Fortitude's attributes are chosen to say that her work is standing rather than charging. The column, often broken and held in her arms, is the load bearing member of a building: the thing whose whole function is to remain where it is under weight. Where the column is shown broken, the reference is usually to Samson pulling down the temple, and the figure then carries both senses at once, the strength that holds and the strength that ends.
The lion is the other standard attribute and it supplies courage in its more obvious form, sometimes as a skin worn over the shoulders in a borrowing from Hercules. Giotto's fortitude in the Arena Chapel at Padua, painted around 1305, has neither: she stands behind a shield with a club, entirely defensive, arrows stuck in the shield and none of them through it. That is the doctrine drawn exactly. Aquinas, following Aristotle, holds that the principal act of fortitude is endurance rather than attack, because standing firm against an evil one cannot remove is harder than striking at one that can be removed.
The column is therefore the more faithful of the two attributes, and the lion the more popular. Cycles that want fortitude legible at a distance use the lion; cycles that want the argument use the column.
The atlas draws the column. It is the attribute that distinguishes fortitude from mere courage, which is the distinction the tradition is careful about.