Second wind means that when you run very fast, you reach a point where you gasp for breath and your muscles burn so much that you feel like you have to slow down, but you try to keep on pushing. After several seconds, you feel recovered and pick up the pace.
We used to think that second wind meant that you slowed down to allow yourself time to recover from your oxygen debt, but research from the University of California in Berkeley gives another explanation (Fed. Proc, 45: 2924-2929, 1986). After you slowed down briefly, you felt better and could pick up the pace because the same lactic acid that caused the burning in your muscles and shortness of breath could be used as an efficient source of energy for your muscles. Since lactic acid requires less oxygen to power your muscles than most other sources of energy, you catch up on your oxygen debt, the concentration of lactic acid in your muscles drops, the burning and gasping lessens, you feel better and you can pick up the pace.
Your muscles get their energy from each of several successive chemical reactions, called the Krebs cycle. The Krebs cycle requires large amounts of oxygen to burn carbohydrates, fat and protein for energy. If you can get enough oxygen to meet your needs, food you have eaten is converted to energy to power your muscles and finally carbon dioxide and water that you blow off from your lungs when you breathe out. However, if you run so fast that your muscles do not get all the oxygen that they need, you develop an oxygen debt that slows down the successive reactions of the Krebs cycle. This causes lactic acid to accumulate in your muscles to make them acidic. It is the acidity that makes muscles burn, and you gasp for air, trying to get more oxygen.
The muscle burning and shortness of breath caused by the accumulation of lactic acid forces you to slow down. However the lactic acid that accumulates in muscles when you run very fast actually is the first choice of fuel for your muscles when you are running so fast that you can’t get all the oxygen that you need (American Journal of Physiology- Endocrinology and Metabolism, June 2006). Your muscles switch to burning more lactic acid for energy, you need less oxygen and then you pick up the pace. You tell everyone that you suddenly got your “second wind”, but actually:
• your body markedly increases its use of lactic acid for energy,
• this requires less oxygen,
• your blood becomes less acidic,
• the burning and shortness of breath lessen, and
• you are able to run faster again.
Of course when you keep on pushing the pace, you can again accumulate large amounts of lactic acid in muscles, which will make them burn and hurt again.
How You Can Use This Information to Have Greater Speed and Endurance
Since you can move faster in races by increasing the rate of forming and removing lactic acid, you must train intensely enough to accumulate large amounts of lactic acid in your body. Exercising with high blood levels of lactic acid stimulates your body to make more enzymes that turn lactic acid into a source of energy and strengthens your heart to be able to pump more oxygen to your exercising muscles. That’s why virtually all athletes in sports that require speed over distance use some form of high intensity interval training.
You also need to eat a lot of carbohydrate-rich foods (fruits and vegetables) to be able to increase the meager amount of sugar that you can store in your muscles and liver. Carbohydrates are the source of the sugar, glucose, that is converted to the energy efficient lactate during exercise. Lactate is used as a very oxygen efficient fuel during exercise and also helps replenish liver sugar stores during exercise.