Stamina is a character statistic in The Last Stand: Union City. It is used when running, jumping, and shoving. Stamina is replenished slowly when the player stands still or walks; the time it takes for Stamina to regenerate is dependent on the character's Fitness level. Sleeping also restores stamina.
Stamina decreases as you sprint
Stamina is also used to shove zombies (you can also shove while crouching)