Lactic Acid

This is a waste product that is produced by your muscles during prolonged activity.  In particular, it is produced during anaerobic activities.  Technically, it is produced during the anaerobic glycolysis of glycogen.  Blah, Blah, Blah.

You'll know you've got lactic acid build-up when you start to feel a burning sensation within your muscles during intense, prolonged exercise.  This waste product is associated with diminishing performance.  Athletes often notice that, in addition to the burning, their arms, legs, etc. begin to feel heavy and start to lack the dexterity and strength that existed when 'fresh'.

Lactic acid needs time to dissipate from the muscles and is removed more quickly through the practice of cooling down and warming up.  Increasing endurance through training will reduce the buildup of lactic acid per unit of time spent exercising.  Proper hydration and nutrient replenishment can also help.