In the 1920s, experiments suggested that the accumulation of lactic acid in the bloodstream interfered with a person’s ability to exercise by causing muscles to stop contracting. However, Carl and Gerty Cori won the 1947 Nobel Prize for discovering the “Cori Cycle,” in which lactic acid produced by reduced oxygen levels from intense exercise may be good for exercisers when it travels from muscles to the liver, where lactic acid is converted to the sugar, glucose, to be used by muscles to supply extra energy (Clinical Nutrition, May 1, 2021;40(5):2988-2998). In the 1980s George Brooks, a professor at Cal Berkeley, showed that intense exercise training causes mitochondria in muscles to grow larger and take up increased amounts of lactic acid to provide exercisers with increased sources of energy so they can exercise longer and harder (Cell Metab, Apr, 2018;27(4):757-785; Med Sci Sports Exerc, Feb,1985;17(1):22-34.

Why World Records Keep Improving
On May 6, 1954, Roger Bannister ran the first sub-four-minute mile. In the 66 years since that world record was set, more than 1600 men have run sub-four-minute miles (Track & Field News, May 2019). The current world record is 3:43.13. The incredible improvement in world records in endurance events in all sports is mostly due to changes in training techniques, with workouts that are now so intense that they cause a lot of muscle fiber damage and the athletes have to spend more days doing slower recovery workouts. For example, 50 years ago, endurance runners would run fast interval workouts twice a week and also race or run long distances fast. That meant that they usually ran intervals on Tuesday and Thursday and a long run or race on weekends. They would allow only one day to recover from an interval workout. Today the interval workouts are so brutal that the athletes usually allow at least two days for slow recovery workouts after each intense day, so they are doing more intense workouts less often.

In intervals, you run a short distance very fast, slow down until you recover your breath, and then repeat alternating the very fast runs followed by much slower recovery runs until your muscles feel stiff and sore. For example, a top runner may run a quarter mile in 60 seconds, followed by a slow jog for an eighth of a mile and repeat it 12 or more times. That means that they are training at 4-minute-mile race pace.

Lack of Oxygen Limits How Fast You Can Go
The limiting factor to how fast you can move over distance is the time it takes for oxygen to go from your red blood cells into your muscles. When you run fast, your muscles use large amounts of oxygen to burn carbohydrates, fat and protein for energy. You get most of 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. The lactic acid and carbon dioxide make your blood acidic and the acid burns your muscles to make them feel hot and painful. (Your non-exercising muscles do not burn because they are not accumulating large amounts of lactic acid inside their cells). You then try desperately to breathe hard enough to get rid of the acidity in your blood by taking in enough oxygen to get rid of the excess lactic acid and blow off the excess carbon dioxide that is accumulating in your blood.

Competitive Athletes Must Run Up Severe Oxygen Debts in Training
Running fast enough to cause severe oxygen debts in training helps you to:
• tolerate higher blood levels of lactic acid,
• strengthen your heart and lungs so you can bring in more oxygen to your muscles, and
• help your muscles to convert lactic acid to be used as energy to fuel your muscles.
The marked accumulation of lactic acid in your muscles during training causes muscles to use more lactic acid as their primary source of energy in races. Lactic acid requires less oxygen than almost anything else to power your muscles, so by doing this, your muscles require less oxygen and you catch up on your oxygen debt. This neutralizes the acidity in your blood, so your muscles stop burning and hurting and you can pick up the pace.

Getting Your Second Wind
The muscle burning and shortness of breath caused by the accumulation of lactic acid forces you to slow down. 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 Cal Berkeley gave another explanation (Fed Proc, 1986;45:2924-2929). Soon after you slow down briefly, you feel better and can pick up the pace because the same lactic acid that caused the burning in your muscles and shortness of breath can 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 and the muscle burning and gasping for breath lessens, so you feel better and can pick up the pace. Of course, when you keep on pushing the pace, you can again accumulate large amounts of lactic acid in your muscles, which will make them burn and hurt again (Am J Physiol-Endocrin Metab, June 2006).

My Recommendations
This knowledge about intense interval training increasing your ability to tolerate and use lactic acid for improved performance applies to all sports requiring speed over distance — swimming, cycling, cross-country skiing and so forth. Since you can move faster in races by increasing the rate of forming and removing lactic acid, you should 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 enlarge your mitochondria, 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 is why virtually all athletes in sports that require speed over distance use some form of high intensity interval training. See Lactic Acid is Good for You: Why Everyone with a Healthy Heart Should Do Interval Exercise