AT: Anaerobic threshold -

The amount of available oxygen is no longer sufficient to meet the body's energy demands and a second pathway called anaerobic ("without oxygen") glycolysis is recruited. The end product of anaerobic glycolysis is lactic acid (lactate). As running pace is increased futher, the lactate concentration in the exercising muscles increases rapidly and this point is referred to as tha anaerobic threshold (AT) or lactate turnpoint. Subjectively, this is where a pace at which breathing becomes more labored and the dreaded burning sensation in the legs begins to appear. Well-trained athletes usually reach their AT at approximately 85-90% of their VO2max heart-rate, but for untrained individuals this threshold is much lower (50-70% of VO2max heart-rate).

An article, Why does lactic acid build up in muscles? And why does it cause soreness? at - Scientific American, says
The body prefers to generate most of its energy using aerobic methods, meaning with oxygen. Some circumstances, however, --such as evading the historical saber tooth tiger or lifting heavy weights--require energy production faster than our bodies can adequately deliver oxygen. In those cases, the working muscles generate energy anaerobically. This energy comes from glucose through a process called glycolysis, in which glucose is broken down or metabolized into a substance called pyruvate through a series of steps. When the body has plenty of oxygen, pyruvate is shuttled to an aerobic pathway to be further broken down for more energy. But when oxygen is limited, the body temporarily converts pyruvate into a substance called lactate, which allows glucose breakdown--and thus energy production--to continue. The working muscle cells can continue this type of anaerobic energy production at high rates for one to three minutes, during which time lactate can accumulate to high levels.

A side effect of high lactate levels is an increase in the acidity of the muscle cells, along with disruptions of other metabolites. The same metabolic pathways that permit the breakdown of glucose to energy perform poorly in this acidic environment. On the surface, it seems counterproductive that a working muscle would produce something that would slow its capacity for more work. In reality, this is a natural defense mechanism for the body; it prevents permanent damage during extreme exertion by slowing the key systems needed to maintain muscle contraction. Once the body slows down, oxygen becomes available and lactate reverts back to pyruvate, allowing continued aerobic metabolism and energy for the body's recovery from the strenuous event. Contrary to popular opinion, lactate or, as it is often called, lactic acid buildup is not responsible for the muscle soreness felt in the days following strenuous exercise. References:

• Why does lactic acid build up in muscles? And why does it cause soreness? - Scientific American