Lactic acid was a substance that naturally occurred in the body tissue and muscles of some species.
Shortly after the formation of the Galactic Empire, an Aurebesh data readout accessed by Clone Force 99 member Tech aboard the Marauder noted that a Zillo Beast at growth level 1 had a high adrenaline level but no lactic acid; while the lactic acid level of a Zillo Beast at growth level 6 was "-0".
During the Galactic Civil War, the human bounty hunter Beilert Valance had extensive cybernetics, including synthetic organs that filtered out any lactic acids and poisons from the system in order to increase his stamina. Shortly after the fall of the Galactic Empire, the city Induchron was endangered by debris from the falling Orbital One space station. As the human X-wing starfighter pilot Yrica Quell was shooting down burning debris in her X-wing, it made her body ache, as every motion released pain and lactic acid in her muscles.
During the First Order-Resistance War, Lexo Sooger, a Dor Namethian masseur at Zord's Spa and Bathhouse in Canto Bight, had chemically sensitive receptors in his eyes that that could detect lactic acid in body tissue; a vital perk for a masseuse. While giving the Heptooinian Joris massage at the spa, Sooger sensed tension on her back and his fingers felt a spot where there was less lactic acid than in the rest of the Heptooinian's back, but still work to be done.
- Star Wars: The Last Jedi: The Visual Dictionary