Anaerobic processes do not require oxygen for reactions while aerobic processes utilize oxygen in the processes of breakdown and synthesis.
All anaerobic processes occur in the sarcoplasm (the cytoplasm/cell fluid of the muscle cell).
All aerobic processes (utilizing the oxidative/aerobic energy system/metabolic pathway) occur in the mitochondria of muscle cells.
The and duration of activity will determine which energy system/metabolic pathway is dominant at any particular time.
Quick fun fact: The word "aerobic" means "with oxygen" (originally coined by Louis Pasteur, "living only in the presence of oxygen," 1884, from the Greek, aero- "air" and bios "life").
As the level of intensity of exercise increases, the body reaches a point where the level of oxygen within the cell's mitochondria is not sufficient.
Since there is not enough available oxygen, the pyruvate produced by glycolysis cannot be oxidized.
Furthermore, since energy production must continue, ATP production from glycolysis must increase.
For glycolysis to continue to produce ATP, certain compounds that were "used up" (i.
e.
reduced) in glycolysis must be regenerated (i.
e.
oxidized) so that the process can continue.
For this to happen the excess pyruvate is reduced to lactate.
Lactate production begins to occur at a rapidly increasing rate (due to maintaining a high level of intensity for either seconds or minutes, depending on conditioning).
This point is known as the Lactate Threshold (LT).
Note: this is also often referred to as the Anaerobic Threshold (AT).
The only essential difference between the lactate and anaerobic threshold is that the lactate threshold is measured through blood concentrations of lactate and the anaerobic threshold is measured by a ventilatory (gas exchange) test.
Lactate accumulation in the blood is signal that there is not enough oxygen getting to the working muscles (i.
e.
"going anaerobic").
During energy production, there are several reactions that cause a release of a hydrogen ion (H+).
This causes a decrease in blood pH (i.
e.
making the blood more acidic).
Without going into detailed biochemistry, when the body is "aerobic" it uses buffering capabilities so that the pH of the blood does not go down.
However, when there is not enough oxygen being supplied to the working muscles, muscle acidosis can begin.
The "burning" sensation in muscles is attributed to this muscle acidosis and correlates to the accumulation of lactate.
The point where so much lactate accumulates that exercise intensity is forcibly decreased (due to muscle acidosis induced fatigue) is known as the Onset of Blood Lactate Accumulation (OBLA).
So, whatever happened to lactic acid? Science used to attribute the "burning" in muscles to lactic acid.
When working at high intensities, scientists saw that there was an accumulation of lactate and hydrogen ions (causing muscle acidosis).
The theory was that pyruvate was reduced to lactic acid (not lactate) and that as soon as is it released in the blood, it separates into lactate and H+ (hyrdrogen) molecules.
We now know that lactate does NOT CAUSE muscle acidosis, but the onset of lactate CORRELATES to acidosis.
So how does this fact affect training? Not much.
We can still use the lactate threshold to determine when an individual is going "anaerobic" and starting to "redline".