The U.S. Army Fitness Training Handbook FM 21-20 is directed at leaders who plan and conduct physical fitness training.This manual covers issues relating directly to the development and maintenance of the five components of physical fitness: cardiorespiratory endurance, muscular strength, muscular endurance, flexibility, body composition.U.S. Army Fitness Training Handbook is an outstanding resource for anyone interested in improving physical health, mental health, and general well-being.