What is lactic acid (lactate)?

In one line

Lactic acid (lactate) is a substance made when muscles split glucose for energy, and the body uses it again as fuel.

What is lactic acid (lactate)?
The stuff people blame for sore muscles
After a hard workout you often hear that 'lactic acid built up and made your muscles sore.' For a long time, lactic acid was seen as a waste product that tires muscles out. Here is what it really is and what it does in the body, step by step.
In one line
A substance made when glucose is split
Lactic acid is a substance made when cells such as muscle split glucose to get energy. Glucose is the sugar that carbohydrates become after digestion. Lactic acid is not thrown-away waste; the body uses it again as fuel.
The name
In Korean it means 'milk acid'
An 'acid' is a substance with a sour taste, like vinegar. When bacteria change the sugar in milk, sour lactic acid forms, and this is exactly how yogurt is made. That is why its Korean name means 'acid that forms in milk'.
Where it forms
On one route that refills ATP
Muscles run on 'ATP', an energy substance often called a 'charged battery', and refill it as fast as they use it. One refill route is splitting glucose. The pieces left after splitting glucose either move on to the oxygen-using route to be burned further, or turn into lactic acid.
When it forms
Even at rest, and more the harder you go
Lactic acid does not form only when oxygen runs short. It forms in small amounts all the time, even at rest when there is plenty of oxygen. The harder you exercise, the faster muscles split glucose, so more lactic acid forms.
Where it goes
During long exercise, most is burned again as fuel
Lactic acid made in a muscle is either used right there or carried by the blood to the heart, brain and other muscles. There it is burned with oxygen and becomes fuel for making ATP again. During long exercise, about 3 out of 4 parts of the lactic acid made was used as fuel this way.
Lactic acid in the liver
Turned back into glucose
Lactic acid that travels in the blood to the liver is made back into glucose. This glucose goes out into the blood and is used again around the body. Lactic acid is the main raw material the body uses when it makes new glucose.
Lactate threshold
Where blood lactic acid starts to climb fast
As you raise exercise intensity bit by bit, blood lactic acid at first stays close to resting levels. Then, from a certain intensity, it starts rising quickly; this boundary is called the 'lactate threshold'. People who ran faster at their lactate threshold had better distance-running times.
Common misconception
Is next-day soreness caused by lactic acid?
No. The idea that lactic acid builds up and causes soreness spread widely, but research overturned it. Running on flat ground raised blood lactic acid yet caused almost no soreness the next day, while running downhill did not raise lactic acid but did cause soreness.
Summary
Made when glucose is split, then used again as fuel
Lactic acid is a substance made when muscles split glucose; most of what forms during exercise is used again as fuel or turned back into glucose in the liver. Next-day muscle soreness is not caused by lactic acid building up. Related terms: ATP, glucose, lactate threshold.