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human muscular system anatomy muscle bodybuilder male Caucasian one person flexed muscles muscular strength health fitness exercise workout training gym weightlifting bodybuilding muscle building muscle growth hypertrophy sarcoplasmic hypertrophy myofibrillar hypertrophy fast twitch muscle fibers slow twitch muscle fibers type I muscle fibers type II muscle fibers red muscle fibers white muscle fibers skeletal muscle smooth muscle cardiac muscle muscle contraction muscle relaxation muscle function muscle physiology muscle anatomy muscle histology muscle pathology muscle diseases muscle injuries muscle pain muscle soreness muscle weakness muscle atrophy muscle loss