Effects of Nattokinase Supplementation on Strength Recovery and Muscle Damage Biomarkers Following Exercise-Induced Muscle Injury: A Pilot Study

This pilot study aims to investigate whether 4 weeks of nattokinase supplementation (2,000 FU/day) affects recovery following exercise-induced muscle damage in healthy adult men. Approximately 12 to 16 participants aged 18 to 40 years will receive either nattokinase or a matching placebo during the supplementation period. After supplementation, participants will complete an exercise protocol designed to induce temporary muscle damage. Recovery will be evaluated using measures of muscle strength and exercise performance, muscle soreness, blood lactate, muscle oxygenation, and blood biomarkers related to muscle damage, inflammation, and oxidative stress. Measurements will be performed before and after the exercise-induced muscle damage protocol, including follow-up assessments during the recovery period. The study is intended to provide preliminary information on the potential effects of nattokinase supplementation on muscle recovery after strenuous exercise.

Inclusion Criteria: * Male participants aged 18 to 40 years. * Regular exercise habits, defined as exercising at least 3 times per week, including at least 1 resistance-training session per week, with a stable exercise routine maintained during the previous 3 months. * No cardiovascular, metabolic, neurological, or musculoskeletal diseases that would prevent safe participation in the exercise testing, as assessed by the research team. * Not currently taking medications or dietary supplements that may affect blood coagulation. * No known allergy to natto or soy products. Exclusion Criteria: * Current use of anticoagulant or antiplatelet medications. * History of bleeding disorders or abnormal blood coagulation. * Major surgery within the previous 3 months. * Known cardiovascular, renal, or other major chronic diseases. * History of severe lower-extremity musculoskeletal injury, such as fracture, ligament injury, or meniscal injury, when judged by the research team to make exercise testing inappropriate. * Current acute inflammation, infection, or another physical condition considered unsuitable for participation in exercise. * Participation in high-intensity or systematic plyometric training within the previous month. * Students currently supervised by the principal investigator or individuals with a relevant conflict of interest with the study.