You are running at a brisk pace that makes you very short of breath and your muscles start to burn. After a while you feel better and pick up the pace. It's called the "second wind", but it really is caused by a combination of:
- slowing down; and
- using lactic acid as a major fuel for your muscles.
HOW MUSCLES GET THEIR ENERGY
Your muscles use carbohydrates, fats and proteins for energy. More than 80 percent of the energy used to power muscles is lost as heat, so burning fuel instantly for energy would produce so much heat that it would burn and destroy your muscles. Your muscles convert food to energy in a sequence of chemical reactions, with each chemical reaction providing a little energy and a little heat. The main source of fuel for your muscles during exercise is called the Krebs Cycle and is located in the mitochondria, small chambers inside every cell in your body except red blood cells.
|Krebs Cycle (image from JustYourAverageJoggler.com)|
WHY MUSCLES BURN WHEN YOU RUN VERY FAST
Each chemical reaction is started by a different enzyme. Enzymes in muscles break down food in a series of reactions that release small amounts of energy at a time. Enzymes require oxygen to turn food into energy. When you exercise so hard that you can't get all the oxygen you need to break down food for energy, the reactions stop at a point where lactic acid accumulates in muscles and spills over into the bloodstream. This makes muscles acidic and it is the acidity that makes muscles burn and forces you to slow down.
HOW LACTIC ACID CAUSES SECOND WIND
The limiting factor to how fast you can run is the time it takes to move oxygen into your muscles. Muscles require less oxygen to turn lactic acid into energy compared to the amount of oxygen required to use carbohydrates, fats or proteins (Sports Medicine. June, 2006;36).
So lactic acid is the most efficient fuel for muscles since it requires less oxygen even than sugar. When you exercise as hard as you can, lactic acid helps you to go harder. When muscles produce lots of lactic acid, they use this chemical for energy which requires less oxygen. As soon as you slow down, you catch up on your oxygen debt and recover. So lactic acid is good for you. It helps you to exercise with less available oxygen and helps you to move faster.
More information from NY Times.