The Stamina Implant allows the player to increase his Stamina Stat by 10. A voucher must be given to Gerard to receive the upgrade. The player receives an implant from Macil each time a mission is completed.
A side note is that in Strife-supported versions of ZDoom, the player can give themselves the Stamina Implant with the GIVE console cheat. Although the player's punch is unaffected by the stat, their natural health cap increases whenever given one, and this works in Doom, Heretic and Hexen games in ZDoom in addition to Strife itself.
An additional note is that in ZDoom, spawning a Stamina Implant voucher with "summon upgradestamina", and running over it automatically adds 10 to the stamina/health cap, unless the player is already at the maximum for their stamina stat. This also works in Doom, Heretic and Hexen games in addition to Strife itself. The console command "give upgradestamina" automatically gives the item without having to pick it up.