Extra Strength is a Power Perk that appears in Futuristic Zombies. The perk is a futuristic successor to Mule Kick.
The icon is represented by a skull with two weapons underneath it. When in its pro form, the icon gains a bright red glowing outline.
Overview
Much like Mule Kick, it allows the player to hold more than two weapons at a time. In the case of Extra Strength, this allows the player to now hold four weapons, but restricts them to only have one Wonder Weapon only. It also increases the player's ammo reserves as well.
After killing 80 Zombies with the third or fourth weapon, the perk will become Extra Strength Pro. In its pro form, lethals have increased reserves, allowing players to hold up to 10 lethals, and ammo reserves now stack with reserve bonuses granted from Pack-A-Punched weapons.