You know... when I was in High School, even though my muscles were a bit less toned I weighed more than all of the other guys. I think endurance is based on the weight of your muscles, not their size... I guess it'd be water in the muscles, which has chemicals in it from the body, because muscle isn't separate water and solid... it's a mixture of both. If you tell a bodybuilder to lift something heavy... they may be able to, but if you tell them to move their arm up and down many times without any weight on it... why do their arms get tired? I could have probably walked from New York to another state, fast, for a day without stopping, though that's a bit of an overstatement. The size of your arms and legs and tone determines strength, but their weight compareable to their size is endurance. Since my legs support my upper body and it weighs on them... I was also carrying that weight, but the thing is that my joints and bones took most of it.