Adrenaline

The Adrenaline powerup in Doom 3 resembles a large hypodermic needle. Once taken, it is immediately used, and the sound of an accelerated heartbeat is heard. For the next 60 seconds, the player will have infinite stamina, allowing him to run continuously.

Whenever the player is in Hell (this includes the Hell level and the final fight with the Cyberdemon), he/she is able to sprint for indefinitely long without exhausting. This is an effect caused by Hell itself (dubbed "the Hercules complex" by the UAC scientists) and is visually indicated by flames shooting across the stamina gauge.