From Slavemaker Wiki
Contests, etc. Below, we've listed the actions that will raise or lower this stat to help you plan out your actions.
- This is is the orange bar that appears in the Constitution stat bar (starting in 3.1.03). It reduces the Tiredness gains from other actions by a % equal to the Fitness number. Raising this reduces the amount of tiredness generated by other actions. As well as increasing the amount that tiredness is recovered by resting. The cap on this stat can increase past what your constitution is, but gets reset down to it, by certain actions. Also decreases with constitution as well. At max, provides a 50% bonus in decreasing fatigue cost, and recovery both via resting and energy potion. This will also affect how many breaks your slave will require after reaching a certain level of tiredness. At max, slaves will no longer need mandatory breaks and fall ill less frequently.