The aerobic threshold (AeT) is a term sometimes used by sports coaches and trainers to describe a level of exercise somewhat below the anaerobic threshold. It, however, is not a defined physiological term. The AeT is sometimes defined as the exercise intensity at which anaerobic energy pathways start to operate and where blood lactate reaches a concentration of 2 mmol/litre (at rest it is around 1). This tends to be at a heartrate of approximately 20-40 bpm less than the anaerobic threshold and correlates with about 65% of the maximum heart rate.