The theory of somatypes developed by William Sheldon, Ph.D., M.D., in the 1940s, describes three classifications of human body types: ectomorph, mesomorph and endomorph. Knowledge of these body types and their associated traits has become a key aspect of most fitness, bodybuilding and fat loss programming.
The ectomorph has a linear physique that has the appearance of making an individual taller than he really is. People in this body type are thin, lack muscle mass and have small joints. Ectomorphs are not naturally strong and have difficulty gaining weight.
The mesomorph has large bones and well-defined muscles. This is the physique commonly associated with athletes. Arms and legs are well-developed, with the torso having a relatively narrow waist compared to the shoulders. Endomorphs gain muscle relatively easily.
Endomorphs are categorized by a round or pear-shaped physique, where much of the mass appears to be concentrated in the abdominal area. The endomorph's hands and feet are relatively small, and her upper arms and thighs are proportionally much larger than the lower parts of the arms or legs. Endomorphs gain weight easily and have a difficult time losing fat.
Sheldon believed the majority of humans belong to mixed categories rather than the three basic body types. Examples would be endo-mesomorphs, who are muscular but gain fat rapidly, and ecto-mesomorphs, who have small joints but are muscularly developed.