Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C₃H₆O₃. It has a hydroxyl group adjacent to the carboxyl group, making it an alpha hydroxy acid (AHA).

In solution, it can lose a proton from the acidic group, producing the lactate ion (to be specific, an anion due to being negatively charged with an extra electron) CH₃CH(OH)COO⁻. Compared to acetic acid, its pK_a is 1 unit smaller, meaning lactic acid deprotonates ten times as easily as acetic acid does. This higher acidity is the consequence of the intramolecular hydrogen bridge between the α-hydroxyl and the carboxylate group, making the latter less capable of strongly attracting its proton.

Lactic acid is miscible with water or ethanol, and is hygroscopic.

Lactic acid is chiral and has two optical isomers. One is known as L-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(−)-lactic acid or (R)-lactic acid.

Lance Edward Armstrong (born Lance Edward Gunderson on September 18, 1971) is an American former professional road racing cyclist who won the Tour de France a record seven consecutive times after having survived testicular cancer. He is also the founder and chairman of the Lance Armstrong Foundation for cancer research and support. He last rode for UCI ProTeam Team RadioShack, a team he helped found.

In October 1996 he was diagnosed as having testicular cancer with a tumor that had metastasized to his brain and lungs. His cancer treatments included brain and testicular surgery and extensive chemotherapy, and his prognosis was originally poor. He went on to win the Tour de France each year from 1999 to 2005, and is the only person to win seven times having broken the previous record of five wins shared by Miguel Indurain, Bernard Hinault, Eddy Merckx, and Jacques Anquetil.

In 1999, he was named the ABC Wide World of Sports Athlete of the Year. In 2000 he won the Prince of Asturias Award in Sports. In 2002, Sports Illustrated magazine named him Sportsman of the Year. He was also named Associated Press Male Athlete of the Year for the years 2002–2005. He received ESPN's ESPY Award for Best Male Athlete in 2003, 2004, 2005, and 2006, and won the BBC Sports Personality of the Year Overseas Personality Award in 2003. Armstrong announced his retirement from racing on July 24, 2005, at the end of the 2005 Tour de France but returned to competitive cycling in January 2009 and finished third in the 2009 Tour de France. He confirmed he had retired from competitive cycling for good on February 16, 2011.