of caffeine supplementation in sports based on the analysis of heart rate variability (HRV)

INTERVENTION: In the study, caffeine consumption is investigated at a dosage of 3 (group A) and 6 mg/kg body weight (group B). Caffeine will be administered orally in a single dose, based on body weight, in clear cellulose capsules (hydroxypropylmethylcellulose 100%). Participants will be instructed to avoid foods containing caffeine for 24 hours prior to testing. All measurements will be taken between 8:00 am and 3:00 pm. The caffeine dose will be randomly selected. All measurements will be performed in university laboratories. After ingestion of caffeine, participants will be continuously supervised until the end of the measurement and for the following 24 hours. CONDITION: Diet and Nutrition ‐ Other diet and nutrition disorders Exercise performance; ; Exercise performance Musculoskeletal ‐ Normal musculoskeletal and cartilage development and function PRIMARY OUTCOME: Changes in heart rate variability measured with a heart rate monitor.; [rest ‐ 0‐20 minutes continuous measurements HRV ; after administration of caffeine/placebo‐ 20‐80 minutes continuous measurements HRV.] Changes in muscle tension of the biceps femoris assessed with tensiomyography (TMG).[30 and 60 minutes after ingestion of 3 and 6 mg of caffeine or placebo; ; ] SECONDARY OUTCOME: .Changes in plasma caffeine concentrations.[30 and 60 minutes after ingestion of 3 and 6 mg of caffeine] INCLUSION CRITERIA: Healthy athletes and healthy volunteers