Medium bone structure with shoulders wider than the hips.
Developed athletic musculature.
Efficient metabolism; mass gain and loss both happen with relative ease.
Endomorph.
Body structure
Stockier bone structures with larger midsection and hips.
Carries more fat throughout the body.
Gains fat fast and loses it slow.
Naturally slow metabolism; potentially due to chronic conditions (e.g., thyroid deficiency, diabetes) but too frequently the result of a sedentary lifestyle and chronically-positive daily energy balance.
Ectomorph.
Body structure
More narrow shoulders and hips in respect to height.
Relatively smaller muscles in respect to bone length.
Naturally fast metabolism makes it difficult for many to gain mass.
Potentially indicative of disordered eating (e.g., anorexia, bulimia) when BMI is ≤17.