Lactate is a chemical that builds up in your muscles during hard work. When you "feel the burn" during intense exercise, you can bet that your muscles are bathing in lactate. You will also feel the need to slow down and end the discomfort.
But what is behind the burn? Muscle cells get most of their energy from a substance called adenosine triphosphate (ATP). ATP gives muscle cells the ability to contract and propel you toward the finish line. While your muscles are using ATP, other chemical reactions are making more ATP so that your muscles won't run out of energy. The body stores energy in substances like fat and glucose, and chemical reactions break down those substances to release ATP [source: National Skeletal Muscle Research Center].
When you're moving at an easy pace, muscles get most of their energy from fat. Fat is an excellent source of stored energy. The human body is very efficient at using fat for energy, and muscles can go for a very long time without tiring out. The catch is that the process of turning fat into energy is not very fast and it takes lots of oxygen [source: National Skeletal Muscle Research Center].
So what happens if you need lots of energy fast -- like during a sprint? Your body can't use fat for quick energy, so it uses glucose, both in your blood and stored within your muscles as glycogen. Muscle cells can use oxygen to change glucose into energy. This process is called aerobic metabolism, and it is very efficient. Muscles can also break down glucose without using oxygen, which is less efficient but lightning-fast. This is called anaerobic metabolism and lactate is one of its by-products [source: National Skeletal Muscle Research Center].
When you're working at maximum capacity and your muscles are producing lots of lactate, you are at your lactate threshold. You know what it feels like, but experts disagree on the details. Scientists used to think that lactate caused muscle fatigue, but now they aren't so sure -- some scientists think the "I can't move another inch" feeling might be caused by other substances that are released around the same time [source: Westerblad et al]. No one is really sure exactly what causes the burn either, but lactate is part of the picture. One thing trainers agree on is that when you train hard enough to push your lactate threshold higher, you can make it to the finish line faster. That is the goal of lactate threshold training.