Complex sentence: Natural bodybuilding is a bodybuilding movement with various competitions that has arisen from the assumption that bodybuilding traditionally includes use of banned performance-enhancing drugs, anabolic steroids, insulin, diuretics and human growth hormone.
Split into simpler sentences: Natural bodybuilding is a bodybuilding movement with various competitions that take place with bodybuilders who do not use performance-enhancing drugs. This includes anabolic steroids, insulin, diuretics and human growth hormone.