Anaerobic exercise
Anaerobic exercise is typically used by athletes in non-endurance sports to build power and by body builders to build muscle mass.  Muscles that are trained under anaerobic conditions develop biologically differently giving them greater performance in short duration-high intensity activities.
Aerobic exercise, on the other hand, includes lower intensity activities performed for longer periods of time. Activities like walking, running, swimming, and cycling require a great deal of oxygen to generate the energy needed for prolonged exercise.
There are two types of anaerobic energy system, the ATP-PCr energy system, which uses creatine phosphate as the main energy source, and the lactic-acid (or anaerobic glycolysis) system that uses glucose (or glycogen) in the absence of oxygen. The latter is an inefficient use of glucose and produces by-products that are thought to be detrimental to muscle function. The lactic-acid system is the dominant energy system during high to maximal intensity exercise over short durations (up to about 1 min), but the lactic acid system can still provide a proportion of the required energy during aerobic exercise, as the body has the capacity to get rid of the anaerobic by-products at a certain rate.  The efficiency of by-product removal by muscles can improve through training.

# Anaerobic biochemistry
Anaerobics are activities that are carried out 'without oxygen'. This terminology refers to the molecular level of respiration, not the respiration of the organism as a whole (i.e., breathing). During anaerobic exercise, the muscles being exercised have insufficient oxygen to meet the demands of the activity, and thus must also use alternate, non-oxygen-dependent processes to produce energy. The muscle does still receive oxygen during anaerobic exercise; the average drop in blood oxygen content throughout the body is likely minimal.
Anaerobic exercise begins with muscles utilizing stored creatine phosphate to generate the ATP that produces muscle contraction. After several seconds, further ATP energy is made available to muscles by metabolizing muscle glycogen into pyruvate through glycolysis, as it normally does through the aerobic cycle. What differs is that pyruvate, rather than be broken down through the slower but more energy efficient aerobic process, is fermented to lactic acid.
Muscle glycogen is restored from blood sugar, which comes from the liver, from digested carbohydrates, or from amino acids which have been turned into glucose.

# Lactate threshold (Now called LIP or Lactate Inflection Point)
The lactate threshold (LT) is the exercise intensity at which lactate  starts to accumulate in the blood stream. This happens when it is produced faster than it can be removed (metabolized).  This point is sometimes referred to as the anaerobic threshold (AT), or the onset of blood lactate accumulation (OBLA). When exercising below the LT intensity any lactate produced by the muscles is removed by the body without it building up.
The lactate threshold is a useful measure for deciding exercise intensity for training and racing in endurance sports (e.g. distance running, cycling, rowing, swimming and cross country skiing), and can be increased greatly with training.
Fartlek (speed-play) training and interval training take advantage of the body being able to temporarily exceed the lactate threshold, and then recover (reduce blood-lactate) while operating at below the threshold, but still doing physical activity. Fartlek and interval training are similar, the main difference being the relative intensities of the exercise, best illustrated in a real-world example: Fartlek training would involve constantly running, for a period time running just above the lactate threshold, and then running at just below it, while interval training would be running quite high above the threshold, but then slowing to a walk or slow jog during the rest periods.
Fartlek would be used by people who are constantly moving, with occasional bouts of speed, such as basketballers, while interval training is more suited to sprinters, who exert maximum effort and then can stop exerting completely. With both styles of training, one can exert more effort before fatiguing and burn more calories than exercising at a constant pace (continuous training), but will emphasize training the anaerobic system rather than the aerobic system. Long duration training below the lactate threshold is recommended to primarily work the aerobic system.
Accurately measuring the lactate threshold involves taking blood samples (normally a pinprick to the finger, earlobe or thumb) during a ramp test where the exercise intensity is progressively increased. Measuring the threshold can also be performed non-invasively using gas-exchange (Respiratory quotient) methods, which requires a metabolic cart to measure air inspired and expired.
Although the lactate threshold is defined as the point when lactic acid starts to accumulate, some testers approximate this by using the point at which lactate reaches a concentration of 4 mM (at rest it is around 1 mM)....