An ectomorph is a lean body type identified by the American psychologist, William Sheldon, in his book, The Atlas of Men, first published in 1954. According to Sheldon, ectomorphs aren't just thin, but have a certain fragility to them. The ectomorph is said to have small shoulders and some muscle, but not enough musculature to have much definition in the body's shape. Ectomorphs tend to have long limbs and a straight, narrow body.
The ectomorph usually has little definition between his or her waist and hip areas. Although it's possible for ectomorphs to gain muscle through exercise, it may take longer than it would for a naturally muscular body type. Ectomorphs usually have a fast metabolism, which means that they process food energy quickly before it turns into excess fat. A problem with this characteristic can be that very thin ectomorphs who need to gain some weight in order to be healthier and less fragile may find it difficult to do so.
The other two body types in Sheldon's theory are the mesomorph and the endomorph. Endomorphs are the opposite of ectomorphs; they have naturally fuller bodies and tend to have short limbs. Endomorphs gain weight easily and have a more difficult time losing excess pounds than the other two body types. Mesomorphs gain muscle much easier than endomorphs or ectomorphs. While the mesomorph usually has a high fat-burning metabolism, he or she will gain weight easier than the ectomorph.
Ectomorphs must be sure to get enough calories each day to avoid becoming too thin and frail. In addition to having longer limbs than the other two body types, ectomorphs are said to have lighter bones and a fragile skeletal system. An ectomorph is usually tall and has a flatter chest than the endomorph and mesomorph.
Even if ectomorphs aren't tall, they tend to look that way due to their lean appearance. Stooped, as well as narrow, shoulders are attributes of the ectomorph body type. They usually have the narrowest hips of all of the body types.
Sheldon's body type theory includes personality characteristics. He views the ectomorph as the type most likely to be nervous, artistic, sensitive, and introverted. Mesomorphs are depicted as adventurous and risk-taking in Sheldon's theory, while endomorphs are said to be sociable as well as relaxed. There are many people who disagree with Sheldon's personality attributes for each body type, although they may agree with his physical analysis of his endomorph, ectomorph, and mesomorph types.