a human physical type (soma- totype) that is marked by greater than average muscular development, as determined by the physique classification system developed by American psychologist W.H. Sheldon. Although the Sheldon (Sheldon, William) system of classification does not make absolute distinctions between types, a person is classed as a mesomorph if mesomorphy predominates over endomorphy and ectomorphy in his body build. The extreme mesomorph has a square, massive head; broad, muscular chest and shoulders; a large heart; heavily muscled arms and legs; and minimal body fat. He tends to develop muscle easily. His muscular development can usually be distinguished from that of one who has developed his muscles through body-building exercises. Compare ectomorph; endomorph.