Robert Maxwell wrote:
What does that even mean? These terms are just descriptors. Some people are skinny and have a hard time putting on weight. Some people are large and put on weight very easily. Other people are of average build and their body's respond in a manner somewhere in between the other two. That's all those terms mean.
Look up William Herbert Sheldon sometime.
Frankly, it astonishes me that people would use those terms yet be totally oblivious to how they originated and the quack science that surrounds them.
I don't use the terms scientifically. I simply use them as a basis in which to understand how diet and exercise will affect different individuals.
It's kind of like the "4 humors", in psychology. The original meaning no longer has any value, but as a starting point to understand someone's psychological profile they help a little.
Like it's been said above, no one is going to fit perfectly into those 3 molds, but most people will find a lot in common with them.