Suppose that a weaker person would push against a brick wall at maximum effort until he literally has no energy to move. It would take time X for this to happen.
A stronger person then push against a brick wall using the same amount of force that the weaker used. Since this stronger person is not using his maximum effort, will he necessarily tire out at time Y, where Y is greater than X?
If the above is true, then the stronger person exerted the same amount of force as the weaker person. However, he was able to exert this force for a longer period of time. Thus, by definition, the stronger person has more stamina?