When you run very fast, you reach a point where you gasp for breath. If you keep on pushing the pace, you continue to feel short of breath, but after a while you start to feel fresh again and can pick up the pace.  That’s called “second wind.” Your discomfort was probably caused by lack of oxygen that caused a buildup of lactic acid in your bloodstream and muscles, and your recovery came when your body used the extra lactic acid that accumulated to power your muscles, which used less oxygen. Lactic acid requires less oxygen than sugar to power your muscles during intense exercise.

Shortness of Breath and Burning Muscles
When you run fast, your muscles use large amounts of oxygen to burn carbohydrates, fat and protein for energy. You get the power to move your muscles from each of several successive chemical reactions, called the Krebs cycle. If you can get enough oxygen to meet your needs, food you have eaten is converted all the way to carbon dioxide and water that you blow off from your lungs when you breathe out. However, If you run so fast that your lungs cannot supply all the oxygen that you need, the series of chemical reactions slows down, you start to accumulate large amounts of lactic acid in your muscles, and the lactic acid spills over into your bloodstream.
• You gasp for breath because you can’t get all the oxygen that you need to supply your muscles with energy.
• Your feel a terrible burning pain in your muscles because the excess lactic acid and carbon dioxide accumulate in your muscles to make them acidic which causes the burning feeling.

You developed a “second wind” because lactic acid requires less oxygen than sugar does to power your muscles, so the large amount of lactic acid that has accumulated in your blood and muscles then becomes a major source of energy to help you catch up on your oxygen debt. You reduce the excess lactic acid and carbon dioxide, so your muscles don’t hurt as much, you become less short of breath and you are able to pick up the pace and run faster.

Explanation of Second Wind
Researchers at the University of California in Berkeley showed that lactic acid requires less oxygen to power your muscles than carbohydrates, fats and protein (Cell Metab, 2018 Apr 3;27(4):757-785). The marked accumulation of lactic acid in your muscles causes muscles to use more lactic acid as their primary source of energy (J Sport Health Sci, Sept 2020; 9(5):446-460). By doing this, your muscles require less oxygen and you catch up on your oxygen debt (Nat Metab, 2020 Jul; 2(7): 566–571). This neutralizes the acidity in your blood, so your muscles stop burning and hurting and you can pick up the pace. You tell everyone that you suddenly got your “second wind,” but actually you started to use huge amounts of lactic acid, which requires less oxygen for energy, and your blood became less acidic so you were able to run faster again.

Endurance Training to Run Faster and Longer
In 1999, George Brooks at Cal Berkley showed that endurance training reduces blood levels of lactic acid when you are running as fast as you can because you are clearing it faster, even though you are producing as much or more. He explained that an “intracellular lactate shuttle” moves lactic acid from the cytoplasm, where lactate is produced, through the mitochondrial membrane into the interior of the mitochondria, where lactate is burned. In 2000, he showed that endurance training increased the number of lactate transporter molecules in mitochondria, to move lactic acid from the cytoplasm into the mitochondria (American Journal of Physiology-Endocrinology and Metabolism, Dec 22, 2021;322(1)).

Using Second Wind for 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. On May 6, 1954, Sir Roger Bannister ran the first under-four-minute mile. Since then almost 1700 runners have run sub-four-minute miles. These fast times are due to the fact that runners today run much faster intervals in training to build up higher levels of lactic acid. This stimulates their bodies to produce higher levels of enzymes to utilize more lactic acid for energy when they compete.

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.

My Recommendations
If you are an athlete, a second wind shows that your training has markedly increased your ability to break down lactic acid rapidly and use it for energy. If you are a casual exerciser, it is far more likely that you got your second wind just from slowing down when you felt short of breath or burning in your muscles.