In human biology and sport sciences , humans are classified according to three body types:

1. People with an ectomorphic body type are naturally thin and lightly build with flat chest and poorly muscled limbs. They can usually eat as much as they want without gaining fat. Extreme ectomorphs are commonly described as "skinny" individuals.
2. People with an mesomorphic or athletic body type are husky, with a hard and muscular body. They are characterized by a large chest, long torso, solid muscle structure. Such individuals find gaining muscle mass much easier than ectomorphs do.
3. People with an endomorphic body type easily gain weight in the form of fat or muscles but often have to work very hard to lose fat.

Almost no one is purely one body type, but elements of all three types are present instead. These body types were first outlined by William Sheldon 's research into possible relationships between body type and personality traits in individuals.

Ectomorph Training

So called ectormorphs tend to have rapidly fast metabolisms (generally unlinked to hyperthyroidism) which keep them naturally "skinny". Although they have a difficult to gain weight and thusly muscle, an ectomorph will find it easier to stay lean than a mesomorph or endomorph. In order to compensate for the metabolism a vastly greater intake of protein, carbohydrates, and calories is required. For those looking to accelerate mass building, 2 grams of protein per pound of bodyweight and 4 grams of carbohydrates per pound of bodyweight is a common formula used by bodybuilders. It should be noted changing to such a diet should be done gradually allowing the body to accomodate the change. In order to gain muscle mass a higher weight with lower repetitions is advisable, for example 6-10 repetitions for 3-4 sets of each exercise (proper form should be adhered to for best results and safety). Larger compound movements and 72 hours rest between training a muscle group is recomended.

Mesomorph Training

The mesomorph will find it easy to build muscle mass but will have to include a variety of exercises in their program so that the muscles develop proportionately and shapely rather than bulky. A combination of heavy power moves and a variety of shaping exercises lead to better quality, proportion and symmetry of the physique. The mesomorphic type responds well to training so super-long sessions are not needed. A balanced diet with a good amount of protein and an even food energy level help to achieve this.

Endomorphs

Sheldon defined endomorphs by the following traits:

• Overdeveloped digestive system, Sheldon likened this to an extremely-powerful boiler room
• Preponderance of body fat, especially around the abdominal area
• Rounded appearance
• Heavy to medium bone structure
• Smooth body contours
• Soft and smooth skin
• Gains weight easily
• Has trouble keeping weight off
• Can gain muscle fairly fast

See also anthropometry.