Most people may think that the reason you begin to fatigue is that you go “anaerobic” or reach an intensity when there is not enough oxygen for your running muscles to contract. That is, that you reach an effort or intensity levels when the demand for oxygen out-strips the available supply of oxygen. This is the fabled concept of “oxygen debt,” and while a convenient way to think of the process, this state doesn’t really ever occur. You simply don’t “go anaerobic”. Watch this quick video.
Compounding this misunderstanding is another non-scientific and dated belief that it is the lack of oxygen that causes the accumulation of lactate in both the cell and in the blood stream. These are both old and disproven concepts. You see, there is plenty of oxygen available to your muscles for contraction when you hit the intensity known as threshold #1 or low threshold, the first point when exercise is no longer sustainable. Technically, the problem isn’t getting enough oxygen in; it’s getting rid of the acid and carbon dioxide, the substances produced during hard exercise, out of the cell and out of the blood stream.
It is the accumulation of lactate that causes the excess acid and carbon dioxide. That’s why lactate — not lack of oxygen— is a more accurate way to measure T1 or low threshold. Some background information can help here:
During higher and higher intensity exercise, the body produces greater amounts of pyruvic acid. (pyruvate is a metabolite, or small molecule that is created by changes or breakdowns in the body’s metabolism.) In order to keep from shutting down one of your metabolic processes, your cell converts pyruvate to lactate, which is then transported out of the cell and used as fuel in other places in the body.
Lactate is a partially broken-down version of glucose, so another cell can convert it back to pyruvate and then go about oxidizing it. There are three main places that lactate is consumed: the heart, the liver, and non-active skeletal muscles. Your heart never stops using lactate as a fuel, but as you exercise harder, you have fewer non-active muscles and the blood flow to your liver decreases, so your ability to remove lactate declines. Given that the rate of lactate production is ever-increasing, the failure to be able to remove lactate is why it accumulates in your blood.
So, if lactate is a fuel, why do we see it as such a bad guy in terms of exercise? When lactate is transported out of the cell, it drags some hydrogen ions (e.g. acid) out of the cell. It is also, by itself, a weak acid. So, as lactate enters the area around the cell, the area becomes more acidic. To protect against an increase in acidity, the body maintains a certain amount of a buffer (bicarbonate) in the fluid around the cell. When the bicarbonate combines with the acid that comes along for the ride with lactate, it produces CO2 and water. What do you do with the extra CO2 that comes from this buffering process? You breathe it out, which leads to a relatively sudden increase in the amount of breathing, which we typically call the ventilatory threshold.