The term 'Muscleman Charles' refers to Charles ATLAS, an Italian-American bodybuilder and fitness expert who was born in 1892. He is best known for his iconic advertisement campaign which featured him as a '97-pound weakling' who was transformed into a muscular man after using his bodybuilding program. ATLAS' nickname was 'Muscleman Charles' and he is credited with popularizing the idea of physical fitness in the early 20th century.