lactic acid

noun1

1. •
   2-Hydroxypropanoic acid (CH₃CHOHCO₂H), a syrupy liquid, soluble in water; found in milk, wine and many fruits; used as a food additive and in many industrial applications.

   • It was [Roger] Federer's ability to pull him around the court that put the lactic acid and debilitating weight in those legs and he should not have been allowed the luxury of his trainer's help. — 2012