Endurance
There is a big difference between being strong and being fit. A muscular person who is not fit could lift something very heavy, but will tire easily and could not run a marathon. Meanwhile, someone who does not look very strong, but is fit, could keep on running long after the unfit person has become exhausted.
This attribute comes into play by reducing the amount of damage sustained in combat, and when testing to see if one can continue doing something strenuous like treading water or holding one's breath. It can also be a measure of one's pain threshold.
A character with a poor Endurance score is a delicate flower who tires easily, is constantly sick, and always gets nauseated on carnival rides. A character with a good Endurance score has an iron stomach, a high tolerance for pain, and an active immune system.
Examples of highly fit people include marathon runners, Greek heroes, soccer players, and masochists.
